Impact of the Langdon effect on crossed-beam energy transfer

Abstract

The prediction that laser plasma heating distorts the electron distribution function away from Maxwellian and towards a super-Gaussian distribution dates back four decades [1]. In conditions relevant to inertial confinement fusion, however, no direct evidence of this so-called "Langdon effect" has previously been observed. Here, measurements of the spatially and temporally resolved Thomson scattering spectrum indicate the presence of super-Gaussian electron distribution functions that are consistent with existing theory [2]. In such plasmas, ion acoustic wave frequencies increase monotonically with the super-Gaussian exponent [3]. Our results show that the measured power transfer between crossed laser beams mediated by ion acoustic waves requires a model that accounts for the non-Maxwellian electron distribution function, whereas the standard Maxwellian calculations overpredict power transfer over a wide region of parameter space. Including this effect is expected to improve the predictive capability of crossed-beam energy transfer modeling at the National Ignition Facility and may restore a larger operable design space for inertial confinement fusion experiments. This is also expected to motivate further inquiry in other areas impacted by non-Maxwellian electron distribution functions, such as laser ministration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and

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