Observation of Plasma Bubble Structures in a GeV Laser-Plasma Accelerator

We measure characteristics of plasma bubbles in GeV-class laser-plasma accelerators (LPAs) using Faraday rotation diagnostics. We extend these techniques, previously demonstrated for LPAs in atmospheric density plasmas (electron density $n_e >10^{19}$ cm$^{-3}$), to LPAs in low-density plasmas ($n_e \approx 5\times10^{17}$ cm$^{-3}$), in which plasma bubbles are $\sim 5$ times larger, and correspondingly easier to visualize in detail. The signals show $\approx 0.5^\circ$ rotation streaks of opposite sign separated by $\sim50$ $\mu$m, consistent with bubble diameter; no on-axis rotation; streaks length consistent with transverse probe pulse duration ($180$ $\mu$m for $500$ fs pulse length, and $600$ $\mu$m for $2$ ps pulse length). We utilized an anamorphic imaging system to obtain a wide longitudinal field of view ($>1$ cm) and a high transverse resolution ($<9$ $\mu$m). We also demonstrated that Faraday rotation signals are sensitive to the stages of acceleration processes using extended 2D Finite Difference Time Domain (FDTD) simulation.