Study of hot electron generation using kilojoule-scale high power lasers in shock ignition relevant conditions

Summary form only given. Shock ignition<;sup>1<;/sup> (SI), an alternative high gain laser fusion scheme, achieves ignition by launching a spherically converging ignitor shock in the latest stage of the compression phase using a spike pulse with absorbed intensity of ~5×10<;sup>15<;/sup> Wcm<;sup>-2<;/sup>. Nonlinear laser plasma instabilities (LPI) and hot electrons are critical issues for SI. Spike pulse generated hot electrons with moderate energies (<;150 keV) could potentially augment ablation pressure leading to gigabar shocks as demonstrated in recent OMEGA experiments<;sup>2<;/sup>.To investigate this potential benefit at SI-relevant high intensities, we have conducted experiments utilizing the high energy OMEGA EP laser to examine the effect of laser wavelength, intensity and plasma conditions on hot electron generation and energy coupling using multilayered planar foil targets. The target was first irradiated by multi-kJ UV beams at relatively low intensity to produce a long scalelength hot (~keV) plasma. The main interaction pulse, either a kJ 1-ns UV pulse with intensity ~1.6×1016 Wcm-2 or a kJ 0.1-ns IR pulse with intensity up to 2×1017 Wcm-2 was injected at varied timing delays. We have successfully demonstrated coupling of IR beam energy to the target after propagating over ~0.5 mm long scalelength plasma. The IR beam was found to break into many filaments near the quarter critical density region followed by propagation of those non-merging filaments to critical density, producing hot electrons with temperature of ~70 keV in a well-contained beam, suitable for electron-assisted SI. Details of the experiments and particle-in-cell simulations will be presented.