Measurements of Non-Maxwellian Electron Distribution Functions and Their Effect on Laser Heating

Area of Science:

• Plasma Physics
• Laser-Plasma Interactions
• Non-equilibrium Electron Distributions

Background:

• Inverse bremsstrahlung (IB) heating is a key process in laser-produced plasmas.
• Maxwellian electron velocity distributions are often assumed for IB absorption calculations.
• Deviations from Maxwellian distributions can significantly alter plasma properties.

Purpose of the Study:

• To experimentally measure electron velocity distribution functions during IB heating.
• To quantify the impact of non-Maxwellian distributions on IB absorption.
• To investigate the influence of laser heating and thermalization rates on distribution function shape.

Main Methods:

• Utilized a novel angularly resolved Thomson-scattering instrument for electron velocity distribution measurements.
• Conducted experiments where laser heating rate dominated electron-electron thermalization.
• Employed the particle code QUARTZ for plasma simulations.

Main Results:

• Measured electron velocity distribution functions were non-Maxwellian.
• Observed super-Gaussian distributions in the bulk (v/v_{th}<3) and Maxwellian in the tail (v/v_{th}>3).
• A ~40% reduction in IB absorption was measured due to the non-Maxwellian distributions.

Conclusions:

• Non-Maxwellian electron velocity distributions significantly reduce inverse bremsstrahlung absorption.
• The shape of the electron distribution tail is determined by laser heating uniformity.
• Experimental measurements validate theoretical predictions of non-equilibrium effects in laser-plasma interactions.