Comparative analysis of tungsten armor and a liquid-lithium film as first walls for the HiPER laser-fusion chamber: thermal response, vapor shielding and helium transmission to the steel

Abstract

Tungsten armor and liquid-metal films, the usual sacrificial first walls of laser-fusion chambers, were modeled separately. Both go through one calculation for the three HiPER scenarios: NRL 154 MJ spectra scaled by yield, binned to keep each ion species' number and energy, a common deposition model, and a one-dimensional enthalpy solver with melting, evaporation and parameterized vapor shielding. In Demo (154 MJ, 6.5 m, 10 Hz), a 1 mm tungsten armor peaks 378 K below melting without neutron heating (275-319 K with it). A degraded near-surface conductivity can erase this margin, which also depends on the ion-spectra construction. The steel interface exceeds the 925 K limit, and the surface accumulates the 10^18-10^19 m^-2 helium fluences associated with the onset of surface modification within 10-98 shots (not a lifetime). A lithium film keeps the steel below 925 K from 44 um and below the 873 K compatibility limit from 56 um, more with partial renewal. Burn ions then implant helium; keeping the steel below 10^18 m^-2 for 10^6 shots needs 184 um, which binds in every scenario. The film evaporates 7.4x10^-4 kg m^-2 per shot (range 4.2-8.4x10^-4 kg m^-2, full renewal). Vapor shielding, with a nearly un-ionized vapor (Saha estimate), changes this by -21 % to +5 % for the Saha-supported stopping factor s = 1 and -34 % to +8 % across s = 1-2. Both walls are thermally viable in Prototype and marginal for tungsten in Demo. The deciding issues are tungsten damage evolution, lithium film survival and chamber clearing.