Initiation of Quasi Spherical Direct Drive capsules for inertial fusion

Summary form only given. Magnetically driven, Quasi Spherical Direct Drive (QSDD) fusion capsules potentially offer much higher efficiency implosions for inertial fusion energy than x-ray driven implosions with the comparable pulsed power generator. However, these solid liners must initiate uniformly to produce a quality implosion. Previous experiments by Stinnett, et al, showed that 20 to 200 nm thick aluminum liners on thick insulating substrates require a local action integral of ~6×10⁸ (A/cm²)²-s or ~8×10¹⁶ A/cm² rising in 7 ns to initiate uniformly. The underlying physical processes are complicated by the presence of ~3 monolayers of adsorbed gas and 7.5 nm of AlO₂O₃. Since QSDD liners are typically 100 micron thick beryllium, we have used a rad-hydro MHD code to analyze the initiation of both thin aluminum liners, to understand the underlying physics, and thick beryllium liners, to predict the drive requirements for uniform initiation of QSDD implosions. Both simulations had 3 monolayers of adsorbed hydrogen on the metal. The simulations provided the local electric field, density, and temperature profile of the metal and gas. The results were analyzed with J. C. Martin´s multi-channel switching relationships. We infer that the metal must reach approximately 1 eV temperature before the gas, which is heated by thermal conduction, can expand to provide an alternative path for current channel formation through the gas. The results also suggest that an ignition-class QSDD capsule will need ~40 ns implosion times with a 40 MA drive pulse. A 2 micron polyimide coating may significantly improve the initiation of solid liners, as it has improved the initiation of wire arrays in work by D. B. Sinars, et. al., and by G. S. Sarkisov, et. al.