Generation of sub-picosecond electron bunches in superconducting RF photocathode injector

A multi-cell superconducting photocathode laser driven RF gun having a longitudinally shorted first cell (the length of the first cell is 0.15 lambda instead of 0.25 lambda) can generate shorted electron bunches with very low longitudinal emittance [V.N. Volkov, D. Janssen" Ultra shirt electron bunches with low longitudinal emittance in a multi-cell superconducting RF guns," these conference.]. These bunches could be considerably longitudinally compressed by additional compressing scheme having a buncher cavity and drift space after them or four dipole achromatic chicane placed just after the buncher cavity. The first scheme with drift space is longer on two meters but has appreciably better effectiveness to compress a bunch. The buncher is composed from 3900 MHz multi-cell TESLA like cavity. Chicane has a modernised geometry to compensate transversal emittance dilution of a bunch. Bunch dynamic calculations in two such compressing schemes (optimized at 10 MeV of bunch energy) show that a bunch rms length down to 7 micrometers (0.02 picoseconds, the laser pulse FWHM duration is 2.5 picoseconds) with a peak current of 1500 A can be reached. The optimised characteristics of two compressing schemes with two different RF injectors (with 3.3 and 5.3 cells) are presented. This work was financially supported by Forschungs Zentrum of Rossendorf.