Theory and simulation of cross-beam energy transfer mitigation through increased laser bandwidth
ORAL · Invited
Abstract
Recently, techniques have been proposed to increase laser bandwidth in inertial confinement fusion (ICF) implosions. These bandwidths could provide transformational improvements in ICF performance by mitigating laser-plasma and hydrodynamic instabilities. This talk will examine the impact of bandwidth on the cross-beam energy transfer (CBET) instability. A generalized linear theory for CBET will be presented that incorporates effects of bandwidth[1]. This is compared with data from linearized fluid simulations performed with the LPSE code, which satisfy many of the assumptions made by the theory and shows good agreement with the theoretical predictions. Particle-in-cell simulations of CBET using the VPIC code have also been performed, allowing an investigation of the nonlinear CBET regime.
It is found that CBET is most effectively suppressed when laser bandwidth exceeds the ion- acoustic wave (IAW) frequency. Such bandwidths reduce the coupling efficiency of laser beams with IAWs and can allow reverse (from lower frequency to higher frequency beams) transfer to occur, which reduces the net energy transfer rapidly as bandwidth is increased. The CBET gain exponent in this regime scales with bandwidth (Δω) as Δω-3 for Gaussian or Lorentzian laser spectra. However, linear analysis also finds that the IAW energy density scales as Δω-1, implying that nonlinear effects may be more difficult to control than the CBET scaling would suggest. Indeed, ion-trapping-induced nonlinear effects, such as modification of the ion wave dispersion, the two-ion wave decay, and ion wave self-focusing, lead to significant departures from linear theory. These nonlinearities can be mitigated through the reduction of intensity spikes in the laser drive, for example via smoothing by spectral dispersion.
[1] A. G. Seaton et al., “Cross-beam energy transfer in direct-drive ICF. II. Theory and simulation of mitigation through increased laser bandwidth”, Physics of Plasmas, 29(4), 042707 (2022).
It is found that CBET is most effectively suppressed when laser bandwidth exceeds the ion- acoustic wave (IAW) frequency. Such bandwidths reduce the coupling efficiency of laser beams with IAWs and can allow reverse (from lower frequency to higher frequency beams) transfer to occur, which reduces the net energy transfer rapidly as bandwidth is increased. The CBET gain exponent in this regime scales with bandwidth (Δω) as Δω-3 for Gaussian or Lorentzian laser spectra. However, linear analysis also finds that the IAW energy density scales as Δω-1, implying that nonlinear effects may be more difficult to control than the CBET scaling would suggest. Indeed, ion-trapping-induced nonlinear effects, such as modification of the ion wave dispersion, the two-ion wave decay, and ion wave self-focusing, lead to significant departures from linear theory. These nonlinearities can be mitigated through the reduction of intensity spikes in the laser drive, for example via smoothing by spectral dispersion.
[1] A. G. Seaton et al., “Cross-beam energy transfer in direct-drive ICF. II. Theory and simulation of mitigation through increased laser bandwidth”, Physics of Plasmas, 29(4), 042707 (2022).
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Presenters
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Alexander Seaton
Los Alamos National Laboratory
Authors
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Alexander Seaton
Los Alamos National Laboratory
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Lin Yin
Los Alamos Natl Lab
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Russell K Follett
Laboratory for Laser Energetics - Rochester, Laboratory for Laser Energetics, University of Rochester
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Ari Le
Los Alamos National Laboratory
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Brian J Albright
Los Alamos Natl Lab