Nonlinear alfv\'enic fast particle transport and losses

TL;DR

This study investigates how multiple Alfvénic modes driven by fast particles interact through resonance mechanisms, affecting particle transport and losses in fusion plasmas, using simulations and experimental comparisons.

Contribution

It provides new insights into double-resonant mode coupling effects and characterizes fast particle losses, including incoherent and prompt losses, with detailed phase space analysis.

Findings

Double-resonance can enhance growth rates and mode amplitudes.

Small radial mode distances can cause nonlinear stabilization.

Losses are consistent with experimental data and include incoherent, prompt, and stochastic types.

Abstract

Magnetohydrodynamic instabilities like Toroidal Alfv\'en Eigenmodes or core-localized modes such as Beta Induced Alfv\'en Eigenmodes and Reversed Shear Alfv\'en Eigenmodes driven by fast particles can lead to significant redistribution and losses in fusion devices. This is observed in many ASDEX Upgrade discharges. The present work aims to understand the underlying resonance mechanisms, especially in the presence of multiple modes with different frequencies. Resonant mode coupling mechanisms are investigated using the drift kinetic HAGIS code [Pinches 1998]. Simulations were performed for different plasma equilibria, in particular for different q profiles, employing the availability of improved experimental data. A study was carried out, investigating double-resonant mode coupling with respect to various overlapping scenarios. It was found that, depending on the radial mode distance, double-resonance is able to enhance growth rates as well as mode amplitudes significantly. Small radial mode distances, however can also lead to strong nonlinear mode stabilization of a linear dominant mode. With the extended version of HAGIS, losses were simulated and directly compared with experimental loss measurements. The losses' phase space distribution as well as their ejection signal is consistent with experimental data. Furthermore, it allowed to characterize them as prompt, resonant or stochastic. It was found that especially in multiple mode scenarios (with different mode frequencies), abundant incoherent losses occur in the lower energy range, due to a broad phase-space stochastization. The incoherent higher energetic losses are "prompt", i.e. their initial energy is too large for confined orbits.