A compact hard X-ray free-electron laser based on velocity bunching

We show that a femto-second hard X-ray radiation pulse can be generated in a free-electron laser amplifier without magnetic bunch compressors. This is possible by adopting a very low charge electron beam followed by the velocity bunching (VB) in two accelerating sections in a linear accelerator. With VB, an electron bunch with a bunch charge of 10 pC can be successfully compressed in the presence of the space-charge effect by a factor of 150, from 0.2 ps to 1.3 fs rms. Multi-gigawatt hard X-ray radiation can be generated from a 5-GeV electron beam, saturating in approximately a 40-m distance from the undulator entrance. Various accelerator and electron-beam parameters were optimized using a multi-objective genetic algorithm.