Ion temperature measurements at MJ yields at the National Ignition Facility using a gamma-based nToF detector

DT ion temperatures above 15 keV are now routinely being measured at the National Ignition Facility (NIF) by the suite of neutron time-of-flight (nToF) spectrometers. Understanding these measurements is crucial to evaluating performance and refining Inertial Confinement Fusion experimental designs. Each NIF nToF has both scintillator and Cherenkov-based detectors, which measure the same neutron beams with different mechanisms. As yields and temperatures at the NIF have increased, discrepancies between the two methods are reaching 25% (5 keV on 20 keV), with scintillator values consistently higher.

A novel, 3^rd nToF approach has been developed to help resolve this discrepancy: measuring the 4.4 MeV 𝛾 from ¹²C(n, n’𝛾) in the scintillator. This gamma-based nToF (gNTOF) has significant advantages in simplicity & speed, and high-quality data has been obtained at two gNTOF test locations at the NIF. gNTOF ion temperatures agree with the scintillator results, as do Magnetic Recoil Spectrometer (MRS) results, giving a high degree of confidence in these measurements. An overview of the gNTOF tests at the NIF will be given. Testing is now focused on the Cherenkov-based nToFs, to determine the cause of their low ion temperatures. Results from this testing effort will be presented.