The advent of chirped-pulse-amplified CO2 lasers  has yielded picosecond, long-wavelength infrared (λ=10 μm) laser pulses of terawatt (TW) peak power suitable for driving laser wakefield accelerators (LWFAs) with high ponderomotive force (∼Iλ2) in low-density (1016 cm-3 < ne <1018 cm-3) plasma . Such pulses can drive GeV plasma accelerating structures large enough (λp up to hundreds of μm) to enable precise injection of <1%-energy-spread lepton bunches from external linacs and detailed 4D imaging of wake density  and field  profiles via optical  and electron  probing. The AE-71 project at Brookhaven’s Accelerator Test Facility (ATF) is devoted to exploring these opportunities. We report time/space-resolved optical measurements of the electron density structure of self-modulated wakes driven by CO2 laser pulses (4 ps, 0.5 J, focus w0 ≈ 25 μm)  in fully self-ionized hydrogen (0.4< ne <3 1018 cm-3) with new experimental and simulation results. Wake amplitude and dynamics were observed by monitoring collective Thomson scattering (CTS) of a co-propagating 532 nm, 4 ps, electronically synchronized probe pulse (jitter ∼200 fs). First- and second-order CTS sidebands, at Δω=±ωp, ±2ωp from the center probe frequency, were observed as a function of pump-probe time delay Δt, plasma density ne, drive laser peak power P, and focus position within the gas jet. SPACE  and OSIRIS  simulations explain key observations, including unexpected spectral splitting of sidebands at Δt≈0, wake lifetimes of ∼10 ps, and dependence of wake amplitude on ne and P.
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|Working group||Laser-driven electron acceleration|