Speaker: Phil Moloney (Imperial)
Title: Impact of Cross-Beam Energy Transfer and Beam-Mode Asymmetries on OMEGA Direct-Drive Implosion Performance
Abstract:
High-performance inertial confinement fusion implosions require highly symmetric fuel compression and efficient coupling of driver energy to the target. In direct-drive implosions, a major source of compression asymmetry arises from the number, layout, and profiles of the drive beams, known as beam-mode (BM) compression asymmetries. On OMEGA, these effects produce a dominant Legendre mode-10 in the power deposition. Cross-beam energy transfer (CBET) is another important degradation mechanism. This is a laser-plasma instability that reduces absorption by transferring energy from incident beams to outward-travelling, unabsorbed “blowby” light from other beams. CBET reduces energy deposition by approximately 20% in typical OMEGA implosions and can further amplify BM asymmetries by modifying the intensity profiles of the incoming beams [1].
This seminar aims to quantify (1) the extent to which OMEGA implosions are degraded by BM asymmetries and CBET, and (2) how these effects modify optimal target and laser pulse-shape designs. To this end, an ensemble of radiation-hydrodynamics simulations is performed in 1- and 2-D (without and with BM degradation) and with and without CBET. These simulations are coupled to the multi-fidelity Bayesian analysis and optimisation toolkit Millefeuille [2]. Surrogate models are constructed for each case over an eight-parameter design space, comprising three target parameters and five laser pulse parameters, enabling identification of optimal implosions and inference of how changes in target design affect performance. The simulations are conducted using the 3-D Eulerian radiation-hydrodynamics code CHIMERA, coupled to the SOLAS ray-tracing library to model the effects of CBET [3].
The results of this simulation campaign demonstrate that the optimal implosion’s generalised Lawson criterion is reduced by approximately 10% due to BM asymmetries and by approximately 30% due to CBET. CBET systematically alters the optimal design, favouring targets with larger outer radii to minimise blowby light and lower target masses to account for reduced energy coupling. As shown in the figure, optimisation of the generalised Lawson parameter using the surrogate models indicates that, for designs with small target outer radii relative to the beam spot size, 2-D performance remains close to 1-D performance and degradation is dominated by CBET. Conversely, for designs with larger outer radii, CBET effects are reduced due to suppressed blowby light, and performance degradation is dominated by BM asymmetries.