Laser wakefield acceleration driven by a few-terawatt laser pulse in a sub-mm nitrogen gas jet

Quasi-monoenergetic electron bunches with energies peaked in 10-20 MeV are generated from laser wakefield acceleration (LWFA) by focusing few-TW laser pulses onto a sub-mm gas jet of dense nitrogen. A 152-μm diameter orifice is used to produce transient (≤ 20 ms), free-flow nitrogen jets, while the plasma electrons with a 860-μm wide Gaussian density profile and a density up to ∼2.8 × 1019 cm-3 enable self-focusing effect and self-modulation instability to develop on the pump pulse, resulting in a high intensity to drive the LWFA. Meanwhile, this Gaussian nitrogen plasma facilitates ionization-induced injection and density down-ramp injection throughout the acceleration process and consequently improves the energy and charge stabilities of output electrons. When 40-fs, 3.2-TW, 810-nm pump pulses are applied, output electrons with a peak energy ∼11 MeV and a charge ∼ 20 pC are routinely generated with ≤20% energy and charge stabilities, ∼20 mrad divergence, and ∼10 mrad pointing variation. A large electron energy spread is attributed to the dominant mechanisms of ionization and down-ramp injections. This scheme represents a viable approach for implementing a high-repetition-rate LWFA, from which stable tens-of-MeV electrons can be generated with less than 150 mJ of on-target laser energy.