Self-modulation of photoemitted bunches at the picosecond timescale
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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.