Advanced Time-Resolved Neutron Time-of-Flight Diagnostics for Burning Plasmas

Project Overview

Understanding the onset of thermonuclear burn and ignition in inertial confinement fusion (ICF) implosions is critical, and time-resolved measurements of ion temperature and fuel density are necessary to do so. There are currently no diagnostics at the National Ignition Facility (NIF) that can constrain the ion temperature as a function of time, T_ion(t). Modulation of the refractive index of an optically transparent medium by ionizing particles can provide a novel and ultra-fast (picosecond) measurement technique that could be adopted for neutron time-of-flight (nToF) spectroscopy. 

Our approach involves probing a detector medium, in this case a Fabry-Perot etalon whose transmission can be very sensitive to the index of refraction, with a CW laser. For the detector medium, candidate materials include silicon and fused silica, which are relatively inexpensive and easy to manufacture. Measurements have been performed at the Alcove TestBed using the 1550 nm probe laser with a silicon etalon. In a DT implosions with yield > 1017, a small signal consistent with the expected time of arrival of the 14 MeV DT neutrons was seen. Continued measurements will confirm that the response was from the expected effect. A follow-on project will be proposed to build a diagnostic insertion manipulator (DIM) based diagnostic, which rests inside of the target chamber closer to TCC, based on the results and designs from this study to obtain novel physics data. 

Mission Impact

The development of a novel and ultra-fast nToF, based on index of refraction modulation techniques, can potentially allow performance characterization through new metrics such T_ion(t) and neutron burn width measurements. Due to its relatively low-cost, the intermediate nToF can potentially be propagated to multiple locations around the target chamber to investigate higher order modes and asymmetries in Tion. Multiple detectors at a single line-of-sight can provide information on the temporal evolution of the neutron energy spectrum. This refractive index modulation technique can be applied to x-ray and gamma measurements to provide a relatively inexpensive and ultra-fast diagnosis of the gamma reaction history. This project supports inertial confinement fusion (ICF) and High Energy Density Science Core Competencies, and Optics Technologies. 

Publications, Presentations, and Patents

Justin Jeet, "BP11.00048 DD-fusion-neutron yield calibrations for NIF nTOF detector systems; BP11.00049 Advanced time-resolved neutron time-of-flight diagnostics via refractive index modulation; BP11.00050 Reaction-in-Flight Measurements using the National Ignition Facility (NIF) Neutron Time-of-Flight detectors; JO07.00012 Preparations for Radiochemical Measurements of Double Shell Implosions; PO04.00004 Neutron imaging estimation of nuclear radchem diagnostics" (Poster Presentation: 65th Annual APS Division of Plasma Physics Conference, 2023, Denver Colorado, Oct. 2023).