Direct measurement of photocathode time response in a high-brightness photoinjector

Abstract

Electron photoinjectors provide high-brightness electron beams to numerous research applications in physics, chemistry, material, and life sciences. Semiconductor photocathodes are widely used here, as they enable the production of low-emittance beams with variable charge at high repetition rates. One of the key figures of merit of photocathodes is the minimum achievable bunch length. In semiconductor cathodes, this is dominated by scattering effects and varying penetration depths of the extracting photons, which leads to a characteristic electron emission function. We present a method to determine this cathode time response with resolution on the tens of femtoseconds level, breaking the resolution barrier encountered in previous studies. The method is demonstrated with cesium-telluride (Cs2Te) and gold cathodes, revealing response times of (184 ± 41) fs up to (253 ± 58) fs for the semiconductor and an upper limit of (93 ± 17) fs for the metal. Monte Carlo simulations of Cs2Te emission benchmarked to these results give detailed information about the cathode material.

Novel approach to push the limit of temporal resolution in ultrafast electron diffraction accelerators

“…The initial temporal and spatial distribution is a replica of an incident laser-pulse on a photocathode. The latest response-time measurement of semiconductor photocathodes 49 shows almost an order of magnitude smaller values than a laser pulse length in SRF Photoinjectors, so this effect is ignored in our calculation. The transverse momentum distribution is chosen so that the normalized transverse emittance of the initial bunch matches the intrinsic emittance of measured for photocathodes 39 .…”

Review of photocathodes for electron beam sources in particle accelerators

“…All Cs 2 Te photocathodes showed prompt response times in 180–250 fs range. 100 Cs 2 Te provides therefore a prompt response time, which is beneficial for ultra short pulse generation.…”

Towards Implementation of 3D Amplitude Shaping at 515 nm and First Pulseshaping Experiments at PITZ

“…The PITZ accelerator is equipped with comprehensive electron beam manipulation and diagnostics, including several sets of quadrupole magnets, scintillator screens, beam emittance measurement systems (EMSY), beam position monitors, and charge measurement instruments. A transverse deflecting structure (TDS) [37][38][39][40] can be used for longitudinal bunch profiling. The electromagnetic field in the TDS is perpendicular to the propagation direction of the electron bunch and the bunch is then encoded to a transverse profile on a view screen, similar to the working principle of a streak camera.…”