The IMAP-Ultra Energetic Neutral Atom (ENA) Imager

Abstract

NASA’s Interstellar Mapping and Acceleration Probe (IMAP), a spinner spacecraft in orbit around L1, is taking in situ observations of thermal, pickup, and energetic particles, while simultaneously remotely sensing the effects that these particles have in the outer heliosphere, by measuring Energetic Neutral Atoms (ENA) emissions produced by neutralized energetic ions when they charge exchange with interstellar neutral particles in that region. The IMAP-Ultra instrument (Ultra), one of the three ENA imagers on IMAP, measures the emission of the highest energy ENAs produced in the heliosheath and beyond. Ultra consists of two sensors with one sensor angled at 90° (Ultra₉₀) and the other at 45° (Ultra₄₅) from the spacecraft’s spin axis. Ultra was designed and optimized to measure hydrogen (H) ENAs from 5 – 40 keV, but the sensors have been demonstrated to measure H from ∼3 – 300 keV. Additionally, Ultra’s large ∼96° × 120° field of view (FoV) is capable of achieving angular resolutions ≤ 6° FWHM for ≥ 10 keV for H ENAs. Ultra provides high spatial resolution, full heliosphere maps, detecting changes in the spatial distribution of ENAs, on time scales sufficient to track both solar cycle as well as other major variations.

Introduction to IMAP Science

The Interstellar Mapping and Acceleration Probe (IMAP) mission (McComas et al. 2025, 2018) provides extensive and well-coordinated observations of the inner and outer heliosphere investigating the acceleration of charged particles and the interaction of the solar wind with the local interstellar medium. These topics are intimately coupled because particles accelerated in the inner heliosphere propagate outward through the solar wind and mediate its interaction with the very local interstellar medium (VLISM). IMAP addresses four fundamental Heliophysics Science Objectives:

1. Improve understanding of the composition and properties of the local interstellar medium (LISM);

2. Advance understanding of the temporal and spatial evolution of the boundary region in which the solar wind and the interstellar medium interact;

3. Identify and advance the understanding of processes related to the interactions of the magnetic field of the Sun and the LISM, and;

4. Identify and advance understanding of particle injection and acceleration processes near the Sun, in the heliosphere and heliosheath.

As the solar system moves through the local interstellar medium (LISM), interstellar neutral (ISN) atoms enter the heliosphere and a fraction of them charge-exchange with the Solar Wind (SW) ions and heated plasma ions. During this process, the SW protons gain an electron and become neutral hydrogen, still flowing outward at the SW velocity, while the newly created ion begins gyrating and advecting outward with the SW under the force of the $\mathit{V}\times \mathit{B}$ electric field, producing the population that is known as interstellar pickup ions (PUIs). The PUIs are further heated at the Termination Shock (TS), while much of the neutral SW hydrogen travels on through the heliosheath (HS) into the interstellar medium, eventually to charge-exchange and begin gyrating in the interstellar magnetic field (ISM). The SW ion population slows down from ∼400 km/s to ∼150 km/s at the TS but remains supersonic in the HS. Most of the SW bulk flow energy removed at the TS goes into the heating of the PUIs. The heated PUIs that populate the HS downstream from the TS form a source of Energetic Neutral Atoms (ENAs), created as they charge-exchange with the interstellar neutral particles in that region. This population ranges from somewhat less to much greater energy than the pre-TS SW flow energy. Those ENAs—unaffected by electric and magnetic fields—can travel freely in space, and some of them make it all the way back to the inner solar system, to IMAP’s L1 orbit. Finally, these ENAs retain critical information about the regions where they are produced, and remotely measuring their characteristics can advance our understanding of these distant regions.

Full-sky ENA intensity maps produced by the Interstellar Boundary Explorer (IBEX) mission (McComas et al. 2009a) showed the existence of a bright and narrow “ribbon” (McComas et al. 2009b; Schwadron et al. 2009) of ENA emissions, of energies ∼0.4 – 6 keV, and intensities up to a factor of two more than the Globally Distributed ENA Flux (GDF), an ENA flux across the entire sky. The ribbon is measured by IBEX-Hi (Funsten et al. 2009; Schwadron et al. 2009, 2018), and IBEX-Lo (Galli et al. 2014, 2022) and is thought to lie beyond the heliopause, formed through a secondary ENA process (e.g., Heerikhuisen et al. 2010; McComas et al. 2009b, 2017). The GDF (McComas et al. 2009b; Schwadron et al. 2011), on the other hand, is largely produced in the HS (Gruntman et al. 2001; Zank et al. 2010; Dayeh et al. 2011; Zirnstein et al. 2020; McComas et al. 2009b, 2020). The Ion Neutral Camera (INCA) (Krimigis et al. 2004) on board the Cassini mission, observed a “belt” of higher energy ENAs (>5 keV). The belt is significantly wider than the ribbon and not organized by the local interstellar magnetic field; it is suspected to originate from within the HS and is constantly replenished by particles accelerated at the TS and possibly in the HS (Dialynas et al. 2013, 2017, 2019).

It is possible that the IBEX ribbon at energies >6 keV may evolve naturally into the belt (i.e., the secondary ENA production rate is overshadowed by emissions from inside the heliopause), but there is still a high uncertainty of this connection (Westlake et al. 2020). Furthermore, recent comparisons between simulated ENAs, and IBEX (∼0.4 – 6 keV) and INCA (∼5 – 55 keV) observations (Gkioulidou et al. 2022; Kornbleuth et al. 2023a,b) showed a notable difference in the INCA 8 – 15 keV energy range, suggesting further acceleration of the PUIs in the HS.

To address IMAP’s scientific objectives and the mysteries discussed above (see Reisenfeld et al. 2026 for a more detailed discussion), IMAP has three energetic neutral atom (ENA) imagers onboard; namely, IMAP-Lo (Lo) (Schwadron et al. 2025), IMAP-Hi (Hi) (Funsten et al. 2026), and IMAP-Ultra (Ultra). Collectively, the three instruments are capable of measuring ENA emissions over an energy range from ∼100 eV to ∼300 keV, produced by neutralized energetic ions when thermal solar wind, pickup ions, and energetic particles charge exchange with interstellar neutrals in the outer heliosphere.

Ultra Overview

The Ultra instrument consists of two identical ENA imagers (Fig. 1) and covers ∼3$\pi$ sr of the full celestial sphere with each spacecraft spin. Ultra was designed and optimized for measuring neutral Hydrogen (H) over the energy range 5 – 40 keV, with an angular resolution as fine as 2° above 30 keV (see Sect. 2). However, Ultra is capable of measuring H over an energy range from ∼3 – 300 keV and can resolve other species, such as He and O. Ultra is the 2nd generation ion and ENA camera designed and built by APL (Mitchell et al. 2016). The first generation consisted of the Ion Neutral Camera (INCA) (Krimigis et al. 2004) on board the NASA Cassini mission, launched in 1997. INCA was followed by the similar design with the High-Energy Neutral Atom (HENA) imager (Mitchell et al. 2000) on board the NASA Imager for Magnetopause-to-Aurora Global Explorer (IMAGE) mission (Burch 2000). Ultra is an almost identical copy of the Jovian Energetic Neutrals and Ions (JENI) instrument (Mitchell et al. 2016) onboard ESA’s Jupiter ICy moons Explorer (JUICE) mission. JUICE was launched in April 2023 and JENI has undergone several instrument checkout campaigns and was fully operational during JUICE’s Lunar Earth Gravity Assist (LEGA). JENI is performing nominally.

Fig. 1.

The two Flight Model (FM) Ultra sensors

Figure 2 shows the two Ultra sensors, Ultra₄₅ and Ultra₉₀ mounted on the IMAP spacecraft, as well as their Fields Of Regard (FOR) and Fields Of View (FOV). Ultra₄₅ is mounted at 45° from the anti-sunward spin axis, covering half of the sky from perpendicular to the spin axis to the anti-sunward spin axis. Ultra₉₀ is mounted perpendicular to from the spin axis covering the sky from 45° to 135° from the Sun. Together, Ultra₉₀ and Ultra₄₅ have ∼35 times more collecting power—defined as the product of the sensitivity and viewing times—compared to INCA.

Fig. 2.

Top panels: Ultra₉₀ (left) and Ultra₄₅ (right) sensors on the IMAP spacecraft, together with members of the Ultra and IMAP engineering teams; Middle panels: Ultra₉₀ (left) and Ultra₄₅ (right) right on the IMAP spacecraft, before encapsulation Bottom panels: FOR and FOV for the Ultra₉₀ and Ultra₄₅ sensors. The spacecraft spin axis is toward the top of the page

The Ultra sensors provide images across the suprathermal energy range with (relative to INCA) lower energy threshold, and improved background rejection and angular resolution, all necessary to resolve heliospheric structures such as the ribbon-to-belt transition (Dialynas et al. 2013), which was left unresolved by INCA and IBEX. Ultra also addresses physical processes regulating the global heliosphere, such as particle acceleration taking place in the outer heliosphere.

The larger collecting power of the two Ultra imagers together, and higher angular resolution (≤6^o FWHM for ≥ 10 keV H ENAs) compared to INCA (8^o FWHM), allow for a more accurate determination of the ENA intensity in the suprathermal energy range, with better spatial and temporal resolution. Higher temporal resolution of 3 months in particular, could potentially enable us to capture not only the response of the outer heliosphere to SW pressure pulse events, like IBEX observations showed (McComas et al. 2017; Sokół et al. 2021), but also solar transients in the heliosheath on shorter timescales. Ultra’s energy range extends up to ∼300 keV, allowing for the first ever measurements of ≥100 keV heliospheric ENAs, dependent on signal-to-noise ratio (SNR), which may allow us to probe further down the heliotail (Zirnstein et al. 2025).

A great advantage of the comprehensive IMAP ENA imager suite is that the Hi and Ultra energy ranges overlap between ∼5 and 15 keV, which is the range over which the discrepancy between the observed and simulated ENAs spectrum at 5 – 8 keV remains, as mentioned above. To ensure measurement consistency between the two instruments, Hi and Ultra underwent a detailed cross-calibration campaign prior to launch (see Sect. 3.5), in the same facility, using the same hydrogen beams in a common energy range, which enabled the accurate characterization of their detection efficiencies. Having two instruments, on the same spacecraft in the L1 environment, away from any magnetospheric influence (such as Earth’s, in IBEX’s case, or Saturn’s, in INCA’s case), capable of making detailed measurements across this critical energy range, is necessary to put to rest questions about the shape of the ENA energy spectrum and address discrepancies that are central to the pursuit of particle acceleration in the outer heliosphere.

Ultra Measurement Requirements and Performance

The Ultra measurement requirements were set so that the combined measurements of Lo, Hi and Ultra would meet the IMAP mission Level 1 requirements related to the creation of ENA full-sky maps (see Fig. 8 of McComas et al. 2025) to address the open questions discussed above. Table 1 summarizes the Ultra Sensor Parameters. Table 2 lists key Ultra measurement requirements along with Ultra’s demonstrated performance, based on results from a comprehensive calibration campaign (Sect. 3), and the sections in this paper where the specific aspects of the instrument performance are being discussed.

Table 1.

Ultra Sensor Parameters

Ultra Sensor Parameters
Parameter (per sensor)	Performance
FOV	MCPs: ∼96° × 120°
	SSDs: ∼70 ° × 120°
Mass	7.66 kg
Volume	454 mm (L) × 359 mm (W) × 283 mm (H)
Power	8.92 W
Telemetry	1530 bps

Table 2.

Ultra Measurement Requirements

Ultra Measurement Requirements
Parameters	Requirements	Performance
Measured particles	ENAs	ENAs
		Ions and electrons
Energy range (Sect. 3.2.2)	5 keV – 40 keV (H)	3 keV – 300 keV (H) 3 keV – 5 MeV (Ions) 30 – 700 keV (electrons)
Energy Resolution, ΔE/E (Sect. 3.2.2)	≤70% for 5 keV – 40 keV (H)	12% - 21% (intrinsic resolution). Note: sky maps will be binned in ∼40% ΔE/E to optimize SNR.
Masses resolved (Sect. 3.2.3)	mass resolution to differentiate H (lights) from O (heavies)	MCP-based: H and He (lights), O (heavies) SSD-based: m/Δm ∼4
Angular resolution (Sect. 3.2.4)	6° FWHM for 10 keV H	≤6° FWHM for ≥10 keV H
Signal to Noise Ratio (SNR) (Sect. 3.2.7.5)	>20 at 10 keV for an assumed H ENA flux of 0.2 cm−2sr−1s−1keV−1	∼180 at 10 keV, but varies between ∼20 – 180 over the required energy range.
		Geometric Factor of different apertures (cm2 sr)	MCPs: 0.006 (pinhole), 0.15 (narrow slit), 0.6 (wide slit)
			SSDs: 0.0006 (pinhole), 0.015 (narrow slit), 0.06 (wide slit)
Flux calibration relative accuracy (Sect. 3.3)	relative accuracy of ±10% in the energy range 5 to 10 keV	relative accuracy of ±10% in the energy range 5 to 10 keV
Flux calibration absolute accuracy (Sect. 3.3)	absolute accuracy of ±25% in the energy range 5 to 10 keV.	absolute accuracy of ±25% in the energy range 5 to 10 keV.

Ultra Sensor Functional Overview

Each of the two identical Ultra sensors is a slit based ENA imager, with a planar axis of symmetry and two separate deflection assemblies and apertures (slits), left and right (Fig. 3 and Fig. 4). These two entrance subsystems have slight overlap in FOV, and their FOVs are projected onto a common detector system that measures ENA TOF and pulse height, incident velocity vector, and, for a subset of ENAs, energy.

Fig. 3.

Top panel: Ultra CAD drawing showing major elements: charged-particle deflection plates; Front (Start-Coincidence) MCP assembly with two 1D start anodes servicing two entrance slits, and one 1D coincidence anode; variable aperture mechanism servicing each entrance slit; two stop MCPs imaged using 2D anodes; and a strip of SSD detectors between the two stop MCP; Bottom panel: Ultra cross-sectional view showing trajectories of particles within the sensor, based on SIMION simulations

Fig. 4.

Upper and Middle panels: Sketches of the Ultra sensor showcasing different examples of primary ENA trajectories hitting either the MCP (upper panel) or the SSDs (lower panel) and secondary coincidence electron trajectories hitting either the coincidence anode (upper panel) or the dedicated SSD stop anode (lower panel). We are also identifying the major components of the Front Anode Assembly (Sect. 2.4.1) and the back plane of the sensor (Sect. 2.4.2); Bottom panel: Ultra sensor geometry

The Ultra sensor is comprised of a charged-particle deflection assembly (Sect. 2.1), two slit entrances with variable apertures (VA) (Sect. 2.2), foil-based TOF microchannel plate imager (MCPs), and solid state detector (SSD) energy subsystems (Sect. 2.4). The top panel of Fig. 3 illustrates these major components.

A cross-section view of the sensor, showing 2D electron optics based on SIMION software (Manura and Dahl 2008) simulations can be seen in the bottom panel of Fig. 3: Particles enter the deflection system, where the strong electric field between alternating grounded and positive HV (up to +8 kV) plates deflects charged particles into the serrated plate surfaces, effectively eliminating their ability to forward scatter into the aperture. This way the charged particle incident flux is reduced by ∼10⁴ for ions and ∼100 for electrons, below the nominal rejection energy (300 keV/q for +8 kV), while the undeflected ENAs enter the sensor (magenta trajectory). ENAs then proceed through a stepper motor-driven variable aperture, which provides a variety of aperture configurations, with different sensitivities and angular resolutions, with or without additional UV filtering (provided by ∼ 5 μg cm⁻² polyimide foil). The ENAs enter the instrument through the start foil (1 μg cm⁻² C) that runs over the length of the entrance slit) producing secondary electrons upon particle traversal. Shaped electrodes, formed in part by a series of harp wires (see Sect. 2.4.2) generate the electric fields (red equipotential contours) to accelerate and steer the secondary electrons into the Start MCP, producing both 1D Start position and Start timing. The particle continues through the sensor to the back plane, where it passes through a 8 μg cm⁻² UV-filtering foil, and strikes the Stop MCP, producing both 2D Stop position and Stop timing. Both Start and Stop use time-delay anodes to measure position to 1 mm spatial resolution, required for the finest intrinsic angular resolution of 2°, limited by secondary electron dispersion in the electron optics, and MCP anode resolution. That finest resolution can only be achieved by energies above 30 keV, where the angular scattering in the foils is not as significant. The combined Start–Stop positions determine the ENA path length (r) between the start and stop foils and combining that with the measured TOF determines the particle velocity (v = r/TOF). The Stop MCP also records pulse height, which, with the TOF, yields a crude determination of particle species, that is, light (e.g., H, He) or heavy (e.g., O, Ne, Fe), a capability inherited from the JENI predecessor, not expected to be utilized on IMAP since we mostly anticipate H ENAs.

Secondary electrons emitted from the entrance surface of the Stop MCP foil are accelerated and guided by electrostatic potentials (red contours in bottom panel of Fig. 3) onto the coincidence region of the start-coincidence MCP (blue trajectories in bottom panel of Fig. 3). The Flight Software valid event (FSW VE) logic (Sect. 3.2.6.1) requires that the coincidence pulse follows the stop pulse within a tight valid event coincidence time window (MCP TOF-only mode).

A row of SSD detectors also located at the back plane, between the two stop MCPs, records the energy deposited for ≥30 keV H particles that hit them. The energy deposited in an SSD, together with TOF derived from the Start time and an SSD Stop taken from the SSD portion of the coincidence anode (middle panel of Fig. 4), yields a so-called TOFxE measurement, sufficient to identify the particle species (H, He, O, Ne, or Fe).

Deflection Assembly

The Ultra charged particle deflection assembly is almost identical to that of JENI. There are 30 deflection blades 0.7 mm thickness, gold plated with a 1016 nm layer, 15 blades for each aperture. The blade material is AlBeMet, as its rigidity to weight ratio is much higher than aluminum, allowing fewer required structural supports to withstand launch loads (Fig. 1). When biased with positive high voltage (HV), charged particles are swept into the blades while ENAs are unaffected and continue their incident trajectories. Up to +8 kV can be applied to the deflection blades, with nominal operations at +4 kV, therefore excluding charged particles up to 120 keV/q.

The plane of each blade intersects the center of the slit aperture. Because the 0.5 mm-wide aperture (nominal aperture; see Sect. 2.2) is thinner than the blade thickness, no serrated surface of any blade has a line-of-sight directly into the aperture, which prevents scattered particles from entering the aperture.

The short ends of the plates are mounted to insulators manufactured from Ultem. This was a design change from JENI, which used Semitron^®, a Thorlon-based thermoplastic, to mount the plates, due to its superior properties to dissipate charge in the radiation environment of the Jovian magnetosphere. The Ultem insulators on Ultra allow for more robust HV stand-off compared to JENI. To minimize the amplitude of mechanical deflection during vibration, the plates are separated by small “nubs” placed around the length-wise center between plates, also manufactured from Ultem. To minimize forward scattering, each plate is serrated on both sides. The serrations are 0.125 mm deep and the central AlBeMet core material is 0.45 mm.

Another design change from JENI is that Ultra deflection assembly openings are not covered with a high-transmission grounding grid. The grid is used on JENI to electrostatically shield the high voltages present on the blades from the rest of the payload.

Variable Aperture (VA)

Once the ENAs go through the deflection assembly they encounter a shutter with a variety of apertures, controlled by a stepper motor (Fig. 5). The shutter can be commanded to go to a position with a certain aperture configuration. Table 3 lists the shutter positions and their corresponding aperture configurations. Note that there is a different GF associated with each of the apertures, as shown in Table 2.

Fig. 5.

Top Left Panel: The Ultra rotating, variable aperture mechanisms are the upper and lower cylindrical structures (shutters). Bottom Left, Top Right, and Bottom Right Panels: CAD models of the variable aperture mechanism

Table 3.

Ultra Shutter Positions and Apertures

Two of the positions are closed, with one presenting a radioactive ²⁴¹Am source of 5.486 MeV alpha particles to the sensor for in-flight calibration, and the Dark one, blocking the sensor with the thickest part of the mechanism to allow background calibrations. The shutter has four kinds of apertures consisting of differing numbers of slits with different dimensions (Ap-1 has a single slit with the largest open area, and Ap-2 through Ap-4, successively reduce the effective area). Finally, three of the positions have a UV filter (∼5 μg/cm² polyimide foil).

The variable aperture mechanisms on each slit can be controlled independently, so that for example the UV filter can be used on one aperture and not on the other, as needed. The nominal Ultra operation is to use the narrow slit without the UV filter foil, i.e., position 1; the narrow slit enables superior angular resolution while maintaining a sufficient geometry factor. The entire mechanism is anchored to the instrument with a duplex bearing. A launch lock is also incorporated to prevent rotation during launch and environmental testing. The variable aperture mechanism is driven by a 2-phase, non-redundant, stepper motor through a two stage planetary gear reduction (60:1) ratio. The mechanism is designed to be fabricated and assembled at +20 °C and operates over a range of temperatures from −50° to +70 °C. JENI’s aperture motor successfully completed life testing with 82,824 motions and the gear motor executed 8,128,197 steps.

Electron Optics

The electron optics shown in Fig. 3 (bottom panel) is the most critical subsystem in the sensor and therefore deserves a detailed overview. The electrostatic potential contours (red lines) are produced by electrodes (at 0 and +2 kV) and thin wires (at 0 V) strung in a harp-like configuration along the length of the sensor (i.e., along $\hat{x}$, as shown in the bottom panel of Fig. 4). The wires are constructed from Tungsten and mounted by spring loaded tensioners in the stop assembly (Sect. 2.5) just below the two entrance apertures. The electrodes are also mounted together with the harp wires in the stop assembly.

The secondary electrons emitted from the exit surface of the start foil, i.e., “start electrons”, are accelerated by a 2 kV start grid just below the foil, which minimizes the thermal spread angular and temporal dispersion of the start electrons exiting the foil. It takes about 1.5 ns for the start electrons to travel from the start foil to the start area of the Start-Coincidence MCP, where the start anode of the coincidence assembly registers their 1D position (along $\hat{x}$, as shown in the bottom panel of Fig. 4). When penetrating the start foil, the primary particle undergoes energy straggling and angular scattering, both of which depend on species, incident energy, and angle of incidence relative to the foil normal direction. Angular scattering is the main factor determining the angular resolution of Ultra (see Table 2).

In ENA mode, the primary particles are neutral, but an energy and species dependent fraction of the neutrals ionize after trasversing the start foil (Allegrini et al. 2016). Thus, these ionized ENAs are affected by the electron steering potentials at low energies (Sect. 3.2.4). At higher energies the ionized fraction increases, but the trajectories of those ionized primary particles are not significantly affected due to their high energy compared to the electrostatic potential in the flight path. Thus, at higher energies, the magnitude of this effect decreases.

Once the primary particle trasverses the stop foil, it emits secondary electrons from the entrance surface of the foil that are accelerated, through +2 kV potential difference, up towards the coincidence area of the Start-Coincidence MCP. The travel time of the secondary electrons from the stop foil varies between about 3.5 ns near the center of the instrument to about 8 ns near the edges of the back plane. Adding the time dispersion caused by the electron emission distributions (in angle and energy), the total window of all trajectories broadens to about 7.5 ns FWHM. The electron optics is designed to produce no forces in the dimension parallel to the slit entrances, (i.e., along $\hat{x}$), so in that dimension the mapping from the stop position and the coincidence position is nearly one-to-one aside from spreading caused by the initial distributions of the secondary electrons. The short time window of the TOF of the stop secondary electrons serves as a second timing coincidence to dramatically reduce backgrounds from accidental coincidences (see Sect. 3.2.6.1 for more details).

Start and Stop Assemblies

The Ultra start assembly (Fig. 6) is identical to that of JENI. It comprises a Start-Coincidence MCP held in an Ultem holder using a Titanium (Ti) compression plate and a wing-like start electron acceleration grid frame mounted to the compression plate on the electron input side which is biased at +2 kV. The entire start assembly is then mounted in the upper part of the housing holding the two entrance slits, where 1 μg/cm² grid-supported carbon foils are mounted. The MCP is backed by two 1-D positioning start anodes, two 1-D position coincidence anodes, and one discrete coincidence anode pad in the middle (Sect. 2.4.1). The Start-Coincidence MCP measures 67.2 cm² (approximately 13.5 × 5 cm) and is a Chevron pair of 1.5 mm thick plates with 25 μm diameter pores at 32 μm pitch.

Fig. 6.

Ultra start assembly consisting of upper housing with the two apertures, respective start foil holders and Start-Coincidence MCP shown. The acceleration grids are the two wing-like structures near the apertures

The Front (Start-Coincidence) Anode Assembly

The Ultra start, coincidence, and SSD stop pulses are registered in a five-part anode (Fig. 7) mounted directly behind the Start-Coincidence MCP. A SSD stop pulse is a detected event registered by the Start-Coincidence MCP from secondary electrons emitted from the entrance surface of the stop foil immediately above the SSD, and following the same trajectories as the coincidence electrons above the stop MCP foils.

Fig. 7.

Ultra start and coincidence anode board. The two start anodes can be seen on the top and bottom. The coincidence anodes are the two anodes bisected by the discrete SSD Stop anode

The two Start anodes (one for each entrance aperture) are the two linear arrays of thin rectangular pads along the top and bottom of Fig. 7. The Front Anode pad dimensions are 0.75 mm × 6 mm and are spaced at 1 mm pitch. In each array, every other pad is connected to a tap of an LC delay line to provide the “Position” pulse at one end while the other interleaved pads are connected directly together to provide the “Full” pulse at the other end (see Fig. 11). The measured time difference between the two pulses identifies the position of the MCP output pulse along $\hat{x}$ (bottom panel of Fig. 4). Due to the proximity of the start acceleration grid to the foil the measured $\hat{x}$ position is < 1 mm of the true $\hat{x}$ position of the particle’s location on the start foil. The TOF Start time for the ENA TOF is derived from the Start Full pulse, corrected for the pulse propagation time along the Full anode (up to 1 ns depending on the $\hat{x}$ position) and the 1.5 ns electron TOF from the Start foil to the MCP.

Fig. 11.

Ultra block diagram

Coincidence electrons emitted from the entrance of the stop foil covering the Stop MCPs are accelerated and guided by the steering potentials onto the coincidence regions of the start-coincidence MCP, where their 1D position is determined in one of two linear arrays of delay line-connected pads forming the coincidence anodes on each side of the SSD stop anode. For particles that traverse the Stop foil above the SSDs and deposit their energy in one of the SSD pixels, the emitted secondary electron trajectories lead to the discrete SSD Stop timing anode located between the two linear array coincidence anodes (Fig. 7; Fig. 4 top and middle panels). In addition to the arrays of pads and delay lines the opposite side of the anode board carries the nine frontend amplifiers needed by the four delay lines and the single SSD stop pad.

Stop Assembly

The stop assembly (Fig. 8) consists of two stop MCPs (Top and Bottom) separated by a strip of eight Silicon SSD pixels (50 nm window, 1.5 mm thick, 8 mm × 8 mm), stop foil, SSD foil and stop grid. The stop grid is mounted to a Ti frame and kept at ∼+150 V to accelerate the secondary electrons released from the stop foil. The stop and SSD foils are held at ground potential. The grid frame, stop foil, and SSD foil are stacked on top of the MCP and separated by insulators. Each stop MCP rests in an Ultem holder and is held in place by a Ti compression frame, on the particle input side, which is biased at ∼+40 V. The stop assembly sits in the sensor housing that also holds the electric field-shaping harp wires seen in Fig. 8. As the primary particle traverses the Stop foil and impacts the stop MCP, it produces both 2D Stop position and Stop timing pulses. The Stop MCP is backed by a 2D delay-line anode consisting of two orthogonal interleaved rows and columns of pads connecting to taps of two delay lines with four readouts; two for position across the figure and two for position out of the figure plane. The discrete frontend amplifiers for the four readout channels are carried on the opposite side of the anode board.

Fig. 8.

Ultra stop assembly consisting of lower housing showing the two stop MCPs, Top and Bottom, intersected by the SSD strip with the eight SSD pixels. The thin electrode wires can be seen in the upper and lower halves of the picture and form the core of the electron optics system. Field-shaping boards with metal contours shaped to minimize the fringing fields in the electron optics are also shown

As in the case of the start-coincidence anode, the TOF Stop time is derived by correcting the position timing pulses for the anode delays. This Stop position and time, together with the Start position and Start time, uniquely determine the particle trajectory and TOF (velocity). The pulse height (PH) from the Stop MCP is also recorded, and paired with the TOF, yields a rough determination of particle species (either light—H, He, or heavy—O). Each Stop MCP is a Chevron pair of 90 mm × 58 mm × 1.5 mm plates having 25 μm diameter pores at 32 μm pitch. The active area defined by the holder is 82 mm × 50 mm.

The two Stop MCPs are separated by the row of eight SSD pixels (Fig. 9) that are mounted directly beneath their own foil. Unlike JENI, which has both large and small pixels, Ultra SSD assembly has only large pixels. Particles that hit one of the SSD pixels do not produce a Stop MCP timing or position pulse, but their position is known from the location of the SSD pixel they enter, and the SSD returns their deposited energy. This energy, together with TOF derived from the Start time and an SSD stop taken from the SSD portion of the coincidence anode (Fig. 7), yields TOFxE, sufficient to identify the particle species (H, He, or O). Frontend amplifiers for the readout channels are mounted on the opposite of the carrier board supporting the eight pixels.

Fig. 9.

Ultra₄₅ and Ultra₉₀ SSD assemblies. The layout includes 8 pixels arranged across the Ultra back plane of the stop assembly as shown in Fig. 8. The diagram at the bottom indicates the detector numbering relative to their location, which is not in sequential order

Foils

Ultra uses i) a total of six ultra-thin start carbon foils with three segments in each aperture (Fig. 10a), ii) a separate ultra-thin carbon foil over each Stop MCP (Fig. 10b), iii) a thicker, individual foil over the SSDs, and iv) a modest thickness UV filter foil mounted directly in the rotating aperture (Sect. 2.2). All foils went through extensive vibration tests successfully showing 95–99% coverage of intact cells. Both the start and stop foils serve the dual purpose of producing secondary electrons as primary particles penetrate through them and blocking UV and visible light so that UV-generated photoelectrons do not swamp the MCP counting rates and visible light does not increase noise levels in the SSDs. At the levels of exposure to UV on IMAP, the Ultra SSDs are not susceptible to UV. Single photons have far too little energy to deposit the energy needed to exceed their thresholds, and the total energy deposited in the SSD over the interval of the front end amplifier time constant is also far below threshold.

Fig. 10.

a) Ultra start foil, b) Ultra stop foil

Although thicker foils would filter the UV photon flux more effectively, the thickness of the foils, particularly the start foil, limits the minimum energy ion or ENA that can traverse both the start and stop foils and reach the stop MCP. Furthermore, those particles that are sufficiently energetic to traverse the foils are nevertheless scattered in angle and lose some energy in transiting the start foil, so the thinner the foil, the lower the energy loss and the less the angular scattering.

The Ultra start foils, therefore, are relieved of any requirement on UV suppression, and so were made as thin as feasible, consistent with surviving launch and providing a source of secondary electrons. They are identical to JENI start foils, i.e 1 μg/cm² carbon on each entrance. The variable aperture mechanism also provides the option to insert a considerably thicker UV filter (∼ 5 μg/cm² polyimide foil) to allow low UV background operation, increasing the minimum particle energy for the hydrogen that can be detected to ∼10 keV.

The Ultra stop foil covering the Stop MCPs is thicker than that of JENI in order to significantly reduce the UV-driven Stop rates, and so reduce the occurrence of accidental background events. It is comprised of 1 μg/cm² C, 1.8 μg/cm² Si, 4.2 μg/cm² polyimide and then another layer of 1 μg/cm² carbon (supported on 200 lines/inch, i.e., 7.875 lines/mm grid, measured to have ∼70% transmission). This foil is designed to reduce the UV light reaching the Stop MCP by a factor of approximately 20. The carbon, again, provides conductivity and secondary electron emission surfaces.

Over the SSDs, the Ultra foil is thinner than that of JENI because, unlike JENI, the Ultra sensor will not receive direct sunlight. It is a multilayer foil of 1 μg/cm² carbon on the SSD-side of the foil, 5 μg/cm² polyimide, and 1 μg/cm² carbon toward the entrance slit.

Electronics

Frontend Electronics

There are 17 timing signals created by the Start, Stop, and Coincidence MCP anodes. These signals are processed in the electronics to create the position-corrected TOF, MCP pulse height, and incoming particle azimuth and elevation measurements. The Start and Stop anode boards are connected to discrete timing preamplifiers and the SSD assembly includes four quad-channel preamplifier-shaping ASICs to shape and amplify the energy signals. Since the design was inherited from the JENI instrument (having twice as many pixels, Sect. 2.4.2) only two of the four channels in each ASIC are used.

Main Electronics

The main electronics is comprised of an Event Board, Low-Voltage Power Supply (LVPS) and a High-Voltage Power Supply (HVPS) (see Ultra block diagram in Fig. 11). The Event Board handles data processing (single RTAX2000 FPGA with embedded custom 16-bit processor, SRAM, MRAM, PROM), spacecraft I/F (UART), energy signal processing (peak detectors and ADCs), timing signal processing, i.e, Constant Fraction Discriminators (CFDs), and Time-to-Digital Converters (TDCs) and FPGA-based event logic.

The LVPS converts primary spacecraft power to all necessary secondary voltages (+3.3 V Digital, +2.5 V Ref, +1.5 V Supply, −3.5 V, +15 V, −40 V, +5 V) for Analog electronics. It also produces the bias voltage (up to +300 V, nominal at +130 V) for the solid state detectors; this voltage output is commandable (∼2 V resolution).

The HVPS generates five independently adjustable HV outputs from the +15 V secondary supply. There are two independent supplies, each of which has commandable voltages up to +8 kV and attached to one of the deflection sides (left or right). The loads are basically static with almost no current consumption, aside from photoelectron current. There is an additional MCP bulk supply, which provides a commandable voltage up to +4.5 kV to the Start-Coincidence MCP assembly. There are two MCP output taps from the bulk supply, each of which supplies a commandable voltage of up to +2.5 kV to one of the Stop MCP assemblies.

Calibration and Performance

This section describes the calibrations performed and the analysis of those results to demonstrate that both Ultra sensors meet and exceed their requirements.

Calibration tests used a variety of methods, e.g., calibrated pulsers, particle beams, and shutter and external radioactive sources, to stimulate the instrument. Calibrations determine all channel specific parameters needed to convert channel specific count accumulations into calibrated intensity images, i.e., the species, energy, and angle dependent response function.

Calibration Facility

Ultra was tested and calibrated using the APL Accelerator Facility covering ∼ 5 – 170 keV ions and radioactive sources. Additional stimulus is provided by the ²⁴¹Am radioactive sources mounted on the variable aperture which provide in-flight calibration (Sect. 2.2). This facility has a Class 5 clean room and has supported multiple flight instrumentation calibrations. For simplicity, Ultra was tested with an ion beam with the deflector at 0 V. In practice, the charge state distribution of the ion beam exiting the foil (and other properties such as energy loss and angular scattering) is independent of its incident charge state. Therefore, the Ultra performance after the start foil is independent of the charge state of the incident ion beam.

Ultra was also cross-calibrated with IMAP-Hi at the LANL Space Plasma Instrument Calibration Facility, using a neutral hydrogen beam where the energy coverage of the two instruments overlap (Sect. 3.5).

Calibrations

Determination of MCP Operating Voltages

Prior to the calibration campaign, it was found that setting CFD thresholds corresponding to a total MCP signal charge $>6\times {10}^5$ electrons gave adequate suppression of system noise and crosstalk. $6\times {10}^5$ electrons is a small fraction (typically a tenth) of the signal expected from a valid particle at an appropriate HV setting. Then, where possible, MCP high voltage (HV) settings were determined relative to where the normalized counting rate of an associated anode signal as a function of the HV of the MCP achieved a plateau. These tests were performed using both shutter sources (see Sect. 2.2) (∼10⁴ counts/s) and 5 keV proton beam (∼ 100 s of counts/s) as the signal. For example, when only one entrance aperture side is stimulated, the efficiency of the Left (Right) start foils and associated regions of the start MCP can be determined by the ratio of the triple-coincidence hits between the Start, Coincidence and Stop anodes, divided by the double-coincidence hits between Stop and Coincidence anodes.

An ionization event could be caused by an electron, photon, ion or even a penetrating background particle. By requiring coincidences the odds favor ions (photons can produce only single ionizing events and electrons or penetrators have much smaller probability of producing multiple ionization events than ions in the range of interest). So, the hits on the Stop and Coincidence anodes within a time window (double coincidence), are given by # of $\text{(StopCoincidence)}=N_{\mathrm{ion}}\cdot {\epsilon }_{\mathrm{sp}}\cdot {\epsilon }_{\mathrm{co}}$, where $N$_ion is the number of ions coming through the entrance aperture and hitting one of the Stop MCPs, and $\epsilon$_sp and $\epsilon$_co are the detection efficiencies of the Stop and Coincidence anodes respectively. Similarly, the hits on Start, Stop and Coincidence anodes within a time window (triple coincidence) are given by # of $\text{(StartStopCoincidence)}=N_{\mathrm{ion}}\cdot {\epsilon }_{\mathrm{st}}\cdot {\epsilon }_{\mathrm{sp}}\cdot {\epsilon }_{\mathrm{co}}$, where $\epsilon$_st is the Start anode detection efficiency. Therefore, assuming the detection is independent, and the particles are coming through only one of the apertures the efficiency for the associated Start anode ($\epsilon$_st) is given by the ratio # of (StartStopCoincidence) /# of (StopCoincidence).

Figure 12 shows this ratio for 5 keV beam and the shutter source on Ultra₄₅ Right side. The chosen operating level, +3.8 kV, is approximately 50 V above the onset of the plateau for 5 keV beam. Higher energy and/or heavier projectiles produce more secondary electrons when they penetrate the foils and would lead to an earlier plateau (see the alpha particle points) partly due to the larger secondary electron input to the MCP. This effect is even larger on the Stop MCP since the particle stops and produces additional secondary electrons on that surface.

Fig. 12.

Start anode efficiency as a function of the Start-Coincidence MCP HV. The front face of the Start MCP is fixed at +2 kV and the anode is fixed at 100 V positive with respect to the output face of the MCP. Therefore the HV at the start anode being 3800 V translates to 1700 V across the MCP

During the calibration campaign the Stop MCP HV levels were varied across scans at a reduced set of angular steps. We found that the HV needed to be raised by ∼ 50 V for proton energies at 10 keV and below. From the reduced set of angular steps at various HV levels correction factors were obtained that were applied to the already collected data.

TOF × PH Energy Range and Energy Resolution

To satisfy IMAP mission requirement of high energy resolution all-sky maps of Hydrogen energetic neutral atoms, “Ultra shall measure Hydrogen neutral atoms of 5 – 40 keV energies with an energy resolution $\mathrm{\Delta }E$/$E$ (FWHM) ≤ 0.7 for an angular resolution ≤18° × 18°”.

To determine the energy range and resolution of Ultra, we used monoenergetic and directional H⁺ beams with high intensities (compared to heliospheric ENAs) in the lab. The measurements were taken at $\varphi =28.5$ and $\theta =0$, that is the incident beam was normal to the start foil surface. This means that our results represent the “best case” geometry.

Figure 13 shows the measured TOF distributions between 5 and 40 keV. The FWHM of the distribution is then determined and compared to the peak energy. The table on the right in Fig. 13 captures the derived $\mathrm{\Delta }$E/E for both instruments and slits. It is important to note that these numbers represent the intrinsic energy resolution of the instrument, which is much better than both the requirement and what we plan to use for building heliospheric sky maps. Unlike IMAP-Lo and −Hi, which have electrostatic analyzers that define the energy resolution, Ultra does not have pre-assigned energy channels and therefore there is more freedom in choosing how to bin the data in energy. A theoretical analysis based on measurements from IBEX and INCA indicate a binning scheme with an $\mathrm{\Delta }$E/E of 0.4 maximizes statistics while optimizing the energy resolution. This also satisfies the SNR requirement (see Sect. 3.2.7.5) for 6^∘ FWHM angular resolution, which is much better than the spatial resolution of the high energy resolution maps.

Fig. 13.

Left: TOF distributions for 5, 7, 10, 20, and 40 keV H⁺ beam tests in the right slit of Ultra₄₅. The FWHM of those distributions are used to derive $\mathrm{\Delta }E/E$ for H. Right: Table with $\mathrm{\Delta }E/E$ for the same energies, for both Ultra FMs and slits. Larger FWHM with decreasing energy is consistent with energy straggling

TOF × PH Mass Resolution

Ultra can differentiate light (H and He) and heavy (masses above He) ENAs with the same velocities by using the pulse height (PH) measured in the MCP. This technique is possible because heavier ions statistically produce more electrons (i.e., larger PHs) when they interact with matter (Baragiola 1991). To demonstrate this technique works for Ultra, we used two ion species representing light and heavy masses and chose their energy such that their velocities (same measured TOF) were the same. Specifically, we used 7 keV H⁺ and 100 keV N⁺ beams. Figure 14 shows their associated PH distributions as measured by Ultra₉₀. Both the Top and Bottom Stop MCPs were illuminated by the ion beams. As it can be seen, the PH distributions for the two different species are fairly separated (with limited crossover). It is important to note that in flight, the energetic ENA populations will be primarily Hydrogen, thus the majority of our PHs will be generated by light ions. This is because the solar wind is ∼96% hydrogen ions (by number density). Therefore, the PH discrimination will be set such that the minority heavy population is not contaminated by the majority hydrogen population.

Fig. 14.

MCP PH distributions for 7 keV H⁺ and 100 keV N⁺ for the Ultra₉₀ Top and Bottom Stop MCPs

TOF × PH Angular Resolution

Per IMAP mission requirement of high angular resolution all-sky maps of Hydrogen energetic neutral atoms, “Ultra shall measure Hydrogen neutral atoms of 10 – 20 keV energies with an angular resolution of 6 × 6 degrees”.

To satisfy this requirement, we performed a series of elevation – azimuth $(\theta -\varphi$) scans (see bottom panel of Fig. 4 for description of $\theta$, $\varphi$ angles), where we rotated the instrument in discrete steps across its full FOV, and slowly translated along the slit for each $\theta$, $\varphi$ pair to fill in the whole slit with the beam. We then performed these elevation – azimuth scans over the required energy range from 5 – 40 keV with H⁺. The elevation – azimuth testing matrix spanned a $\theta$ range from −44^∘ to +44^∘and a $\varphi$ range from approximately −55^∘ to +55^∘. To balance testing time with sufficient angular granularity, the $\theta$ range was broken up into 8^∘ steps except for the range between ±6^∘ to ±12^∘ where the angular steps are 3^∘. This increased resolution was chosen to characterize the region in the response function with more structure (Fig. 23). The $\varphi$ range was broken into approximately 9^∘ steps. Additionally, at every discrete angle a “X-scan” was performed, where Ultra was translated along the slit (along $\hat{x}$, as shown in the right panel of Fig. 4), from −60 mm to +60 mm, to ensure the beam illuminates the full aperture (80 mm) and thus the MCP back plane. Figure 15 (left panel) shows the various motions that Ultra underwent during the elevation – azimuth scans. Figure 15 (right panel) illustrates the angular distributions of 40 keV H⁺ over all the discrete angles. Note that distributions at 0^∘ in $\varphi$ overlap between the two slits. The angular distributions between 0 and ±20^∘ in $\theta$ appear to overlap only because we performed fine angular steps in this region since we wanted to characterize the evolution of the response function in this particular region.

Fig. 23.

Geometric function calculated by SIMION simulations, including the grid transmission effects

Fig. 15.

Left panel: Depiction of rotations in $\theta$ and $\varphi$ angles, as well as translation of Ultra, while in the chamber performing elevation – azimuth tests. Occasionally UV LEDs are used to evaluate performance in the presence of background (see 3.2.6 for those tests). Right panel: angular distributions of 40 keV H⁺ ions at all the discrete angles tested

Figure 16 shows color coded angular resolutions across the sensor’s FOV.

Fig. 16.

$\varphi$ - $\theta$ maps of Ultra’s angular resolution for both sensors from 10 – 40 keV. The topmost row illustrates the range of measured angular resolutions across the FOV (we use the highest value between the FWHM values of the $\theta$ and $\varphi$ distributions). Blue color dots indicate locations in the instrument where the requirement of FWHM ≤ 6^o is satisfied. All other rows have color-coded points showing the angular resolution range across the FOV

Each data point in Fig. 16 represents the largest FWHM (resolution) in either $\theta$ or $\varphi$, which were calculated by approximating the angular distributions like the one depicted in Fig. 15 (right panel) as Gaussians. The topmost panel in Fig. 16 utilizes a diverging color bar to indicate the range of angular resolution relative to requirements at 10 keV incident energy. Blue color dots indicate locations in the instrument where the requirement of FWHM ≤ 6^o is satisfied, while the warm colors indicate FWHM > 6^o. We can therefore identify that, for 10 keV there is a limited FOV within which Ultra meets the requirement of FWHM ≤ 6^o.

All other panels use a color bar to highlight the sensors angular resolution across its FOV. At 20 keV the requirement is satisfied throughout, except at the extremes of the FOV. And at 40 keV, the angular resolution is as fine as 2^o for certain parts of the FOV. As expected, the resolution worsens as the energy decreases due to the increased scattering of low energy particles in the start foil. Another effect that contributes to the degrading resolution around the edges of the FOV (high $\varphi$ and $\theta$ angles) is the “splitting” of the charged and neutral particle distributions coming out of the start foil (only one is deflected by the electrostatic steering potentials (Sect. 2.3)).

Ultra MCP Sensitivity Within the FOV

In order to determine the bounds of the Ultra FOV, we use the ²⁴¹Am shutter sources (Sect. 2.2; Fig. 5) to stimulate both the MCP and SSDs in the back plane. Figure 17 (top) shows counts on the back plane MCP in $\theta$, $\varphi$ coordinates illustrating the sensitivity of the MCP and its full FOV across: $-{60}^{\circ }\le \varphi \le +{60}^{\circ }$ and $-{48}^{\circ }\le \theta \le +{48}^{\circ }$. Figure 17 (bottom) shows the response of the SSDs, which are arranged along the y-direction $-{60}^{\circ }\le \varphi \le +{60}^{\circ }$ and $-{35}^{\circ }\le \theta \le +{35}^{\circ }$.

Fig. 17.

(top): counts on the back plane MCP in $\varphi$, $\theta$ coordinates; (bottom): counts on the backplane SSDs

UV Rejection Capability

The main background source for Ultra is the random coincidences of counts on the Start and Stop MCPs from interplanetary Hydrogen Lyman-$\alpha$ UV. The Lyman-$\alpha$ emission is distributed broadly across the entire sky, generated by stars as well as by the scattered interstellar neutral hydrogen stimulated by solar Lyman-$\alpha$. In order to verify the SNR requirement, we needed to test the Ultra UV rejection capability. This test involves measuring low intensity ion signals (our proxy for ENAs) in the presence of laboratory UV sources.

Ultra FM Testing with Simultaneous UV Sources and Ion Beams

The Ultra FM was tested using UV LED sources attached to the right deflection blade assembly (see Fig. 15) while simultaneously illuminating the left slit with a 10 keV proton beam. There were 60 LEDs arranged as 6 strips of 10 LEDs mounted over every other collimator blade opening. The LEDs have a built-in dome lens that forms a loosely collimated beam. The entire collection was operated at a DC current of a few $\mu$A per LED to achieve the desired counting rate.

By illuminating the two slits with different sources, we can distinguish accidental counts caused by UV, masquerading as valid events (VEs), from true VEs caused by the 10 keV proton beam, since they register different start positions, from the right and left slit respectively. Since the sensor tags events with their entrance slit, we could tell which recorded events were caused by the 10 keV proton beam and which were caused by accidental coincidences from UV photons. The UV LEDs were set to produce 10⁵ counts/s Start anode rates, on the LED-illuminated slit and start foil, from the photoelectrons produced by the UV exiting the start foil and reaching the Start area of the Front MCP. This is a theoretically predicted Lyman-$\alpha$ UV rate (this rate has been subsequently confirmed by UV rates measured in space by the very similar JUICE/JENI sensor). The proton beam intensity was ∼10 counts/s, i.e., comparable to the expected Ultra sensor ENA rate in the high intensity regions in the sky. UV tests with the Ultra EM (not shown here) have demonstrated that in addition to the strictly random accidental counts resulting from starts, stops, and coincidence pulses triggered by UV, a second source of UV-associated accidental counts (and the prevailing one) are caused by a start pulse produced by photoelectrons, emitted when UV trasverses through the Start foil, combined with a correlated pair of stop-coincidence pulses, caused by a beam (foreground) particle and the subsequently emitted secondary electrons, within the TOF window.

Figure 18 shows how VE filtering in flight software (FSW VE criteria) eliminates invalid events caused by accidental coincidences. Figure 18a shows the counts registered at the Ultra MCP backplane, in $\varphi$, $\theta$ coordinates, without the FSW VE criteria applied. The LEDs are mounted between pairs of blades, so they are spaced at regular intervals in $\varphi$. The UV illuminates the back plane in stripes according to those angles. Figure 18b shows consistent position and correct timing criteria between the Stop and Coincidence (Coin) anode signals. More specifically, the counts registered as FSW VEs need to satisfy certain criteria with respect to the difference between their Coincidence and Stop positions along $\hat{x}$ dimension (CoinXpos – Xstop), and the time it takes for the secondary electrons released from the Stop foil to reach the Coincidence anode (eTOF). Figure 18 c shows counts registered after the position and timing criteria have been imposed, as shown in Fig. 18b. Figure 18 c also includes a TOF criterion of < 65 ns to remove the ∼2 keV H⁺ beam seen in Fig. 18a. Those ∼2 keV H⁺ (TOF ∼ 65 ns) are produced when water molecules in the MCP are dissociated via the accelerated start electrons. They are subsequently accelerated via the 2 kV potential between the Start and Stop assemblies and register as a valid event. Since the energy is outside the measurement requirement for Ultra, we can easily discard by imposing a maximum TOF limit.

Fig. 18.

a) counts registered at the Ultra MCP backplane, in $\varphi$, $\theta$ coordinates, without valid event criteria applied; b) shows counts registered, with valid event criteria being applied, in the (CoinXpos – Xstop) vs. eTOF space; c) the counts registered at the Ultra MCP backplane, in $\varphi$, $\theta$ coordinates, after the valid event criteria have been applied

Rate in Vs. Rate Out (Dead Time) Corrections

In addition to UV generated accidentals (Sect. 3.2.6.1), UV also generates high rates on the start MCP (via the photoelectrons emitted when the UV passes through the Start foil) that can encumber the processing electronics. These start signals trigger the sensor’s front-end logic to open an analysis time window in anticipation of particle-associated coincidence and stop signals. This produces an interval of “dead” time, during which no more counts can be registered on the Start anode, and thus no more can be registered within the sensor. If the start was generated by UV, in most cases no coincidence or stop pulses follow, and after a prescribed interval (of “dead” time) the system resets, and can once again be ready for a new event. This affects the total FPGA valid events (FPGA VEs), and thus the resulting events require scaling—also known as the Rate in versus Rate out (R vs. R) or dead time corrections—to properly represent actual incident event rates. FPGA VEs are those that satisfy acceptance timing logic applied at the FPGA clock time resolution. Specifically, the first hit on a Start Anode initiates an 8 clock period (800 ns) window during which one and only one of each of Start, Stop and Coincidence Anodes have hits on all channels, and all required TDCs have measurements within range. More stringent criteria are applied to this small subset of events once they are transferred to the DPU (FSW VE criteria; Sect. 3.2.6.1). The TOF electronics is non-paralyzable in the 10⁵ counts/s rate range but becomes paralyzable as the rate approaches $5\times {10}^6$ counts/s. This is because the FPGA pulse detection and synchronization logic runs at 10 MHz and requires a minimum of one clock cycle between successive synchronized pulses.

We conducted a number of experiments with the Ultra Engineering Model (EM) to confirm the successful implementation of those R vs. R or dead time corrections. The setup for those experiments is shown in Fig. 19. It includes the EM sensor without the deflection blades, with two LED UV lamps attached on its right slit. Attached to the sensor there is also an arm with a series of alternating degraded and undegraded disk alpha sources, capable of rotating through the sensor $\varphi$ angle from −85^o to 0 (right slit), and from 0 to +85^o (left slit), while covering the full range in theta with multiple sources on the arm. The undegraded alpha disk source consists of ²⁴¹Am embedded in a Au matrix such that the resulting alpha spectrum peaks at ∼4.5 MeV (rather than 5.486 MeV) with a 10% FWHM. In the degraded alpha disk sources, the alpha energies were further degraded by covering every other disk source with a thin mylar sheet spreading the alpha energies down further (<100 keV) with a low but reasonable rate.

Fig. 19.

(top left) Ultra EM sensor without the blades together with the source arm in the chamber; (top right) source arm with alternating degraded and undegraded disk alpha sources attached to it; (bottom left) schematic of the sensor and the source arm apparatus operation; (bottom right) LED UV sources attached to the right slit of the sensor

We performed four different tests: i) sweep of alpha source arm from $\varphi ={60}^{\circ }$ to $\varphi =0$ and back on the left slit with the LED UV sources turned off, ii) sweep of alpha source arm from $\varphi ={60}^{\circ }$ to $\varphi =0$ and back on the left slit, with LED source set at 10⁵ counts/s rate on the right slit Start anode, iii) source arm on the left slit at $\varphi ={60}^{\circ }$ with LED source set at $4\times {10}^5$ counts/s on the right slit Start anode, and iv) source arm on the left slit at $\varphi ={60}^{\circ }$, with LED source set at 10⁵ counts/s on the right slit Start anode.

Figure 20 shows a piece-wise timeline of the various tests described above. The magenta curve shows the changes in the right slit Start Rates triggered by the UV when the LEDs are on. The ratio of FPGA VEs (blue curve) from the alpha source arm sweep with LED set at 10⁵ counts/s rate over those of the sweep without LED is 1300/1650 ∼ 0.79, that is, in the presence of UV rates, the FPGA VEs decrease because of the aforementioned dead time. After applying the dead time correction, the corrected FPGA VE rates (green curve) are at the same level as the FPGA VEs without the LED (blue curve). The ratio of FPGA VEs (blue curve) from the source arm at 60° with LED set at $4\times {10}^5$ counts/s, over those of the source arm at 60° with LED set at 10⁵ counts/s, is 400/600 ∼ 0.66, that is, the higher UV rate results in a further decrease of the registered FPGA VEs due to dead time. It should be noted that $4\times {10}^5$ counts/s is an over-estimate for UV rates in-flight. Also shown in Fig. 20 are the FSW VE rates (black curve). The dead time corrected FSW VEs (black curve) are that fraction of the dead time corrected FPGA VEs that pass the more stringent VE criteria applied by the FSW, discussed above in the description of Fig. 18. The final FSW VE rate, dead time corrected, should be independent of the LED UV-driven Start rate, as it is (black curve is not being affected by the LED rates throughout the test). Note that the shape of the VE rate curves up to ∼951 seconds is due to the increase and decrease of the counts as the source arm sweeps on the left slit from $\varphi ={60}^{\circ }$ to $\varphi =0$ and back. After 951 seconds, when the source arm is static at 60^o, the rate curves are straight lines.

Fig. 20.

Timeline of R vs. R correction tests

In practice, the intensity of the ENA emission is determined by accumulating FSW VE events into bins (angular pixels in the sky, of prescribed energy bandwidth), and dividing those counts by the sensor geometric function, efficiency, and accumulation time. The R vs. R dead time correction is applied as an adjustment to the accumulation time for each pixel. This accounts for the VEs that have been missed while the sensor was busy waiting following a UV-induced triggering of the front-end logic.

The full R vs. R correction applied to these test data involves an expression that takes into account the various state machines running in the FPGA and is rather complex. However, a good approximation to that complex expression is provided by a simple exponential of the Start rate multiplied by a characteristic time constant of 2.4 μs (yellow curve in Fig. 20). The reason this works as well as it does is that the vast majority of the Start counts that trigger the FPGA state machines are UV-driven, therefore, no real particle is present, and no Stop or Coincidence pulses occur within an analysis window. Such triggers all behave the same, and result in a dead time represented by a 2.4 μs window. While in flight we apply the more complex expression for this correction, we include the approximation here to demonstrate the general nature of the correction.

Maximum UV rates for the narrow (nominal) shutter slit are expected to be ∼10⁵ counts/s (see Sect. 3.2.6.1). Nonetheless, there are mitigation strategies in the case that UV background is too high, such as using the shutter filter foil to reduce the UV, even though that would compromise the lowest energies Ultra can measure, as well as the angular resolution.

Measurement Efficiencies and SNR

The main goal of the Ultra calibration campaign is to define the Ultra species, energy, and angle dependent response function necessary to convert counts into physical units, that is, differential intensities.

Critical to the success of deriving the full response function, is to independently monitor the ion beam intensity, i.e., its stability, uniformity, and divergence. Before the Ultra FM units were installed in the chamber for calibrations, the beam was characterized using an Absolute Beam Monitor (ABM; Funsten et al. 2006) and an MCP beam imager. The ABM is used for the beam stability measurements (i.e., temporal variations) and the MCP imager maps out the uniformity and divergence of the beam. MCP beam imagers are placed both at the location of the ABM and the location where Ultra’s aperture will be fixed (Fig. 21). Comparing the spatial profile between these two locations aids in the interpretation of the calibration results. The beam intensity and uniformity was spot checked throughout the elevation – azimuth tests described in Sect. 3.2.4 to ensure any variability in the ion beam is properly folded into the analysis and derivation of the response function. MCP 1 in Fig. 21 is used throughout the FM elevation – azimuth testing to check beam uniformity, while MCP 2 is used only during the beam characterization part to determine: i) the uniformity of the beam at the Ultra location, and ii) the particle density at Ultra location relative to the ABM.

Fig. 21.

Simple illustration of test setup for APL’s accelerator ion beam characterization, and measurements taken by the two MCP beam imagers monitoring beam uniformity. Components are not to scale. 2D histograms represent beam measurements between the two MCPs. Yellow projection on 2D histograms depicts the dimension of Ultra’s slit. 1D histograms show beam uniformity along slit (yellow projection) in the instrument X and Y coordinates

The energy ($E$) and angle ($\theta$, $\varphi$) dependent efficiencies for certain species can be derived by equating the ion flux measured at the ABM to the incident ion flux at Ultra’s aperture. The difference between these two numbers is determined by many factors and accounting for them all allows us to determine Ultra’s absolute measurement efficiency as a function of angle and energy, as we describe in the following sections. The number of particles per time measured by Ultra, properly weighted by the instrument response dependences is given by:

$$R(E,\theta ,\varphi )=\frac{N_{\mathrm{Ultra}}(E,\theta ,\varphi )A\left(\theta \right)}{ϵ(E,\theta ,\varphi )\times g_d(\theta ,\varphi )\mathrm{\Delta }t}$$ 1

$N_{\mathrm{Ultra}}(E,\theta ,\varphi )$ represents the total number of FSW VEs (i.e., events that meet the FSW valid event logic; see Sect. 3.2.6.1), $A(\theta )$ is the projected area of Ultra’s slit, $\mathrm{\Delta }t$ is the accumulation time, $g_d(\theta ,\varphi )$ is the solid angle area of the instrument, and $ϵ(E,\theta ,\varphi )$ is the measurement efficiency. Deriving the quantity $ϵ(E,\theta ,\varphi )$ is one of the main purposes of these calibrations and depends on how well we can estimate the other quantities. Therefore, we discuss those estimates in detail next. The relation of $R(E,\theta ,\varphi )$ to the rate measured by the ABM is:

$$R(E,\theta ,\varphi )=R_{\mathrm{ABM}}\times \langle PY\rangle \times \mathrm{\Delta }y$$ 2

The parameters $R_{\mathrm{ABM}}$, $\langle PY\rangle$, $\mathrm{\Delta }y$ are explained in detail in the next section.

Determining $\mathit{R}(\mathit{E},\mathit{\theta },\mathit{\varphi })$

As discussed above, $R(E,\theta ,\varphi )=R_{\mathrm{ABM}}\times \langle PY\rangle \times \mathrm{\Delta }y$, where $R_{\mathrm{ABM}}$ is the average rate reported by the ABM over an approximate three-minute average. The rates are read out at ∼1 Hz but are very stable over the accumulation period, so taking an average is justified. Modest beam rate drift occurs over timescales of hours, but periodic measurements of $R_{\mathrm{ABM}}$ can correct for this known behavior. The ABM consists of two channel electron multipliers (CEMs) on either side of a thin foil used for secondary electron production. As the primary ion traverses this foil it can produce secondary electrons that are steered to the CEMs. $R_{\mathrm{ABM}}$ can be calculated by taking a ratio of the product of the individual CEM rates to the coincidence rate. The equation is: $R_{\mathrm{ABM}}=R_{\mathrm{CEM}1}{R}_{\mathrm{CEM}2}/R_{\mathrm{CEM}1,2}$ (Funsten et al. 2006). The benefit of this approach is that it divides out the detection efficiencies of the CEMs.

Figure 22 (upper panel) is a plot of $R_{\mathrm{ABM}}$ over a full elevation – azimuth test. Error bars are calculated assuming Poisson statistics and the horizontal axis is effectively time, but it is remapped into Ultra’s phi angle.

Fig. 22.

$R_{\mathrm{ABM}}$ as a function of instrument angle (upper panel). This represents a roughly eight-hour testing period. The beam density or $\langle PY\rangle$ distribution along the y-coordinate (bottom panel). Note, only the flattop intersects Ultra’s slit since the narrow slit is 0.5 mm wide and the wide slit is 2 mm wide

Figure 22 (bottom panel) shows $\langle PY\rangle$, i.e., the normalized counts per width along $\hat{y}$ in Ultra’s coordinate system (bottom panel of Fig. 4), for a beam of 20 keV H⁺. This quantity is calculated using a 2D MCP beam imager that is located upstream from Ultra and is co-located with the ABM (MCP1 in Fig. 21). The most critical dimension is the one along $\hat{y}$ since that intersects Ultra’s slit in the dimension that is not being translated. To estimate the density we take the histogram counts along $\hat{y}$ and bin them in spatial coordinates along the MCP. We use an oscilloscope to measure the signals coming from the 2D anode located behind the MCP. The signals provide the timing that needs to be converted to a spatial coordinate. For that reason, we put a physical mask of known dimensions over the MCP, to define the time range associated with the edges, and divide that by the dimensions of the mask to get a scaling of ns per mm. The oscilloscope binned the signals into 0.4 ns bins, so the minimum step size is 2.675 mm. Then $\langle PY\rangle =(countsperbin/\sum countsperbin\times 0.4\text{ ns}/2.675\text{ mm})$. As it shown in Fig. 22 (bottom panel), it is only the flattop of the $\langle PY\rangle$ distribution that intersects with the Ultra 0.5 mm slit, therefore these are the values we are using. Those values vary between ∼0.15 and 0.3, generally increasing toward the higher energies. Finally, $\mathrm{\Delta }y$ is the width of Ultra’s slit.

Determining $\mathit{A}(\mathit{\theta })$ and $\mathbf{\Delta }\mathit{t}$

The projected area of the beam onto the slit at different $\theta$ angles can be calculated simply by the physical area of the slit (length× width, or $L_x\times \mathrm{\Delta }y$) multiplied by the $cos(\theta )$. Therefore, $A\left(\theta \right)=L_x\mathrm{\Delta }ycos\left(\theta \right)$. $L_x$ is a constant 80 mm and $\mathrm{\Delta }y$ depends on the slit configuration used during the test. The narrow slit $\mathrm{\Delta }y$ is 0.5 mm, and wide slit is 2 mm. In general, the narrow slit is used for beam energies ≥10 keV and the wide slit is used for $<10\text{ keV}$ due to the low intensities in the beam. When alternating between slits we ensured that the measured count rate differences between the two configurations were purely geometric, i.e., 2 mm / 0.5 mm or a factor of 4. This was important to check because the wider slit could intersect a non-uniform portion of the beam and therefore bias our calibrations.

The accumulation time, $\mathrm{\Delta }t$, is calculated based on the translation speed, $v_{\mathrm{translate}}$, of Ultra’s slit across the beam or $\mathrm{\Delta }t=N\times (L_x/v_{\mathrm{translate}})$, where $N$ is the number of scans that were performed. For the majority of elevation – azimuth tests we used $N=2$, which represents the instruments slewing in one direction, stopping, and returning. If the beam intensity was low, sometimes $N=4$ and/or a slower $v_{\mathrm{translate}}$ was used to increase the accumulation time and therefore the number of VEs. The average $v_{\mathrm{translate}}=0.997\text{ mm}/\text{s}$ with a standard deviation of ∼1%.

Determining ${\mathbf{g}}_{\mathbf{d}}(\mathit{\theta },\mathit{\varphi })$

The geometric function $g_d(\theta ,\varphi )$ is essentially the solid angle weighted effective area at a given combination of polar angles. The unit of $g_d(\theta ,\varphi )$ is cm² and it relates to the total geometric factor GF (cm² sr) by taking the integral over solid angle. The product of $g_d$ and $ϵ$ is usually referred to as the response function and together they map how to properly weigh counts of a particular species, energy, and incident angle (in both $\theta$, $\varphi$) to an incident ENA intensity. In theory, $g$ and $ϵ$, are easily separated, but this is nearly impossible to do experimentally. In short, the goal is to determine $ϵ=ϵ(E,\theta ,\varphi )$ by dividing out $g_d(\theta ,\varphi$)—which does not depend on species and $E$—using incident flux measurements from the ABM upstream of Ultra’s aperture.

The geometric function was determined both analytically, using detailed CAD drawings, and from detailed particle tracing SIMION simulations. The SIMION software allows CAD models to be directly imported for ion and electro-optical simulations. A two-step process was used to simulate the particle trajectories that would make it into the instrument. First, we inject particles along the full slit, carefully modeling their angular and position distributions, launch them in the direction of the MCP back plane, and record their positions. Next, we take the particles that hit active areas of the MCP and fly them backwards tracking the ones that exit beyond the deflection blades. This subset of particles represents those that would define the active area and thus total GF. The number of particles detected are binned in $\theta$, $\varphi$ and are normalized by the total number of particles flown in that elevation – azimuth bin. The GF factor calculated by the two methods agree to within 2%. Note that for this comparison, the grid transmission, is not included. Grid transmission has a dependence on incident angle of the particles, and includes the transmission through start foil, start acceleration, stop acceleration, and stop foil grids. Figure 23 shows the geometric function as calculated by the SIMION simulations including the grid transmissions. The sharp increase at $\varphi =0$ is because beams with that configuration illuminate both slits at the narrow overlap of the FOVs of the left and right apertures, while the fine structure along $\theta$, including the large dip at $\theta =0$, is due to blockages from a combination of sources that include the deflection blade nubs, grid holders within TOF section, and partial blockages at the extremes of the deflection assembly.

Determining $\mathit{ϵ}(\mathit{E},\mathit{\theta },\mathit{\varphi })$

Rearranging Eq. (1) for the efficiency, $ϵ(E,\theta ,\varphi )$, and expanding the terms discussed in Sects. 3.2.7.1-3.2.7.2 provides:

$$ϵ(E,\theta ,\varphi )=\frac{N_{\mathrm{Ultra}}(E,\theta ,\varphi )\times cos\left(\theta \right)\times v_{\mathrm{translate}}}{R_{\mathrm{ABM}}\times \langle PY\rangle \times g_d(\theta ,\varphi )\times N\times 0.8}$$ 3

The only new term in Eq. (3) is the constant 0.8, which represents the ratio of the number particles, in the $\hat{y}$ direction, between the MCP1 located at the ABM and the MCP2 located at Ultra’s slit (Fig. 21). The 20% reduction in the number of particles per $\mathrm{\Delta }y$ bin is due to the beam divergence and remains nearly constant across the measured energies.

Figure 24 shows the efficiencies as a function of energy for $\varphi ={28.5}^{\circ }$ and $\theta ={20}^{\circ }$. Error bars represent the propagation of uncertainties associated with the various quantities discussed between Sects. 3.2.7.1–3.2.7.2 (i.e., $\delta N_{\mathrm{Ultra}},\delta R_{\mathrm{ABM}},\delta \langle PY\rangle ,\delta v_{\mathrm{x}}$). The magenta points depict measurements at additional energies that were only taken at $\varphi ={28.5}^{\circ }$ and $\theta ={20}^{\circ }$. Those points were then scaled, based on their relative differences to adjacent energies, to the other angles. The blue curve represents an empirical fit with the functional form:

$$\epsilon (E,\theta ,\varphi )=\frac{L(\theta ,\varphi )}{1+e^{(-k(\theta ,\varphi )(E-x_o(\theta ,\varphi )))}}$$ 4

where $k$, $x_o$, and $L$ are the free fitting parameters.

Fig. 24.

Left panel: Efficiencies for H as a function of energy for $\varphi ={28.5}^{\circ }$ and $\theta ={20}^{\circ }$. The error bars represent propagation of errors associated with various measurements needed to determine an efficiency. The blue solid curve represents the fit based on Eq. (4). The red dashed curve is the square of the electronic stopping power. The black dashed curve represents cumulative grid transmissions; Right panel: Efficiencies for H as a function of energy based on Eq. (4) for all angle combinations tested in the lab. The arbitrary color coding is simply showing energy dependent variations of efficiencies for different $\varphi$, $\theta$ pairs

Equation (4) was chosen as the interpolation function for Ultra because the steepness parameter ($k$) follows the theoretical electronic stopping power of H particles in matter ($dE/dx$ curve) well, and the asymptote can be set by the $L$ parameter and is well behaved over the energy range. In general, at energies below ∼20 keV, the Ultra efficiencies appear to be primarily driven by the probabilities of generating secondary electrons since they match the square of electronic stopping power curve well (red dashed curve in Fig. 24). We used the PSTAR tool from National Institute of Standards and Technology (NIST) to approximate the electronic stopping power of protons on amorphous carbon. We had to square the electronic stopping power since there are two foils responsible for generating start and stop secondary electrons. Although the stopping power curve keeps rising well above 20 keV, the measurement efficiencies flatten out near 30 keV and are limited by the cumulative grid transmissions (dashed black curve in Fig. 24). The right panel of Fig. 24 shows the empirical function across all possible angle combinations tested in the laboratory.

In-flight we apply a calibration matrix to our data that uses Eq. (4) to set the energy dependence of the efficiencies and cubic interpolation for the $\theta$ and $\varphi$ angle dependence (Fig. 25). We only show the cubic interpolation map for 40 keV, but we have a measurement efficiency data cube that depends on both angles and energy that act as a lookup table for each event.

Fig. 25.

Measurement efficiencies as a function of $\theta$ and $\varphi$ for 40 keV to illustrate how the efficiencies are interpolated in angle. Together with the energy function, we have successfully mapped out the efficiencies across a broad range of measurement parameters. The black dots represent the original angles used in the interpolation. Interpolations near the extremes of the FOV require careful interpretation

The secondary electron yield is proportional to 1/cos(A), where A is the angle between the incident beam and foil normal. Thus the detection efficiency increases nonlinearly at higher angles of incidence relative to the foil normal. Considering that the start and stop foils lie in different planes, a large $\varphi$ angle (away from the overlapping FOV of the left and right sides) and larger $\theta$ angle should result in increased detection efficiency. This angle dependence is reflected in the results presented in Fig. 25.

SNR

Using the efficiencies determined by our calibration campaign, and the ENA spectra derived from IBEX (Schwadron et al. 2011) and INCA (Dialynas et al. 2013) data for low ENA intensity regions (i.e., the so-called lobes or basins respectively) we calculate the expected foreground counts that Ultra will measure. Figure 26, left panel, shows the associated energy spectra of H ENAs (black solid curve), from which we calculate expected foreground counts per 2^∘ resolution and over the full instrument, represented as the red dashed and solid curves, respectively. Note that these counts are based on the narrow slit configuration without the UV filter. Based on those inputs and our detailed measurements efficiencies derived from laboratory measurements, we can estimate the SNR based on both the FPGA VEs and the FSW VEs (red and black curves respectively in the right panel of Fig. 26). The additional spatial and timing criteria imposed by the FSW can provide a factor of ∼2.5 improvement of the SNR. Recent flight data from JENI (not shown) have also verified the expected foreground counts that we have estimated here for Ultra.

Fig. 26.

(left panel) ENA and count spectra derived from IBEX and INCA data for low ENA intensity regions in the sky; (right panel) Expected SNR based on ENA basin spectrum and Lyman-$\alpha$ UV rates for both the FPGA VEs and FSW VEs

Inter-Calibration Between Ultra₉₀ and Ultra₄₅

The two Ultra sensors were calibrated independently, therefore, we performed a test at an uncharacterized energy to see how well the two sensors estimated the incident beam flux. This test demonstrated the fidelity of the empirical fits in capturing the energy and angle dependent efficiencies. In this case, the proton energy was set to 15 keV. This energy was chosen because it is both within the measurement requirement range and happens to be where the response function is changing relatively rapidly (see left panel of Fig. 24) and therefore, is a gauge of how well we can interpolate. Black circles in Fig. 27 show derived ABM rate, by inverting the efficiency Eq. (3) in Sect. 3.2.7.4, while red triangles are the actual ABM rates. The rates were normalized to 1, so that relative changes between the derived and measured rates can be easily interpreted. Both sensors were able to come to within ∼10% relative accuracy (between Ultra₄₅ and Ultra₉₀) and absolute accuracy of each sensor with respect to the measured ABM rate within 25%.

Fig. 27.

Derived (black circles) and actual (red triangles) ABM rates. The rates were normalized to 1 so that relative changes between the units and derived/measured rates can be easily interpreted

Environmental Testing Activities

Environmental testing consisted of a series of vibration and thermal cycling tests to ensure Ultra could survive launch and the thermal environment of space with no degradation to its performance. The vibration tests were conducted first and included sine and random frequency testing. The sine testing covered 27 g peak perpendicular, 18 g peak parallel, over a 5–100 Hz frequency range. Random testing was comprised of 14.1 grms General Environmental Verification Standard (GEVS) and used force washers for response limiting. Thermal testing consisted of a thermal balance test used for instrument bakeout, heater power verification, and long duration thermal balances. Thermal cycling consisted of a long bakeout with six thermal cycles ranging from −45 °C cold plateaus to +45 °C hot plateaus. Figure 28 depicts the test set up for vibration (left panel) and thermal (right panel).

Fig. 28.

Pictures of Ultra’s vibration test set up (left panel) and thermal test set up (right panel)

Ultra and Hi Cross-Calibration

As discussed in Sect. 1.1, the Ultra and Hi instruments measure ENAs with an overlap in energy. To assure that both instruments are measuring the same ENA intensities in those energies while in flight, they were cross-calibrated at the LANL Space Plasma Instrument Calibration Facility using a neutral hydrogen beam. Figure 29 shows photos of the two instruments’ setup in the chamber. They are placed on a motion stage that allows rotation of the two instruments in and out of the same beam, as well as horizontal translation for Ultra, so that the whole 80 mm slit was filled with the beam.

Fig. 29.

Pictures of Ultra and Hi in the LANL facility chamber right before cross-calibration measurements begun

The measurements occurred over overlapping energy channels, namely 8.4, and 12.7 keV, and over one entrance location. Since the Ultra instrument does not have pre-assigned energy bands, the beam energies were chosen so that they match the centers of the Hi energy bands. ABM measurements were taken throughout the process to be compared against the intensities that the two instruments were measuring. Figure 30 shows the results of the cross-calibration tests. More specifically, it shows the ratio of the incident flux that each instrument measured, over the ABM reported flux, as a function of the beam energy. The flux measured by each instrument is calculated by dividing the incident rate by the instrument response function at a specific energy, aperture location, and aperture angle, as it has been determined during the calibration campaigns for each instrument (see Sect. 3.2.7 for Ultra’s response function). As it can be seen, the two instruments agree with each other within 10–15%, depending on the energy, thus satisfying the cross calibration requirement of 15% absolute flux accuracy at applied calibration energies. The very minimal discrepancy could be attributed to the fact that the two instruments used different ABMs at their own facilities to determine their response functions. There is likely a systematic offset between ABMs due to the two units not being identical copies.

Fig. 30.

Ratio of the incident flux that each instrument measured, over the ABM reported flux, for 8.4 and 12.7 keV H beam

Ultra Sensor Operations and Data Products

Ultra operations are quite straightforward. The two sensors continuously collect data to produce: i) onboard processed 2D Histograms, ii) basic rates of various sensor counters, iii) raw direct events, and iv) housekeeping data. Although we produce onboard processed 2D histograms (i.e., onboard binned data) we plan to use raw direct events to produce all-sky ENA intensity maps after processing through the data pipeline.

All data processing is carried out at the IMAP Science Operations Center (SOC; Reno et al. 2026). Figure 31 shows a high-level Ultra data flow diagram:

• Level 1A (L1A) product is decommutated L0 packages comprising of housekeeping data, raw direct events (that is, FPGA VEs) and onboard processed 2D histograms.

• Level 1B (L1B) product is comprised of i) annotated direct events, which are processed raw events using timing, attitude, and ephemerides information provided by the s/c, as well as calibration tables for species discrimination, and after the FSW VE criteria have been applied ii) a “bad times” list, identifying spins when we expect the data to be unsuitable for heliospheric imaging, because of the configuration of the instrument, activities of the spacecraft or periods of enhanced backgrounds, iii) a culling mask to eliminate “bad times” from further processing, and iv) an “extended spin table” product, which is a table with all the relevant spin information, including flags.

• The main Level 1C (L1C) product is the “pointing sets”, that is L1B annotated events binned by estimated energy, direction in the sky, as well as exposure time on a pre-defined Pointing set grid in the sky. The pointing sets are the building blocks for our maps. At this level, we also produce i) spun instrument sensitivities on the pointing grid based on sensitivity tables input produced from our calibration campaigns (Sects. 3.2.7.3 and 3.2.7.4), ii) estimated background rates on the same pointing set grid, and spatiotemporal culling masks.

• Using the L1C pointing sets together with the sensitivity information on the same grid, we produce the Level 2 (L2) product, which is our basic all-sky ENA intensity maps accumulated over 3, 6, and 12 months, at 2^o, 4^o, and 6^o grid resolution. Along with that product, we are also providing i) background rate maps, ii) corrected count rate maps (after subtracting the background), iii) uncertainty maps, including statistical uncertainties, and spatial uncertainties due to scattering of particles in the foils, and iv) exposure time and sensitivity maps (which is an intermediate product, used to produce the all-sky ENA intensity maps).

• Finally, our Level 3 (L3) product is comprised of i) survival probability corrected maps, which are inferred ENA intensity maps at ∼ the termination shock (as opposed to 1AU maps, which is our basic map product), ii) spectral index maps, and iii) combined Ultra₉₀ and Ultra₄₅ ENA intensity maps.

Fig. 31.

Ultra Data pipeline flow

Our usual operations are performed in two telemetry modes, the ENA Heliospheric Spin Survey and the ENA Heliospheric Sector Survey. Although our main mode is the Spin Survey, we switch to Sector Survey three times per Pointing (∼per day), for 20 min. each time, to produce image mode basic rate packets, with information used to correct the exposure time based on the instrument dead time (Sect. 3.2.6.2) for our L1C pointing set product.

Summary

The Ultra instrument is a slit based, TOF ENA imager. It consists of two identical sensor heads, Ultra₄₅ and Ultra₉₀, covering ∼3$\pi$ sr of the full celestial sphere with each spin, over the energy range 3 – 300 keV, with variable angular resolution, as fine as 2° for Hydrogen (H) above 30 keV. Ultra is the 2nd generation ion and ENA camera, coming from a long history of ENA imagers designed and built by APL.

Ultra underwent comprehensive standalone calibration, and cross-calibration with Hi campaigns on the ground, to accurately determine the instrument response and to confirm that the instrument meets, and sometimes exceeds, all of its measurement requirements.

Ultra is set to measure the emission of primarily Hydrogen ENAs produced in the heliosheath and beyond, across the suprathermal energy range, and provides high spatiotemporal resolution images, detecting changes in the spatial distribution of ENAs on time scales sufficient to track both solar cycle as well as other major changes. The overlap between the Ultra and Hi imagers in the critical energy range of 5 – 15 keV, addresses current discrepancies in the observed and simulated ENA energy spectra, that are central to the understanding of particle acceleration in the outer heliosphere.

Declarations

Competing Interests

The authors declare no competing interests.