E.A. Williams

Modeling with backscatter and transmitted light of high-power smoothed beams with pF3D: a massively parallel laser plasma interaction code

Using the 3D wave propagation code, F3D, Berger et al., Phys. Plasmas 5,4337, and the massively parallel version pF3D, we have computed the transmitted and reflected light for laser and plasma conditions in experiments that simulated ignition hohlraum conditions. The frequency spectrum and the wavenumber spectrum of the transmitted light are calculated and used to identify the relative contributions of stimulated forward Brillouin and self- focusing in hydrocarbon-filled balloons, commonly called gasbags. The effect of beam smoothing, smoothing by spectral dispersion and polarization smoothing, on the stimulated Brillouin backscatter from Scale-1 NOVA hohlraums was simulated with the use of nonlinear saturation models that limit the amplitude of the driven acoustic waves. Other experiments on CO2 gasbags simultaneously measure at a range of intensities the SBS reflectivity and the Thomson scatter from the SBS-driven acoustic waves that provide a more detailed test of the modeling. These calculations also predict that the backscattered light will be very nonuniform in the near field which is important for specifying the backscatter intensities to be tolerated by the National Ignition Facility laser system.

Simulating NIF laser-plasma interaction with multiple SRS frequencies

Understanding the energetics of a NIF ignition hohlraum is important to achieving ignition. Laser-plasma interactions (LPI) can reduce the radiation drive if backscatter occurs, and can also affect the hohlraum energetics by modifying the laser beam energy deposition which in turn can alter the implosion symmetry. The addition of a second SRS frequency to the modeling code pF3d can capture physics which would otherwise have been omitted. In the case of a wide or bi-modal SRS spectrum, this physics can be important. We discuss the modifications to the pF3d computational model, and exhibit its effect in a NIF ignition-relevant LPI simulation.