Magnetic recoil neutron spectrometry for inertial confinement fusion: To ignition and beyond (invited)

For more than 15 years, the magnetic recoil spectrometer (MRS) has been used to measure the DT fusion neutron spectrum in inertial confinement fusion (ICF) implosions at OMEGA and the National Ignition Facility (NIF). These measurements have provided essential information about implosion performance that has helped guide the mainline ICF programs to the recent demonstration of hydro-equivalent ignition (χ > 1) at OMEGA and to ignition and target gain (G > 1) at the NIF. Today, there are variants of the MRS being built across private industry, including spectrometers on the SPARC tokamak (expected Q > 1, 100 MW) and the Pacific Fusion Demonstration System (expected G ∼10, 100 MJ), both with planned electronic detection systems capable of measuring neutron spectra at high processing rates. In addition, an upgrade of the existing MRS on the NIF is being planned in which a collimator will be incorporated to extend the MRS measurement ceiling to ∼30 MJ. Meanwhile, electromagnet upgrades (MRSnext) have been designed to replace, or complement, the existing MRS systems on OMEGA and the NIF. These, plus other upgrades, will result in several notable improvements from a scientific and/or engineering perspective: significantly smaller focal planes, improved resolution, conversion foils positioned close to the magnet system, and enhanced signal-to-background. Here, we summarize the MRS motivation and history, concept, demonstrated performance, and many of the recent developments, highlighting the versatility of the technique for diagnosing fusion performance across approaches on the road to high energy gain and commercialization.