Simulations of racetrack microtron for acceleration of picosecond electron pulse

Low emittance sub-picosecond electron pulses are expected to be used in a wide field, such as free electron laser, laser acceleration, femtosecond X-ray generation by inverse Compton scattering, and pulse radiolysis, etc. In order to produce the low emittance sub-picosecond electron pulses, we are developing a compact racetrack microtron (RTM) with a new 5 MeV injection system adopting an laser photocathode RF gun. The operation of the RTM is kept under the steady state of beam loading for long pulse mode so far. We have investigated for the first time by numerical simulation the case of short- and single-pulse acceleration. The RTM is also useful to accelerate a picosecond electron pulse under a transient state of beam loading. In the simulation, a picosecond electron pulse is accelerated to 139 MeV in the RTM for the injection of about a 5 MeV pulse with a pulse length of 3 picoseconds, a charge of 1 nC per pulse, and an emittance of 1.8 πmm mrad, which corresponds to the output of the RF gun