An RF photogun for external injection of electrons in a laser wakefield accelerator

We have developed a 2.5 cell, 3 GHz RF accelerator specifically to inject electrons in a laser wakefield accelerator (LWA). The electron bunches are accelerated to around 3.5 MeV and focused at 1.14 m from the cathode of the accelerator using a pulsed solenoid. Bunches between 0 and 33 pC were focused onto a phosphor screen at the position of the entrance of a plasma channel. The (RMS) bunch size was 32 mu m at 1 pC and increases to 61 mu m at 33 pC. The energy of the bunches at the chosen settings was measured to be 3.71 MeV with 0.02 MeV energy spread (at 10 pC). Energy fluctuations were less than 2 keV. The pointing stability of the focused electron bunches was determined from 100 consecutive shots at 1 Hz to be 5 mu m (RMS). GPT (General Particle Tracer) simulations have been performed using the measured bunches as input for LWA. The simulations show that up to 1 pC of charge can be accelerated to energies of around 1 GeV using realistic plasma and laser parameters. The measured bunch parameters in combination with the simulations show how external injection of pre-accelerated electrons can be a viable alternative to other injection mechanisms.