Plasma electron acceleration driven by a long-wave-infrared laser

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 CO₂ laser (λ_L ≈ 10 μm). Through optical scattering experiments, we observed wakes that 4-ps CO₂ pulses with <  1/2 terawatt (TW) peak power drove in hydrogen plasma of electron density down to 4 × 10¹⁷ cm^-3 (1/100 atmospheric density) via a self-modulation (SM) instability. Shorter, more powerful CO₂ pulses drove wakes in plasma down to 3 × 10¹⁶ 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.