To optimize and control GeV-level laser plasma acceleration (LPA), it is important to visualize transient LPA structures in a single shot. In this set of LPA experiments using the Texas Petawatt (~120fs, ~120J) in a plasma of He of density ne < 5 × 1017 cm−3, electrons to (~0.6)GeV were produced in (300pC) bunches. The low repetition-rate and slight fluctuations of the laser motivates capturing the dynamics of bubble structures at multiple points along their path in a single shot. For single snapshots, the Faraday effect has been used1,2, where the magnetic field of the accelerated electrons preferentially magnetizes the relatively dense plasma of the surrounding bubble wall, which in turn rotates the polarization of a probe beam moving transverse to the LPA drive pulse1. Coupled with polarization and imaging optics on the probe beam, the plasma density can be visualized.
Here, we describe and show results from a multiplexed Faraday rotation diagnostic. Our Faraday rotation diagnostic uses multiple non-overlapping transverse probe beams of 1cm diameter, each timed to propagate transversely through the bubble at different positions in its path. Together the probes extended over a field of view of 2.7cm. To acquire this wide horizontal field of view while resolving features vertically, we multiplexed the probes through an anamorphic imaging system to de-magnify the horizontal dimension (30μm resolution) while magnifying the vertical dimension (9μm resolution with 300μm depth of field).
The results demonstrate the feasibility of this single-shot diagnostic for capturing GeV-class LPA structures in tenuous plasma. By exploiting the ~3 kilo-Tesla magnetic fields of this high-charge, high-energy electron bunch to magnetize the bubble’s dense walls, we were able to image the plasma bubble structure via Faraday rotation of the transverse probe beams despite low plasma density. The bubble evolution involved its birth, initial rapid evolution, and eventual stabilization. These stages resulted from a “mismatched” focusing geometry, which prompted initial bubble size oscillations and self-injection of up to nCs of electrons from surrounding plasma into the bubble.
 Kaluza, PRL 105, 115002(2010)
 Buck, Nature Phys. 7, 543 (2011)