Assessment of the Cross-Beam Energy Transfer (CBET) Risk for Polar Direct-Drive Wetted-Foam Designs

The polar direct-drive wetted-foam concept[1] is a promising design for achieving high-gain in inertial confinement fusion (ICF). The baseline target is composed of a spherical shell of 3D-printed (2pp) lattice that is wetted with liquid deuterium-tritium (DT) fuel, and surrounds a central DT gas region. A thin layer of CH typically coats the exterior of the target, and prevents leakage of the fuel. Such designs are thought to be advantageous for a variety of reasons, including high laser-target coupling, and good hydrodynamic efficiency.

One important aspect of the PDD-WF concept is that for much of the pulse, the laser will be ablating the wetted-foam, in contrast to the CH ablator typically used for conventional direct-drive targets. It is therefore important to understand the susceptibility of this ablator material to laser-plasma instabilities (LPI).

In this presentation, we discuss the risk posed by cross-beam energy transfer (CBET) to PDD-WF designs proposed for the National Ignition Facility. We investigate this via a combination of radiation hydrodynamics modelling, laser ray-trace calculations, and theoretical considerations. We find that ablation of wetted-foam leads generally to increased CBET losses relative to CH, and discuss the reasons for this. Finally, we consider possible mitigation strategies for CBET.

[1] R. E. Olson et al., “A polar direct drive liquid deuterium–tritium wetted foam target concept for inertial confinement fusion,” Physics of Plasmas 28(12), 1–9 (2021).