Constraints on Ion Velocity Distributions from Fusion Product Spectroscopy

TL;DR

This paper develops a theoretical framework to analyze non-Maxwellian ion velocity distributions in fusion plasmas using spectral moments, aiding the interpretation of experimental spectroscopy data.

Contribution

It introduces a classification of non-Maxwellian distributions based on spectral moments and derives constraints for isotropic and anisotropic cases.

Findings

Spectral moments constrain the space of isotropic velocity distributions.

Derived expressions for anisotropic spectral moments.

Experimental data can identify plasma kinetic states.

Abstract

Recent inertial confinement fusion experiments have shown primary fusion spectral moments which are incompatible with a Maxwellian velocity distribution description. These results show that an ion kinetic description of the reacting ions is necessary. We develop a theoretical classification of non-Maxwellian ion velocity distributions using the spectral moments. At the mesoscopic level, a monoenergetic decomposition of the velocity distribution reveals there are constraints on the space of spectral moments accessible by isotropic distributions. General expressions for the directionally dependent spectral moments of anisotropic distributions are derived. At the macroscopic level, a distribution of fluid element velocities modifies the spectral moments in a constrained manner. Experimental observations can be compared to these constraints to identify the character and isotropy of the underlying reactant ion velocity distribution and determine if the plasma is hydrodynamic or kinetic.