Suppression of Alfvén Modes on the National Spherical Torus Experiment Upgrade with Outboard Beam Injection

Abstract

Abstract. In this paper we present data from experiments on NSTX-U where it is shown for the first time that small amounts of high pitch-angle beam ions can strongly suppress the counter-propagating Global Alfvén Eigenmodes (GAE). GAE have been implicated in the redistribution of fast ions and modification of the electron power balance in previous experiments on NSTX. The ability to predict the stability of Alfvén modes, and developing methods to control them, is important for fusion reactor like the International Tokamak Experimental Reactor (ITER) which are heated by a large population of non-thermal, super-Alfvénic ions consisting of fusion generated alphas and beam ions injected for current profile control. We present a qualitative interpretation of these observations using an analytic model of the Doppler-shifted ion-cyclotron resonance drive responsible for GAE instability which has an important dependence on k ∞ ρ L . A quantitative analysis of this data with the HYM stability code predicts both the frequencies and instability of the GAE prior to, and suppression of the GAE after the injection of high pitch-angle beam ions.

Analytic stability boundaries for compressional and global Alfvén eigenmodes driven by fast ions. I. Interaction via ordinary and anomalous cyclotron resonances

“…This enables systematic analysis and prediction of scenarios like those of the cntr-GAE stabilization observed in NSTX-U. 10,46,48 VIII. ACKNOWLEDGMENTS…”

“…Previous studies related to NBI-driven CAEs/GAEs 55,56 also overlooked this regime by implicitly assuming v ,res v 0 . Consequently, their results were used to interpret experimental observations in NSTX(-U) 3,10,46 and DIII-D 15 in cases where they may not have been valid. In contrast, this new instability regime can more consistently explain the excitation and suppression of cntr-GAEs observed in NSTX-U, 46,48 and also suggests that the properties of high frequency modes previously identified as CAEs in DIII-D 15 would in fact be more consistent with those of GAEs.…”

“…[40][41][42][43][44][45] During early operations of NSTX-U, robust stabilization of GAEs by the addition of a small amount of power in the new off-axis neutral beam source was discovered and subsequently reproduced with numerical modeling and analytic theory. 10,[46][47][48] Further understanding of these processes will be aided by the new stability conditions derived here.…”

Hybrid simulations of sub-cyclotron compressional and global Alfvén Eigenmode stability in spherical tokamaks

“…A detailed simulation study of multi-beam effects on CAE/GAE stability is a compelling next step, especially when considering the very robust stabilization of GAEs found with the off-axis, tangential beam sources on NSTX-U. 9,34,56 While this paper focused on the influence of fast ion parameters, the stability properties also depend on the equilibrium profiles, which could be explored in future work. Development of a complete nonlocal analytic theory including both fast ion drive and relevant background damping sources would be the next step forward for the local theory used here which works well qualitatively but does not give reliable values for the total growth rate.…”

Energetic-particle-modified global Alfvén eigenmodes

“…The basic picture of an energetic beam driving an MHD mode of the thermal plasma without modifying its attributes breaks down in conditions where J beam is comparable to J plasma . Even with the nominal factor of two increase in toroidal field in NSTX-U which will tend to decrease v 0 /v A , these modes may still be unstable due to the increase in beam power 49 , though early operations indicate they can be suppressed with the addition of off-axis injection 50 .…”