Abstract
We propose an ultra-compact soft X-ray superradiant FEL at 30 nm driven by a 150 MeV beam. The total length of such a FEL is less than 30 m, which is 10 times shorter than the length of a typical soft X-ray FEL. The key concept is to laser modulate the electron emission at the photoinjector and then compress the electron macro-bunch by linac and chicane magnet to achieve a soft X-ray bunching frequency in front of an FEL undulator. With an initial 10-ppm bunching factor, we calculated a sub-GW radiation power at 32.2 nm from a 3m long undulator.