FEL simulations for few-cycle pulse generation

Few-cycle optical pulses can be generated in a FEL oscillator operated with high-gain and small-loss parameters. We proposed application of such FEL pulses to high-harmonics-generation (HHG) to explore atto- and zepto-second science and X-ray frequency combs. For detail design of a FEL oscillator to generate few-cycle pulses, we discuss techniques of FEL simulations in this paper. We describe validity of the carrier-envelope expression in the few-cycle regime of FEL oscillator and introduce FEL simulations based on the 'unaveraged model', which does not require averaging of electron phase and momentum within a bunch slice. (author)