Generation of ultra-short, high brightness electron beam for single spike SASE-FEL operation

In this paper, we study the production of low charge-in the 1 pC range-high brightness, ultra-short electron bunches, with a length shorter than I micromn, to produce sub-ferntosecond pulses in an X-ray FEL We show that the electron bunches have a brightness one or two orders of magnitude larger than the current photoinjectors run in their design regime. The ultra-short bunches can be used to drive a high gain SASE X-ray FEL with a small gain length, to produce femtosecond to attosecond X-ray pulses. This method to produce such short X-ray pulses has the advantage over other proposed methods in that it can be free from longer pulse duration background radiation. We also show, using nascent SPARX SASE FEL as an example, that the electron bunch thus produced can be shorter than the cooperation length of the X-ray FEL, leading to the production of a single spike, fully coherent, X-ray pulse. The proposed system for the production of ultra-short bunches uses the same injector hardware configuration of X-ray FELs like SPARX or the LCLS, operated with a different set of parameters, and does not require any new technical development in the injector and compressor systems.