Proposal of an operation mode of FELiChEM for generating high-power ultra-short infrared FEL pulses

FELiChEM is a new infrared free-electron laser (FEL) oscillator user facility in Hefei, and has achieved the first lasing at 15μm in June, 2019. To improve the temporal resolution for fulfilling the user demands, in this context, we propose an operation mode of generating high-power ultra-short FEL pulses by shortening the electron bunch and enhancing the FEL gain with a small cavity length detuning. The existing magnetic chicane is used to compress the electron bunch and increase its peak current. Taking the 10μm FEL as an example, start-to-end simulations demonstrate that the FEL power can be greatly enhanced while the pulse duration is as short as one hundred femtoseconds. An energy slit provides an additional option that can further shorten the bunch length and reduce the energy spread by cutting off the bunch head and tail. With the energy slit on, all the minor spikes inside the FEL pulse will be further suppressed and only the main spike will be preserved.