Experimentally Estimating the Density Evolution of Laser Driven NIF Gas Pipe Targets using Bremsstrahlung Emission

Laser pre-heating DT gas mixtures of a few mg/cc to keV temperatures is a key step in the Magnetized Liner Inertial Fusion (MagLIF) ICF concept. Lasers can couple an ignition relevant amount of energy (20-30 kJ) into these gas fills in 0.1-1 cc volumes, such that once compressed via a Z-pinch the MagLIF target can achieve pressures sufficient to undergo nuclear fusion. At the National Ignition Facility (NIF), a laser quad is propagated through at scale gas pipes to experimentally quantify the behavior of MagLIF laser pre-heating in D2 and neopentane gas fills. As the laser propagates through the gas fill it will produce both a shockwave and Bremsstrahlung radiation. Bremsstrahlung as viewed by a gated x-ray detector (GXD) can be used to measure the laser energy coupling [1], but as shown here it can also be used to estimate the density of the shock created by the driving laser. Using the measured emission and assuming an electron temperature and geometry of the emitting regions, a spatial map of the density of the laser produced shockwave in magnetized neopentane and un-magnetized D2 filled targets can be inferred. The same estimations can then be applied to later time laser interactions used to measure wall mix. This will help better inform simulations and future MagLIF experiment designs.