Design Of A High-Current Injector For The Nist Nrl Free-Electron Laser

The electron beam of the NIST-LANL racetrack microtron (RTM) is to be used to drive a free electron laser and for other applications. We have designed a new injector for the RTM to provide the 3.5 ps long, 7–14 pC electron beam pulses required for lasing. The new injector consists of a pulsed, 120 keV thermionic electron gun, a subharmonic chopping and bunching system, and the existing 5 MeV RTM injection linac. It has been designed to produce a continuous train of electron pulses at 5 MeV with a normalized transverse emittance of 5 mm mrad and a longitudinal emittance of 20 keV deg for 95% of the beam (for 7 pC) at repetition rates of 66.111 or 16.528 MHz. These two frequencies are subharmonics of the fundamental accelerator frequency of 2380 MHz. The new injector will also produce lower-emittance beams at lower peak currents at the fundamental frequency.