Laser-induced electron trapping in plasma-based accelerators

Trapping of plasma electrons in the self-modulated laser wakefield accelerator (LWFA) via the coupling of Raman backscatter to the wake is examined analytically and with three-dimensional (3-D) test particle simulations. The trapping threshold for linear polarization is much less than for circular and occurs for wake amplitudes of δn/n∼25%, which is well below wave breaking. Self-channeling provides continuous focusing of the accelerated electrons which, along with relativistic pump laser effects, can enhance the energy gain by a factor ⩾2. The colliding pulse method for injecting electrons in the standard LWFA is examined. Simulations of test electrons in 3-D fields indicate the production of relativistic (⩾25 MeV) high-quality electron bunches with ultrashort durations (a few femtoseconds), small energy spreads (a few percent), and low normalized emittances (1 mm mrad).