Collective fast ion instability-induced losses in National Spherical Tokamak Experiment

E D Fredrickson,J E Menard,N A Crocker,S A Sabbagh,S S Medley,A L Roquemore,D Stutman,W Peebles,K Tritz,W W Heidbrink,N N Gorelenkov,F M Levinton,S Kubota,R E Bell,G Y Fu,H Park,K C Lee,D S Darrow,B P Leblanc

doi:10.1063/1.2178788

Abstract

A wide variety of fast ion driven instabilities are excited during neutral beam injection (NBI) in the National Spherical Torus Experiment (NSTX) [Nucl. Fusion 40, 557 (2000)] due to the large ratio of fast ion velocity to Alfvén velocity, Vfast∕VAlfvén, and high fast ion beta. The ratio Vfast∕VAlfvén in ITER [Nucl. Fusion 39, 2137 (1999)] and NSTX is comparable. The modes can be divided into three categories: chirping energetic particle modes (EPM) in the frequency range 0 to 120kHz, the toroidal Alfvén eigenmodes (TAE) with a frequency range of 50kHz to 200kHz, and the compressional and global Alfvén eigenmodes (CAE and GAE, respectively) between 300kHz and the ion cyclotron frequency. Fast ion driven modes are of particular interest because of their potential to cause substantial fast ion losses. In all regimes of NBI heated operation we see transient neutron rate drops, correlated with bursts of TAE or fishbone-like EPMs. The fast ion loss events are predominantly correlated with the EPMs, although losses are also seen with bursts of multiple, large amplitude TAE. The latter is of particular significance for ITER; the transport of fast ions from the expected resonance overlap in phase space of a “sea” of large amplitude TAE is the kind of physics expected in ITER. The internal structure and amplitude of the TAE and EPMs has been measured with quadrature reflectometry and soft x-ray cameras. The TAE bursts have internal amplitudes of ñ∕n=1% and toroidal mode numbers 2&lt;n&lt;7. The EPMs are core localized, kink-like modes similar to the fishbones in conventional aspect ratio tokamaks. Unlike the fishbones, the EPMs can be present with q(0)&gt;1 and can have a toroidal mode number n&gt;1. The range of the frequency chirp can be quite large and the resonance can be through a fishbone-like precessional drift resonance, or through a bounce resonance.

Highlights

Thermonuclear plasmas are heated by the products of the D-T fusion reaction, 3.5 MeV He++, or ␣’s, which are much more energetic than the particles in the thermal plasma
Toroidal Alfvén eigenmodes are seen in most operational regimes on NSTX, including shots with ␤tor up to 35%
A broad spectrum of fast ion driven instabilities are excited in NSTX by the super-Alfvénic neutral beam ions

Summary

INTRODUCTION

Thermonuclear plasmas are heated by the products of the D-T fusion reaction, 3.5 MeV He++, or ␣’s, which are much more energetic than the particles in the thermal plasma. In NSTX beam heated plasmas we see transient neutron rate drops, correlated with bursts of fast ion driven instabilities, including modes identified as TAE and fishbone-like EPM.. Together with TAE and EPM, are seen in most regimes of NSTX NBI heated operation, up to the highest ␤, highest densities, and at the highest plasma current. 5. ͑Color Spectrogram of magnetic fluctuations for three shots showing “low” frequency fast ion driven instabilities, including toroidal Alfvén eigenmodes and various forms of energetic particlechirpingmodes. As seen in this figure, the TAE typically manifest as a sequence of short bursts, which often exhibit weak downward frequency chirps.

COMPRESSIONAL AND GLOBAL ALFVÉN MODES

SCALING OF LOSSES

Findings

CONCLUSIONS