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

摘要

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 μm at 1 pC and increases to 61 μm 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 μm (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.