High energy, low energy spread electron bunches produced via colliding pulse injection

Electron beams of high energy and narrow, controllable energy spread using significantly reduced laser power are demonstrated by combining control of Laser-Plasma Accelerator (LPA) structure and injection. A high-energy accelerating structure was formed by controlling the phase front of the drive laser in order to obtain collimated propagation over the length of the plasma. This produced electron energies nearly double those previously achieved using comparable lasers. Injection into the accelerator was controlled by using the beat between “colliding” laser pulses to kick electrons at a controlled location into a plasma wave that was operated below the threshold for self injection. This resulted in the production of bunches with controllable energy. Stability of charge, pointing, and energy were demonstrated. With the injection location fixed by the colliding pulses, beam energy up to 200 MeV was obtained using 10 TW drive laser pulses, controlled by plasma density and by target location with respect to the collision. Instrument-limited energy spreads below 1.4% FWHMtogether with a divergence of 1.5 mrad were measured at 150 MeV. Production of such high quality bunches using low-laser energies is an important step for transportable or laboratory scale LPAs to power photon sources.