Self-modulation of photoemitted bunches at the picosecond timescale

Abstract

Photoemission sources are essential for operation of high-power accelerators. The successful operation of such electron accelerators demands the appropriate choice of photocathode material that can produce low emittance electron beams with high brilliance and high quantum efficiency. The time response of the cathode is also an important factor for the accelerator applications, and the pulse response must be within the longitudinal acceptance of the accelerator. Any part of the electron bunch not contained in the longitudinal acceptance of the accelerator contributes to a longitudinal halo which must be strictly minimized. The losses associated with longitudinal halo result in induced accelerator component radioactivity and can create background which masks the physics processes in the detectors. In the case of the MESA accelerator, the maximum accepted pulse length is 200 ps. Therefore, photocathodes capable of producing fast responses are desirable. In addition, many accelerator systems require spin-polarized beams. In MESA, beams of high spin-polarization are required for the precision measurements of the electroweak mixing angle (P2). Generating the spin-polarized electron beam can be achieved with GaAs-based photocathodes in negative electron affinity state. The strained superlattices, in particular, increase the strain-induced splitting which makes them superior sources of polarized electrons. The present work is composed of two parts. First, investigating pulse responses of the bulk GaAs as the quantum efficiency decays. The dependency of the pulse shape on quantum efficiency over many orders of magnitude was observed. The results showed that as the quantum efficiency dropped, the general trend was such that the intensity of the halo decreased. At very low quantum efficiency (< 10−4 %), when high-intensity illumination was needed, a surface photovoltage developed and limited the extracted current due to space charge. The surface photovoltage effect at the picosecond timescale was investigated in photoemission for the first time. This characteristic can be approximated to a continuous pump-probe in which self- modulation of the electron pulses at picosecond timescale is reached. Second, the time response and polarization of the strained GaAs/GaAsP su- perlattice was investigated in a time-resolved manner. In order to conduct the polarization measurements, a Wien filter spin-rotator was built and commissioned. The investigation delivered a pulse with a longitudinal halo in the order of 3 × 10−4 of the peak intensity. The results indicated that GaAs/GaAsP, when compared to other superlattices, can be considered a better choice of cathode for investigations in accelerator systems, in particular, for experiments with spin-polarized electron beams such as the P2 experiment in MESA.