Mitigation of stimulated Brillouin scattering in NIF experiments*

Stimulated Brillouin scattering (SBS) is a laser-plasma instability (LPI) that can lead to high levels of back-scattered laser energy in inertial confinement fusion (ICF) or high-energy density physics experiments on laser facilities like the National Ignition Facility (NIF). In addition to its impact on laser-target coupling and drive symmetry, SBS can also constrain experiments on NIF due to its potential to damage laser transport mirrors as the scattered light travels back through the laser chain. Recent indirect-drive ICF designs have experienced a resurgence of SBS on the outer cones of NIF beams. This is attributed to the lower gas-fill used in these hohlraums, that allows the formation of a large volume ("bubble") of gold filling up the hohlraum that is prone to SBS growth.  The experiment designs also have less reliance on crossed-beam energy transfer (CBET) to tune the implosion symmetry by transferring power to the inner cones, leading to increased power on the outer cones beams. Risk of optics damage due to SBS restricts the design space of most of these recent platforms and must be considered for operation of NIF at higher power and energy.

In this talk, we will review our recent efforts to understand and mitigate SBS and optics damage. Mitigation strategies have been developed and tested in simulations using the code pF3D coupled to crossed-beam energy transfer calculations; these include laser-based LPI control (higher laser bandwidth, multi-wavelength upgrade, or optimized phase plate design), target optimization (e.g. changing the hohlraum wall composition to increase ion wave damping and reduce SBS), and facility protection against SBS scattered light. We will present the estimated impact of these mitigations on current ICF platform performance, and on scaled designs at higher laser energy.