High-brightness photo-injector with standing-wave buncher-based ballistic bunching scheme for inverse Compton scattering light source

We report our recent progress in the design and simulation of a high-brightness S-band photo-injector with a ballistic bunching scheme aimed at driving an inverse Compton scattering (ICS) X-ray source. By adding a short standing-wave buncher between the RF gun and first booster in a conventional S-band photo-injector, electron bunches with a 500 pC charge can be compressed to the sub-picosecond level with very limited input RF power and an unchanged basic layout of the photo-injector. Beam dynamics analysis indicates that fine tuning of the focusing strength of the gun and linac solenoid can well balance additional focusing provided by the standing wave buncher and generate a well-compensated transverse emittance. Thorough bunching dynamics simulations with different operating conditions of the buncher show that a buncher with more cells and a moderate gradient is suitable for simultaneously obtaining a short bunch duration and low emittance. In a typical case of a 9-cell buncher with a 38 MV/m gradient, an ultrashort bunch duration of 0.5 ps (corresponding to a compression ratio of $$> 5$$ ) and a low emittance of $$< {1}\,{\hbox {mm}\cdot \hbox {mrad}}$$ can be readily obtained for a 500 pC electron pulse. This feasible ballistic bunching scheme will facilitate the implementation of an ultrashort pulse mode inverse Compton scattering X-ray source on most existing S-band photo-injectors.