Study of fast electron generation and transport in interactions of intense laser pulse with pre-plasma produced by long laser pulse on foil targets

The influence of pre-plasma on fast electron generation is a critical issue for fast ignition. Multi-layered targets containing a Cu fluorescence layer were irradiated by an intense laser pulse ($ I \leq 2 \times 10^{18} $ W cm$^{-2}$) at the ILE, Japan. Various scale lengths of pre-plasma were generated using a long laser pulse. $K_\alpha$ x-rays spectroscopy and imaging were employed to characterize electron generation and transport. The Cu $K_\alpha$ x-rays were viewed from the front (laser irradiated) side through an Al ablator layer that prevented direct heating of the fluor. The presence of pre-plasma caused an x-ray yield reduction ($\leq 50\%$) and created a ring structure around an x-ray bright spot. Results compared against radiation hydrodynamics and hybrid/PIC modeling will be presented.