Crossed-beam energy transfer in direct-drive implosions

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.

Refractive Index Seen by a Probe Beam Interacting with a Laser-Plasma System

“…Recent targets have lowered the initial gas fill in large part to reduce CBET and other laser-plasma instabilities, but these targets rapidly fill with wall material such that symmetry control remains a key challenge, and furthermore the range of available design options is severely constricted [16][17][18][19]. In direct-drive ICF, CBET reduces the implosion hydrodynamic efficiency, and its mitigation is considered essential for demonstrating performance improvements [20][21][22]. In both cases, validating linear crossed-beam energy transfer theory with direct measurements is a first step toward demonstrating its controllability and expanding the operable parameter space for integrated ICF experiments.…”

Multiple-view spectrally resolved x-ray imaging observations of polar-direct-drive implosions on OMEGA

“…each beam had the same, averaged power and hence effects due to power imbalance were not taken into account in the calculations. Hydra's total laser energy was decreased by 20% to approximately account for cross beam energy transfer (CBET) [29,30] and other absorption effects not included in the simulations. This adjustment was also needed in order to improve the matching between simulation and experimental bang times.…”

Laser requirements for a laser fusion energy power plant

“…The second concern is called cross-beam energy transfer (CBET) [15] , which can also be viewed as seeded stimulated Brillouin scattering. When two laser beams cross at an angle in the plasma, their interference pattern will produce an ion acoustic wave.…”