CO₂-laser-driven laser-wakefield acceleration experiments at Brookhaven's Accelerator Test Facility

Advances in CO₂ lasers[1] open mid-IR wavelengths for laser wakefield accelerators, a regime favorable at low plasma densities n_e[2], with large accelerating structures enabling precise external lepton injection and optical[3] and electron[4] probing of wake density and field structure. The AE71 project at BNL's Accelerator Test Facility is devoted to these studies. We report optical measurements of the plasma density structure, amplitude and dynamics of self-modulated wakes driven by CO₂ laser pulses (4ps, 2J, focus w₀≈25µm)[5] in H2 (0.4<n_e<3e18cm^-3), observed by collective Thomson scattering (CTS) of a 532nm, 4ps, probe. 1^st and 2^nd order CTS sidebands were studied vs. n_e, pump-probe time delay ∆t, drive laser peak power P, and focus position within the gas jet. Simulations, detailed in other co-author presentations at this DPP, explain key observations, including unexpected spectral splitting of sidebands at ∆t~0, ~10ps wake lifetimes, and wake amplitude dependence on n_e and P.

[1] M.N. Polyanskiy et al. Optica 2, 675 (2015)

[2] I. Pogorelsky et al, Plasma Phys. Control. Fusion 56, 084017 (2014)

[3] S. P. LeBlanc et al., Phys. Rev. Lett. 77, 5381 (1996)

[4] C. Zhang et al., Plasma Phys. Control. Fusion 60, 044013 (2018)

[5] N. Andreev, Phys. Rev. STAB 6, 041301 (2003)