Resonance line broadened quasilinear (RBQ) model for fast ion distribution relaxation due to Alfv\'enic eigenmodes

TL;DR

This paper introduces the RBQ1D model, a generalized resonance line broadened quasilinear approach, to simulate fast ion distribution relaxation caused by Alfvénic eigenmodes in fusion plasmas, validated with DIII-D experiments.

Contribution

The paper develops a new 1D RBQ1D code incorporating resonance line broadening for fast ion diffusion, extending previous QL models to include radial diffusion effects.

Findings

RBQ1D successfully models fast ion relaxation in DIII-D.

The model captures stiff transport properties of beam ions.

Validation shows good agreement with experimental observations.

Abstract

The burning plasma performance is limited by the confinement of the superalfvenic fusion products such as alpha particles and the auxiliary heating ions capable of exciting the Alfv\'enic eigenmodes (AEs). In this work the effect of AEs on fast ions is formulated within the quasi-linear (QL) theory generalized for this problem recently. The generalization involves the resonance line broadened interaction of energetic particles (EP) with AEs supplemented by the diffusion coefficients depending on EP position in the velocity space. A new resonance broadened QL code (or RBQ1D) based on this formulation allowing for EP diffusion in radial direction is built and presented in details. We reduce the wave particle interaction (WPI) dynamics to 1D case when the particle kinetic energy is nearly constant. The diffusion equation for EP distribution evolution is then one dimensional and is solved simultaneously for all particles with the equation for the evolution of the wave angular momentum. The evolution of fast ion constants of motion is governed by the QL diffusion equations which are adapted to find the fast ion distribution function. We make initial applications of the RBQ1D to DIII-D plasma with elevated q-profile where the beam ions show stiff transport properties. AE driven fast ion profile relaxation is studied for validations of the QL approach in realistic conditions of beam ion driven instabilities in DIII-D.