Theory and design of a photoinjector-driven chirped pulse free-electron maser

An overview of the design parameters of a compact high-gradient high-luminosity X-band (8.568 GHz) photoinjector is followed by a more detailed description of each of its major subsystems: X-band RF gun, GHz repetition rate synchronously modelocked AlGaAs quantum well laser oscillator, and eight-pass Ti:Al₂O₃ chirped pulse laser amplifier. The photoinjector uses a high quantum efficiency (~5%) Cs₂Te photocathode, 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 that a normalized rms transverse emittance ε_n=0.75 π mm-mrad is possible at 0.1 nC charge, while 2.5 π mm-mrad is obtained at 1 nC. One of the most interesting applications of the photoinjector, namely the generation of ultrashort pulses of coherent synchrotron radiation, is then addressed in detail. The spectral and temporal radiation characteristics of an axially extended (finite-size) transversely accelerated charge distribution propagating on fixed helical trajectories through a wiggler are derived for a cylindrical waveguide. At grazing, where the axial bunch velocity matches the electromagnetic wave group velocity, the single output radiation pulse is extremely short, and chirped over the full interaction bandwidth; the pulse duration is determined by group velocity dispersion. A 5-MeV 1.4-nC 1-ps photoelectron bunch is then considered. The grazing frequency is adjusted to 175 GHz for the TE₁₂ cylindrical waveguide mode, corresponding to an 8.5-kG 30-mm-period helical wiggler. The instantaneous bandwidth of the chirped 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 full width at half maximum