Experimental Results and Numerical Predictions of Coherent Transition Radiation (CTR) for the Characterization of High-Current, Fast-Electron Beams

CTR imaging is a technique for analyzing the properties of high-current, relativistic electron beams created in laser--solid interactions. Al, Cu, Sn, and Au foils of thickness ranging from 5 to 100 \textit{$\mu $}m were irradiated with an intensity of $\sim $10$^{19}$ W/cm$^{2}$. Based on the measured signal, the fast-electron-beam temperature ($T_{hot})$ and divergence were estimated to be $\sim $1.4 MeV and $\sim $10.8\r{ }. Collisional effects were found to influence the estimate of $T_{hot}$ and are quantified using a Monte Carlo code. High-resolution, $\sim $1.4-\textit{$\mu $}m imaging of the rear-surface emission reveals small-scale structures $\sim $2 \textit{$\mu $}m in size, embedded in a larger ring-like structure, suggesting electron-beam filamentation and annular propagation. The interpretation of the experimental observations requires numerical calculations. Using the particle-in-cell (PIC) code \textit{OSIRIS} and the hybrid PIC \textit{LSP}, the distribution of CTR has been simulated. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.