Laser wakefield accelerators offer the possibility of compact electron acceleration. However one of the key outstanding issues with the results reported to date is the electron beam stability. Experiments on two laser systems reveal that the contrast ratio between the ASE pedestal and main pulse is an important factor in determining the quality of the electron beam. With a high contrast ratio (108) the electron beam profile is a well collimated single beam having a low pointing instability (<10 mrad rms). With a lower contrast (106) the beam profile contains multiple beamlets which exhibit a large pointing instability (≃50 mrad rms). A high contrast ratio not only improves the beam pointing stability (≃6 mrad) but also stabilizes the electron beam energy reproducibility (5%).
Controlling beam loading to produce large-charge high-quality electron beams by tuning the laser profile in laser wakefield acceleration