Space charge effect for short electron bunches in an ALPHA magnet

Sub-picosecond electron pulse with relativistic energy may perform a source of coherent synchrotron radiation in terahertz (THz) frequency region [1]. In addition, such short pulse will be used directly in pulse radiolysis experiment for radiation chemistry, and also equally short X-ray pulse can be produced via Compton backscattering. On the other hand, compression of the electron bunch to a few hundreds femtosecond is crucial for SASE-FEL, accordingly various schemes of bunch compression have been studied [2]. Usually magnetic chicane is used as a bunch compressor for energy chirped electron bunches manipulated by RF beam-conditioning, i.e., off-crest acceleration in accelerator structures. However this scheme makes the system large, and then relatively low energy and compact system require another compression scheme. Combined use of RF gun and a-magnet seems to be very much effective for production of sub-picosecond bunch length in a compact injector system [1]. Nevertheless space charge effects including wakefield in a-magnet have not been well studied. In this paper we discuss emittance growth in both the longitudinal and the transverse phase spaces (actually those are coupled in a-magnet) by showing numerical simulations employing finite difference time domain (FDTD) method for a Maxwell's equation solver [4].