Hard x-ray spectrometer for hot electron measurements on the National Ignition Facility

The upcoming National Ignition Campaign (NIC) on the National Ignition Facility (NIF) will attempt to achieve fusion ignition and gain using a ≈1 MJ laser-driven hohlraum. In order to maintain the fuel on a low enough adiabat so it reaches adequate compressed areal density, the amount of ≫ 170 keV hot electrons generated, those that can penetrate to the DT fuel [1] has to be below 0.25 J for the first 7 ns of the drive pulse, and stay below 500 J during the peak of the pulse at 13 ns. On the NIF, hot electron spectra and yields are inferred from the ≪ 20 keV bremsstrahlung spectrum generated by the hot electrons depositing energy in the hohlraum walls using an absolutely calibrated Filter-Fluorescer (FFLEX) diagnostic [2]. An early version of FFLEX successfully characterized 20 – 120 keV brems-strahlung spectra generated by electrons having 30 keV temperature on vacuum hohlraum experiments using 4 NIF beams [3]. For the upcoming NIC experiments, however, the FFLEX must be capable of measuring x-ray spectra above 170 keV characteristic of electrons above that energy. We propose high energy (100 – 300 keV) channel upgrades that will allow us to estimate the generated hot electron populations with the required accuracy.