pF3D simulations of filamentation in Au foil reflection experiments at the NIF

Recent experiments at the NIF were aimed at designing a transmitted beam diagnostic for characterizing a beam passing through an ICF plasma. In the experiments, the beam sprayed upon reflection from a plasma created by ablating an Au foil. The beam impinged on the foil at intensities of order 1E13 W/cm^2 and incidence angles of 8 and 26 degrees relative to the plane of the foil.



According to linear theory, thermal filamentation in the experiments occurs at a significantly lower threshold than ponderomotive filamentation. We use the code Python pF3D to simulate beam spray and filamentation in the experiments. For our initial conditions, we use one-dimensional plasma profiles extracted from radiation hydrodynamic simulations.



Our pF3D simulations show the beam is unstable to thermal filamentation. Although nonlocal heat transport enhances beam spray, thermal filamentation occurs with classical Spitzer-Härm heat transport. Finally, the beam is stable to ponderomotive filamentation in the simulations, which is consistent with linear theory.

The simulated spread in f# at a given intensity and incidence angle is in good agreement with experimentally measured values. The simulations also recover the trend of decreasing beam spray toward more glancing angles of incidence at a given intensity.