Performances of the Alpha-X RF gun on the PHIL accelerator at LAL

The Alpha-X RF-gun was designed to produce an ultra-short (<100fs rms), 100pC and 6.3MeV electron beam with a normalized rms transverse emittance of 1πmmmrad for a gun peak accelerating field of 100MV/m. Such beams will be required by the Alpha-X project, which aims to study a laser-driven plasma accelerator with a short wavelength accelerating medium. It has been demonstrated on PHIL (Photo-Injector at LAL) that the coaxial RF coupling, chosen to preserve the gun field cylindrical symmetry, is perfectly understood and allows reaching the required peak accelerating field of 100MV/m giving beam energy of 6.3MeV. Moreover, a quite low beam rms relative energy spread of 0.15% at 3.8MeV has been measured, completely agreeing with simulations. Dark current, quantum efficiencies and dephasing curves measurements have also been performed. They all show high values of the field enhancement factor β, which can be explained by the preparation of the photocathodes. Finally, measurements on the transverse phase-space have been carried out, with some limitations given by the difficult modelization of one of the PHIL solenoid magnets and by the enlargement of the beam transverse dimensions due to the use of YAG screens. These measurements give a normalized rms transverse emittance around 5πmmmrad, which does not fulfill the requirement for the Alpha-X project.