THEORY AND DESIGN OF A PHOTOINJECTOR-DRIVEN CHIRPED PULSE FREE-ELECTRON MASER

An overview of the design parameters of a compact high-gradient high-l uminosity X-band (8.568 GHz) photoinjector is followed py a more detai led description of each of its major subsystems: X-band RF gun, GHz re petition rate synchronously modelocked AlGaAs quantum well laser oscil lator, and eight-pass Ti:Al2O3 chirped pulse laser amplifier. The phot oinjector uses a high quantum efficiency (similar to 5%) Cs2Te photoca thode, and is capable of producing high-charge (>1 nC) relativistic (5 MeV) ultrashort (<1 ps) electron bunches at 2.142-GHz repetition rate in burst mode (100 photoelectron bunches), Design studies indicate th at a normalized rms transverse emittance epsilon(n) = 0.75 pi mm-mrad is possible at 0.1 nC charge, while 2.5 pi mm-mrad is obtained at 1 nC , One of the most interesting applications of the photoinjector, namel y the generation of ultrashort pulses of coherent synchrotron radiatio n, is then addressed in detail, The spectral and temporal radiation ch aracteristics of an axially extended (finite-size) transversely accele rated charge distribution propagating on fixed helical trajectories th rough a wiggler are derived for a cylindrical waveguide, At grazing, w here the axial bunch velocity matches the electromagnetic wave group v elocity, the single output radiation pulse is extremely short, and chi rped over the full interaction bandwidth; the pulse duration is determ ined by group velocity dispersion, A 5-MeV 1.4-nC l-ps photoelectron b unch is then considered, The grazing frequency is adjusted to 175 GHz for the TE(12) cylindrical waveguide mode, corresponding to an 8.5-kG 30-mm-period helical wiggler. The instantaneous bandwidth of the chirp ed output pulse after 10 wiggler periods extends from 125 to 225 GHz, The corresponding power level is 2.2 MW, with a pulsewidth of 15 ps fu ll width at half maximum.