Impact of the Langdon effect on crossed-beam energy transfer

• Published in: Nature physics Vol. 16; no. 2; pp. 181 - 185
• Main Authors: Turnbull, David, Colaïtis, Arnaud, Hansen, Aaron M., Milder, Avram L., Palastro, John P., Katz, Joseph, Dorrer, Christophe, Kruschwitz, Brian E., Strozzi, David J., Froula, Dustin H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group 02.12.2019; Nature Publishing Group (NPG)
• Subjects: Distribution functions; Electron distribution; Electrons; Energy transfer; Experiments; Inertial confinement fusion; Ion acoustic waves; Ion beams; Laser beam heating; Laser plasmas; Lasers; Normal distribution; Physics; PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; Plasma heating; Plasma Physics; Power transfer; Predictions; Thomson scattering; X-ray spectroscopy
• ISSN: 1745-2473; 1745-2481; 1476-4636
• DOI: 10.1038/s41567-019-0725-z

The prediction that laser plasma heating distorts the electron distribution function away from Maxwellian and towards a super-Gaussian distribution dates back four decades1. In conditions relevant to inertial confinement fusion, however, no direct evidence of this so-called ‘Langdon effect’ has previously been observed. Here we present measurements of the spatially and temporally resolved Thomson scattering spectrum that indicate the presence of super-Gaussian electron distribution functions consistent with existing theory2. In such plasmas, ion acoustic wave frequencies increase monotonically with the super-Gaussian exponent3. 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 modelling at the National Ignition Facility in California and may restore a larger operable design space for inertial confinement fusion experiments. This is also expected to motivate further inquiry in other areas affected by non-Maxwellian electron distribution functions, such as laser absorption, heat transport and X-ray spectroscopy.In inertial confinement fusion experiments, the effect of the overlapping laser beams on the plasma is predicted to lead to a distortion of the electron distribution function, which has now been observed in experiments.