Crossed-beam energy transfer in direct-drive implosions

TLDR

OMEGA direct‑drive implosions exhibit a 10–20 % reduction in laser coupling efficiency due to discrepancies between simulated and measured scattered light. The study evaluates three mitigation strategies—narrow beams, multicolor lasers, and higher‑Z ablators—to improve laser coupling. Crossed‑beam energy transfer (CBET) via low‑gain stimulated Brillouin scattering scatters energy from the central beam to outgoing light, lowering absorption and hydrodynamic efficiency. Including CBET in one‑dimensional simulations reproduces all OMEGA observables, and narrow‑beam experiments confirm improved implosion performance.

Abstract

Direct-drive-implosion experiments on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have showed discrepancies between simulations of the scattered (non-absorbed) light levels and measured ones that indicate the presence of a mechanism that reduces laser coupling efficiency by 10%–20%. This appears to be due to crossed-beam energy transfer (CBET) that involves electromagnetic-seeded, low-gain stimulated Brillouin scattering. CBET scatters energy from the central portion of the incoming light beam to outgoing light, reducing the laser absorption and hydrodynamic efficiency of implosions. One-dimensional hydrodynamic simulations including CBET show good agreement with all observables in implosion experiments on OMEGA. Three strategies to mitigate CBET and improve laser coupling are considered: the use of narrow beams, multicolor lasers, and higher-Z ablators. Experiments on OMEGA using narrow beams have demonstrated improvements in implosion performance.