Initial testing of neutron-induced refractive index modulation in optical materials at the National Ignition Facility

Understanding the onset of thermonuclear burn and ignition in inertial confinement fusion implosions motivates the pursuit of time-resolved measurements of ion temperature and fuel density. At the National Ignition Facility (NIF), there are currently no diagnostics that can constrain ion temperature as a function of time. A single neutron time-of-flight (nToF) detector at an intermediate distance, or multiple detectors along a single line of sight, can in theory constrain the time evolution of the neutron energy spectrum. However, ultrafast time response is required. Refractive-index modulation in optically transparent media induced by ionizing radiation offers a novel ultrafast (sub-ps) measurement technique that could be applied to nToF spectroscopy. A related approach is used in RadOptic detectors, which have previously demonstrated x-ray detection. Here, the RadOptic technique is adapted to evaluate the sensitivity of refractive-index modulation in different materials to 14 MeV neutrons, a response that remains broadly uncharacterized. This work summarizes ongoing efforts toward development of a refractive-index-modulation-based nToF spectrometer at the NIF. To date, measurements include a pump-probe experiment conducted 27 m from the target chamber center in which silicon and fused silica etalons serve as RadOptic media, which are pumped by neutrons and probed with a continuous-wave laser. The measured response at 27 m shows low signal-to-noise levels, however. A change in the refractive index in silicon of ∼5× 10-7 is measured and steps are outlined for improving sensitivity toward development of an in-chamber diagnostic that can constrain the temporal evolution of the neutron spectrum.