Study of the likelihood of Alfv\'enic mode bifurcation in NSTX and predictions for ITER baseline scenarios

TL;DR

This study investigates Alfvénic mode bifurcation in NSTX, applying a predictive criterion for fast ion redistribution, and extends findings to ITER scenarios, highlighting the borderline nature of most modes between steady and chirping phases.

Contribution

It combines experimental observations with gyrokinetic simulations and predictive modeling to understand Alfvénic mode bifurcation and spectral behavior in NSTX and ITER.

Findings

Anomalous fast ion transport likely mediates bifurcation.

Most modes in ITER scenarios are borderline between steady and chirping phases.

Gyrokinetic simulations reveal turbulence effects on mode behavior.

Abstract

Rare Alfv\'enic wave transitions between fixed-frequency and chirping phases are identified in NSTX, where Alfv\'enic waves are normally observed to exhibit either chirping or avalanching responses. For those transitions, we apply a criterion [Duarte et al, Nucl. Fusion 57, 054001 (2017)] to predict the nature of fast ion redistribution in tokamaks to be in the convective or diffusive nonlinear regimes. For NSTX discharges in which the transition is not accompanied by changes in the beam deposited power or modifications in the injected radiofrequency power, it has been found that the anomalous fast ion transport is a likely mediator of the bifurcation between the fixed-frequency mode behavior and rapid chirping. For a quantitative assessment, global gyrokinetic simulations of the effects of electrostatic ion temperature gradient turbulence and trapped electron mode turbulence on chirping were pursued using the GTS code. The investigation is extended by means of predictive studies of the probable spectral behavior of Alfv\'enic eigenmodes for baseline ITER cases consisting of elmy, advanced and hybrid scenarios. It has been observed that most modes are found to be borderline between the steady and the chirping phases.