The energy density deposited in a highly nonlinear “blowout” regime wake reaches that of the rest energy density of plasma electrons . This energy relaxes through a complex redistribution between e.g. accelerated electrons, undirected hot electrons, ion-channel formation, ionization and excitation of surrounding gas and radiation over ns or longer time scales. These relaxation dynamics ultimately govern the repetition rate of plasma accelerators. Although simulations have predicted that strongly nonlinear electron wakes can spawn “ion wakes” of unique structure and dynamics [1-3], experiments have not yet explored this long-term evolution. Here, we present ps-time-resolved optical shadowgraphic measurements of cylindrically symmetric ion channels that emerge from broken plasma wakes generated in singly self-ionized lithium (Li) plasma (ne=8 1016 cm-3) of meter length, by SLAC’s 20 GeV, 2 nC electron bunches (σx=σy=30 μm, σz=50 μm). Results show that the plasma column remains peaked on axis and grows continuously in radius from <10 μm at time delay Δt<10 ps after passage of the drive bunch to several hundred μm at Δt=1.5ns. Measurements at longer Δt show that a strongly refracting plasma column persists at microsecond delays. Simulations using the fully relativistic particle-in-cell code OSIRIS  and the quasi-static LCODE  model the evolving plasma column out to Δt∼1.4 ns, and yield an evolving density profile consistent with measurements.
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