Measuring the hot-electron population using time-resolved hard x-ray detectors on the NIF

In laser-driven inertial confinement fusion, hot electrons can preheat the fuel and prevent compression of the capsule to ignition conditions. Measuring the hot-electron population in these high-intensity, laser-driven experiments is key to understanding the laser-plasma interaction and the resulting target evolution. This can be inferred from the bremsstrahlung generated by the interaction of the hot electrons with the target. At the National Ignition Facility (NIF), the filter-fluorescer x-ray diagnostic (FFLEX), a multichannel, hard x-ray spectrometer operating in the 20- to 500-keV range, was recently upgraded to provide time-resolved measurements of the bremsstrahlung spectrum. Characterization data is presented for the upgraded setup, as well as recent results from ignition-scale experiments.