Suprathermal ion distributions in burning plasmas on the NIF*

For hydrodynamic plasmas described by Maxwellian ion distributions, a well-described relationship exists between the mean (<E>) and variance of the neutron energy spectrum¹. On the National Ignition Facility (NIF), the addition of the quartz Cherenkov neutron time-of-flight (nToF) spectrometers and a fifth nToF line-of-sight has extended the ability to measure <E> to a precision of 5 parts in 100,000 for high yield DT shots. Observations using this new capability on NIF shots with the highest neutron yield have revealed a significant deviation from the hydrodynamic relationship and disagreement with radiation-hydrodynamic simulations.

Experimental results are described and compared to predicted neutron spectra for a range of candidate ion distributions. This supports the claim that suprathermal ions are required to increase the mean of the neutron kinetic energy faster than the variance. The result suggests that the significant a-heating present in a burning plasma is sufficient to generate non-Maxwellian DT ion distributions. LLNL-ABS-824483

¹. D. H. Munro, Nucl. Fusion 56 036001 (2016) https://doi.org/10.1088/0029-5515/56/3/036001