Characterizing laser-heated plasma energy density distributions for magnetized liner inertial fusion
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Magnetized Linear Inertial Fusion (MagLIF) is an inertial confinement fusion (ICF) concept being pursued at Sandia National Laboratories . It uses fusion fuel preheating via a laser. A subsequent cylindrical liner implosion and magnetic flux compression brings this fuel to fusion conditions. Experiments on the Z-facility at Sandia have produced compelling neutron yields in the range of 10¹² n/shot . Maximal and axially uniform energy density in the preheat plasma is important for MagLIF yield  . We developed a preheat test platform at the Z-beamlet laser facility that includes surrogate helium gas cell targets. We fielded 2D multi-frame imaging diagnostics including shadowgraphy and x-ray pinhole cameras (PHC). Sound speed and blast wave plasma expansion provide methods for temperature and energy per unit length measurements, giving distributions across the axial length of the plasma. We used these measurements to compare energy density beyond a benchmark of 4 mm across a range of laser parameters, enabling comparisons between preheat configurations. Initial preheat configurations with a 2 ns early prepulse and a 3 ns long heating pulse produced very low energy densities of less than 200 J beyond 4 mm axially. We observed approximately a factor of 4 improvement in coupled energy beyond 4 mm from implementation of a 20 ns early prepulse. Combining a magnetic field, early prepulse, and 5 ns main heating pulse produced an approximate factor of five improvement in coupled energy beyond 4 mm. Without a magnetic field, the energy per unit length descends steeply with axial length. With a 17 T axially-oriented magnetic field, the axial energy density and temperature distribution becomes more uniform up to 6-8 mm, and total coupled energy beyond 4 mm increases by a factor in the range of 1.5-2. Transverse x-ray PHC images on magnetic field shots indicate that the laser heated region is narrower than the laser width. This is evidence for cross-field electron thermal conduction suppression and self-focusing. A Gaussian radial temperature distribution solves the diffusion equation with a diffusion coefficient that is a constant function of temperature, which is approximately the case for electron conduction above a certain temperature across magnetic field lines. This approximate temperature distribution produces a qualitatively similar x-ray profile to data if peak temperatures exceed 1.5 keV during heating. To our knowledge, we have the first indirect measurements of temperatures and energy densities in surrogate preheat plasmas, both with and without a magnetic field. The energy density and temperature distributions we have measured will provide a valuable comparison to simulations and inform both the design simulators of MagLIF and the experimental planners.