Abstract
A compact rf cavity was constructed that simultaneously resonates at many harmonic modes when excited by a repetitive bunched electron beam passing through its bore. The excitation of these modes provides a Fourier description of the temporal characteristics of the bunch train. The cavity was used to noninvasively characterize electron bunches produced from thin and thick GaAs photocathodes inside a DC high voltage photogun illuminated with 37 ps (full width half maximum, FWHM) laser pulses at repetition rates near 1500 MHz, at average beam current from 5 to $500\text{ }\ensuremath{\mu}\mathrm{A}$, and at beam energy from 75 to 195 keV. The cavity bunch-length monitor could detect electron bunches as short as 57 ps (FWHM) when connected directly to a sampling oscilloscope, and could clearly distinguish bunches with varying degrees of space-charge induced growth and with different tail signatures. Efforts are under way to detect shorter bunches by designing cavities with increased bandwidth. This demonstration lends credibility to the idea that these cavities could also be used for other applications, including bunching and shaping, when driven with external rf.
Highlights
It is extremely important to understand the temporal characteristics of electron beams at accelerator photoinjectors
Many parameters affect the temporal characteristics of electron bunches at the photoinjector including the drive laser optical pulse, the photogun bias voltage, beam current and bunch charge, the field strength of focusing magnets, and the phase and field strength of rf bunching and accelerating cavities
The first measurements were made using electron beam delivered from the strained-layer GaAs/GaAsP superlattice photocathode with 100 nm thick active layer
Summary
It is extremely important to understand the temporal characteristics of electron beams at accelerator photoinjectors. Many parameters affect the temporal characteristics of electron bunches at the photoinjector including the drive laser optical pulse, the photogun bias voltage, beam current and bunch charge, the field strength of focusing magnets, and the phase and field strength of rf bunching and accelerating cavities Optimizing these parameters can be a time consuming process, during accelerator commissioning. The shape of the cavity was tuned such that the TM0N0 modes are harmonic This was accomplished by iteratively modifying the cavity’s geometry and solving for the first 20 TM0N0 mode frequencies with the field solver Poisson Superfish [4]. The bunch-length monitor that employed the bolted-together cavity halves operated closer to the desired resonant frequency because slight adjustments to cavity dimensions could be made by applying different levels of pressure to the intervening wire seal. Monitor with welded cavity halves was sent to Jefferson Lab for beam-based evaluation
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