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

Abstract

A wide variety of fast ion driven instabilities are excited during neutral beam injection ͑NBI͒ in the National Spherical Torus Experiment ͑NSTX͒ ͓Nucl. Fusion 40, 557 ͑2000͔͒ due to the large ratio of fast ion velocity to Alfvén velocity, V fast / V Alfvén , and high fast ion beta. The ratio V fast / V Alfvén in ITER ͓Nucl. Fusion 39, 2137 ͑1999͔͒ and NSTX is comparable. The modes can be divided into three categories: chirping energetic particle modes ͑EPM͒ in the frequency range 0 to 120 kHz, the toroidal Alfvén eigenmodes ͑TAE͒ with a frequency range of 50 kHz to 200 kHz, and the compressional and global Alfvén eigenmodes ͑CAE and GAE, respectively͒ between 300 kHz 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 Ͻ n Ͻ 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 q͑0͒ Ͼ 1 and can have a toroidal mode number n Ͼ 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.

Intermittency in the scrape-off layer of the National Spherical Torus Experiment during H-mode confinement

“…In the case of the NBI heated, early sequence data the skewness near the separatrix remains low (near Gaussian skewness, i.e., ~0) while the fluctuation level throughout the profile and skewness in the SOL are high. This is may be due to n=3 TAE MHD modes [19] affecting the edge and sporadically triggering blobs that propagate into an otherwise low intensity D  profile creating a large RMS and skewness region in the SOL.…”

Radial Transport Characteristics of Fast Ions Due to Energetic-Particle Modes inside the Last Closed-Flux Surface in the Compact Helical System

“…On the other hand, energetic-particle modes (EPMs) are determined by characteristics of the fast-ion motions such as transit, bounce and precession, and can be destabilized with the frequencies inside the shear Alfvén continuum when the energetic-ion drive is strong enough to overcome the continuum damping [9]. Actually, EPMs are experimentally observed on the condition that the fast-ion pressure is fairly high [3]. EPMs often show nonlinear phenomena such as periodic burst and frequency chirping, which are considered to be related to fast-ion behavior, for example, change of fast-ion profile.…”

“…EPMs often show nonlinear phenomena such as periodic burst and frequency chirping, which are considered to be related to fast-ion behavior, for example, change of fast-ion profile. The investigation of EPM properties and interaction with fast ions is still in progress [3,10,11].…”

“…EPMs often show nonlinear phenomena such as periodic burst and frequency chirping, which are considered to be related to fast-ion behavior, for example, change of fast-ion profile. The investigation of EPM properties and interaction with fast ions is still in progress [3,10,11].Recently, a directional Langmuir probe (DLP) method was applied to plasmas heated by a neutral beam (NB) for fast-ion measurement, and fast-ion behavior inside the last closed-flux surface (LCFS) can be measured with high time and spatial resolutions [12]. In this Letter, we focus on the experimental studies on the fast-ion-behavior interacting with bursting EPMs.…”

A tangentially viewing fast ion D-alpha diagnostic for NSTX

“…6,7 Due to the quasivertical view, the present diagnostic is sensitive to fast ions with large velocity in the direction perpendicular to magnetic field B; hence, it is sensitive to the p ϳ 0 region of phase space. On the other hand, NSTX fast ions originate from ionization of NB neutrals that are injected tangentially in the cocurrent direction.…”