Ray-based calculations of backscatter in laser fusion targets

TL;DR

This paper introduces a 1D steady-state model and the DEPLETE code for calculating backscatter in laser fusion targets, comparing it with experiments and other simulations to understand laser-plasma interactions.

Contribution

The paper presents a new kinetic model and the DEPLETE code for backscatter calculation, including effects like pump depletion and plasma fluctuations, and compares results with experiments and other models.

Findings

Laser speckles significantly increase backscatter reflectivity.

DEPLETE provides lower bound estimates for backscatter in fusion designs.

Re-absorption of Raman light is important in ignition target analysis.

Abstract

A 1D, steady-state model for Brillouin and Raman backscatter from an inhomogeneous plasma is presented. The daughter plasma waves are treated in the strong damping limit, and have amplitudes given by the (linear) kinetic response to the ponderomotive drive. Pump depletion, inverse-bremsstrahlung damping, bremsstrahlung emission, Thomson scattering off density fluctuations, and whole-beam focusing are included. The numerical code DEPLETE, which implements this model, is described. The model is compared with traditional linear gain calculations, as well as "plane-wave" simulations with the paraxial propagation code pF3D. Comparisons with Brillouin-scattering experiments at the OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, p. 495 (1997)] show that laser speckles greatly enhance the reflectivity over the DEPLETE results. An approximate upper bound on this enhancement, motivated by phase conjugation, is given by doubling the DEPLETE coupling coefficient. Analysis with DEPLETE of an ignition design for the National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, p. 755 (1994)], with a peak radiation temperature of 285 eV, shows encouragingly low reflectivity. Re-absorption of Raman light is seen to be significant in this design.