Physics

Electron

Distribution function

Laser

Optics

Inertial confinement fusion

Gaussian

Atomic physics

Nuclear physics

National Ignition Facility

Thomson scattering

Quantum mechanics

Plasma

Computational physics

Nature Physics

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.

Impact of the Langdon effect on crossed-beam energy transfer