Optical measurements and simulations of nanosecond-scale plasma channel evolution excited by beam-driven plasma wakes at FACET

Energy density contained in highly nonlinear “blowout” regime plasma wakes can reach electron rest energy density (~n_emc²) [1]. The ns, and longer, dynamics of the subsequent redistribution and diffusion of this enormous energy density sets ultimate repetition rates for plasma accelerators. Although simulations have predicted unique structure and dynamics of such “ion wakes,” [1-3] experiments have not yet explored this long-term evolution. We report optical measurements of cylindrically symmetric ion channel structures emerging from broken plasma wakes generated in singly self-ionized lithium (Li) plasma (n_e=8e16cm^-3) by SLAC’s 20GeV, 2nC electron bunches (σ_x=σ_y=30µm, σ_z=50µm), that show the plasma column remains peaked on axis and grows continuously in radius from < 10 µm at ∆t < 10 ps after passage of the drive bunch to several hundred µm at ∆t = 1.5 ns. Strongly refracting plasma column persists at ms delays. Simulations using the fully relativistic PIC code OSIRIS [3] and the quasi-static LCODE [1], model the evolving plasma column to ∆t ~ 1 ns, yielding a density profile consistent with measurements.

[1] K. V. Lotov et al., Phys. Rev. Lett. 112, 194801 (2014)

[2] A. Sahai et al., ArXiv.1504.03735 (2015)

[3] J. Vieira et al., Phys. Rev. Lett. 109, 145005 (2012)