Measurement of preheat due to fast electrons in laser implosions of cryogenic deuterium targets

Preheat due to fast electrons generated by nonlinear laser-plasma interactions can reduce the gain in laser-imploded fusion targets. The first measurements of electron preheat in directly driven, imploding cryogenic-deuterium targets are reported. The preheat level is derived directly from the measured hard-x-ray spectrum. The hard-x-ray detectors were calibrated in situ with a parylene-coated molybdenum solid-sphere target where both the hard x rays and the Mo Kα line were measured. Using electron and radiation transport calculations to relate the measurements to preheat, the fraction of the incident laser energy that preheats the cryogenic deuterium fuel is found to be less than 0.1%, suggesting that preheat will have a negligible impact on target performance. The results are encouraging for the success of planned high-gain direct-drive-ignition experiments on the National Ignition Facility [W. J. Hogan, E. I. Moses, B. E. Warner, M. S. Sorem, and J. M. Soures, Nucl. Fusion 41, 567 (2001)].