Progress of a 140 GHz, 1 kW Confocal Gyro-TWT Amplifier

Introduction

The current status of the 140 GHz, 1 kW gyro-traveling wave amplifier is presented. The amplifier operates in the high order HE₀₆ mode of a confocal mirror waveguide. The gain of the amplifier is predicted in simulation to be 53 dB, producing pulses up to 2 kW at 30 kV beam voltage and 2A beam current at a beam pitch factor of 0.75 and estimated total velocity spread of 6%. The amplifier [1] will replace a 140 GHz gyrotron oscillator [2] in use for Electron Spin Resonance (ESR) spectroscopy experiments in the Francis Bitter Magnet Laboratory at the Massachusetts Institute of Technology, Cambridge, MA.

Design of 140 GHz gyro-amplifier

The amplifier has a quasioptical interaction structure comprised of two confocal mirrors. The mode selectivity of the confocal waveguide will allow operation at a higher order (HE₀₆) mode which is analogous to a TE₀₃ mode of a cylindrical waveguide without mode competition. The overmoded interaction structure improves the power handling capability and is easy to fabricate.

Two mirrors with equal radii of curvature R_c, separated by distance L_┴ have mirror aperture width 2a. Fig. 1 shows this geometry in cross section along with the resulting normalized power contours for the case of a confocal waveguide, R_c=L_┴=6.9mm, with aperture size 2a=6.0mm. The resulting field patterns are nearly Gaussian in the x-direction. The electron beam is positioned to interact with the intermediate mode peaks.

Figure 1.

The confocal geometry and normalized power contours for the HE₀₆ mode. The annular electron beam position is shown by the dashx-dot circle.

The design parameters and simulation results are listed in Table 1. The amplifier consists of three 7cm amplifier sections separated by two quasi-optical severs to limit the gain in each section. Photo 1 shows the completed amplifier structure prior to installation.

Table 1.

Parameters & Simulation

Frequency	140 GHz
Operating Mode	HE06 Confocal
Output Power	2 kW peak
Input Power	10 mW
Saturated Gain	53 dB
Voltage	30 kV
Current	2.0 A
Pitch Factor	0.75
Perp. Vel. Spread	6.0 %
3dB Bandwidth	2.4 GHz (1.7%)

Photo 1.

Completed three-section confocal interaction circuit. Total circuit length is 25 cm.

Using generalized interaction formalism [3], a code was developed to calculate the gain and bandwidth of the confocal structure. The amplifier section lengths were limited to 7cm to suppress backward wave oscillations.

The interaction circuit was assembled and carefully aligned to within ±0.03 mm for the mirror separation. Fig. 2 shows the resulting S₁₁ as measured on a vector network analyzer. Good coupling of the input power is ensured by keeping S₁₁ below −14 dB near 140 GHz.

Figure 2.

Measured S₁₁ of the completed amplifier structure as shown in Photo 1.

Confocal Cold Test Verification

We have fabricated and cold-tested confocal waveguide structures supporting the HE₀₆ mode as a proof of concept. Fig. 3 shows six main peaks as captured by a Spiricon Pyrocam III [4] pyroelectric camera located 3.7mm from the output of the test structure. Electromagnetic fields at this position matched well with simulations predicted by HFSS from Ansoft. The EM fields of the overmoded structure have been directly imaged in a single frame capture at a frequency of 140 GHz. In addition, the S₁₁ reflectance of the input coupler was measured using a Vector Network Analyzer. The S₁₁ value measured −35 dB near 140 GHz, showing good coupler performance as desired.

Figure 3.

Pyroelectric camera image showing six main peaks, A–F. Inset: Confocal cold test structure. Normalized power contours shown.

Conclusion

The design and fabrication of the 140 GHz, 1kW gyro-TWT amplifier has been completed. The hot test of the tube will begin shortly.