Chiron: A ray based laser-plasma instability model for radiation-hydrodynamics codes

A new approach to modelling laser-plasma instabilities (LPI) has been developed that is designed to run in-line with ray tracing routines alongside radiation-hydrodynamics simulations of inertial confinement fusion (ICF) implosions. Accurate modelling of ICF experiments is a crucial aspect to advancing the field. Many approaches to LPI modelling exist [1–4] because, although predicting LPI levels is essential to ICF simulations, developing models that encompass the physics of the highly complex, nonlinear LPI processes is very difficult.

The model developed in this work, called Chiron, is based on analytic theory for the
steady-state solutions to LPI levels. Chiron is unique in that it combines the simultaneous
growth of stimulated Raman scattering, stimulated Brillouin scattering, and two-plasmon decay along with the inverse bremsstrahlung absorption mechanism, while featuring saturation by convection, Langmuir decay and pump depletion for the LPI models.

The famous Rosenbluth gain [5] has been adapted into a new expression for convective
gain called the “bounded” gain. A new expression for saturation of instabilities via Lang-
muir decay has also been derived. The LPI models are saturated both convectively and
absolutely depending on which mechanism the model determines is dominant. The regimes
of applicability of both Rosenbluth and bounded gain are defined by comparing to a more
physically complete numerically (rather than analytically) integrated LPI model. An iter-
ative method has been developed to deal with how pump depletion nonlinearly affects LPI
saturation levels, which has allowed the pump amplitude to be treated as a constant in the
analytic theory, keeping it tractable.

Chiron was tested by simulating a planar target experiment carried out on the OMEGA
laser facility. The spectra generated by stimulated Raman scattering shows strong agreement with the experimental scattered light data. The model also indicates two-plasmon decay was the dominant instability in the experiment, which can not be verified from the currently available experimental data.