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Abstract
Even at a continuous wave facility such as CEBAF at Jefferson Lab, an electron beam with long time intervals (tens of ns) between individual bunches can be useful, for example to isolate sources of background via time of flight detection or to measure the energy of neutral particles that cannot be separated with a magnetic field. This paper describes a demonstrated method to quickly and easily deliver bunches with repetition rates of 20 to 100 MHz corresponding to time intervals between 10 to 50 ns (respectively). This is accomplished by changing the ON/OFF frequency of the RF-pulsed drive laser by a small amount (f/f < 20%), resulting in a bunch frequency equal to the beat frequency between the radio frequencies of the drive laser and the photoinjector chopper system.
Highlights
A DC high-voltage GaAs photogun [1] provides the electrons at the Continuous Electron Beam Accelerator Facility (CEBAF) [2] at Jefferson Lab
Three drive lasers, one for each experimental hall, emit 40 ps (FWHM) optical pulses with 499 MHz pulse repetition rate corresponding to the third subharmonic of the CEBAF accelerating cavities
In ‘‘beat-frequency’’ mode, most of the extracted beam is dumped at the photoinjector chopper, as illustrated in Fig. 1, bottom, which depicts operation with one laser at 467.8125 MHz
Summary
A DC high-voltage GaAs photogun [1] provides the electrons at the Continuous Electron Beam Accelerator Facility (CEBAF) [2] at Jefferson Lab. Three drive lasers, one for each experimental hall, emit 40 ps (FWHM) optical pulses with 499 MHz pulse repetition rate corresponding to the third subharmonic of the CEBAF accelerating cavities The light from these lasers is combined and directed at the same location on a GaAs photocathode within the 100 kV DC high-voltage photogun to produce three independent electron pulse trains interleaved in time and each experimental hall receives an electron beam with 2 ns bunch spacing. Repetition rates from 20 to 100 MHz were demonstrated, with bunch separation ranging from 50 to 10 ns This beat-frequency technique is considerably easier to implement compared to installing a completely new laser, and it provides only a few microamperes of low duty factor beam, we expect it will become a valuable and widely used tool for nuclear physics experiments at CEBAF
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