Abstract
Laser-driven plasma accelerators provide tabletop sources of relativistic electron bunches and femtosecond x-ray pulses, but usually require petawatt-class solid-state-laser pulses of wavelength λ(L) ~ 1 μm. Longer-λ(L) lasers can potentially accelerate higher-quality bunches, since they require less power to drive larger wakes in less dense plasma. Here, we report on a self-injecting plasma accelerator driven by a long-wave-infrared laser: a chirped-pulse-amplified CO(2) laser (λ(L) ≈ 10 μm). Through optical scattering experiments, we observed wakes that 4-ps CO(2) pulses with < 1/2 terawatt (TW) peak power drove in hydrogen plasma of electron density down to 4 × 10(17) cm(-3) (1/100 atmospheric density) via a self-modulation (SM) instability. Shorter, more powerful CO(2) pulses drove wakes in plasma down to 3 × 10(16) cm(-3) that captured and accelerated plasma electrons to relativistic energy. Collimated quasi-monoenergetic features in the electron output marked the onset of a transition from SM to bubble-regime acceleration, portending future higher-quality accelerators driven by yet shorter, more powerful pulses.