FIG. 2.

Experimental beam properties of photoelectron pulses. (a) Exemplary beam caustics are plotted for three different $400-\text{nm}$ laser pulse durations and two different electron bunch charges, characterized by the number of electrons counted in the sample plane. The laser pulse duration is varied by dispersive broadening in a $10-\text{cm}$ BK7 or SF6 glass bar. The solid curves are adapted to the experimental data, considering the caustic behavior of an electron beam with a beam quality factor $M^2$ [cf. figure (b)]. (b) and (c) Transverse electron beam emittance and energy width depending on the electron pulse charge in the sample plane (estimated bunch charge at the emitter: up to 200 electrons per pulse). As a reference for a space-charge-free beam, the emittance and energy width of a continuous photoelectron beam and for continuous thermal electron emission are given (orange symbols and gray line, respectively). (d) Focal spot profiles of the electron pulses for selected initial pulse durations and bunch charges exhibiting a close-to Gaussian shape. (e) Electron beam brightness depending on beam current shows a linear scaling in the low-charge regime and saturation at higher bunch charges. The saturation level depends on the initial pulse duration. An equal color coding for the initial pulse durations is used in all panels.