A beat frequency RF modulator for generation of low repetition rate electron microbunches for the CEBAF polarized source

Fiber-based drive lasers now produce all of the spin- polarized electron beams at CEBAF/Jefferson Lab. The flexibility of these drive lasers, combined with the existing three-beam CEBAF photoinjector Chopper, provides a means to implement a beat frequency technique to produce long time intervals between individual electron microbunches (tens of nanoseconds) by merely varying the nominal 499 MHz drive laser frequency by < 20%. This submission describes the RF Laser modulator that uses a divider and heterodyne scheme to maintain coherence with the accelerator Master Oscillator (MO), while providing delay resolution in increments of 2 ns. Some possible uses for such a beam are discussed as well as intended future development.