Feasibility of 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 1.3 m from the accelerator using a pulsed solenoid. The critical parameters for bunches injected into a laser wakefield accelerator are the bunch charge that can be focused into the plasma (channel), its energy, timing and pointing stabilities. Bunches between 0 and 50 pC were focused onto a phosphor screen at the position of the entrance of a plasma channel. The (RMS) bunch size was 30 µm at 0.1 pC and increases to 70 µm at 50 pC. The energy of the bunches at the chosen settings was measured to be 3.71 MeV with 0.01 MeV energy spread (at 10 pC). The 3 GHz oscillator is synchronized with the Ti:Sapphire laser system which is used for photoemission (and will later also be used to drive the laser wakefield). The synchronization between the two systems is better than 20 fs. The measured energy fluctuations were less than 2 keV, which is consistent with the stability of the output power of the modulator of 0.05% and a negligible contribution due to timing jitter between the laser and the accelerator. The jitter in arrival time between the accelerated electron bunches and the laser caused by these fluctuations is less than 50 fs. Finally, the pointing stability of the focused electron bunches was determined from 100 consecutive shots at 1 Hz to be 6 µm (RMS). GPT (General Particle Tracer) simulations have been performed using the measured bunches as input for LWA. The simulations show that up to 5 pC of charge can be accelerated to energies of around 500 MeV 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.