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

TL;DR

This study visualizes plasma bubble structures in GeV laser-plasma accelerators using advanced Faraday rotation diagnostics, revealing larger bubbles in low-density plasmas and demonstrating the technique's sensitivity to acceleration stages.

Contribution

It extends Faraday rotation diagnostic techniques to low-density LPAs, enabling detailed visualization of larger plasma bubbles in GeV-class accelerators.

Findings

Plasma bubbles are approximately 5 times larger in low-density plasmas.

Faraday rotation streaks correlate with bubble diameter and acceleration stages.

The diagnostic system achieves high transverse resolution and wide field of view.

Abstract

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.