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.

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In this paper we report on observations and interpretations of a new class of global MHD eigenmode solutions arising in gaps in the low frequency Alfvénacoustic continuum below the geodesic acoustic mode frequency. These modes have been just reported (Gorelenkov et al 2007 Phys. Lett. 370 70-7) where preliminary comparisons indicate qualitative agreement between theory and experiment. Here we show a more quantitative comparison emphasizing recent NSTX experiments on the observations of the global eigenmodes, referred to as beta-induced Alfvén-acoustic eigenmodes (BAAEs), which exist near the extrema of the Alfvén-acoustic continuum. In accordance to the linear dispersion relations, the frequency of these modes may shift as the safety factor, q, profile relaxes. We show that BAAEs can be responsible for observations in JET plasmas at relatively low beta <2% as well as in NSTX plasmas at relatively high beta >20%. In NSTX plasma observed magnetic activity has the same properties as predicted by theory for the mode structure and the frequency. Found numerically in NOVA simulations BAAEs are used to explain the observed properties of relatively low frequency experimental signals seen in NSTX and JET tokamaks.

The pedestal structure in NSTX is strongly affected by lithium coatings applied to the PFCs. In discharges with lithium, the density pedestal widens, and the electron temperature (Te) gradient increases inside a radius of ψN ∼ 0.95, but is unchanged for ψN > 0.95. The inferred effective electron thermal and particle profiles reflect the profile changes: is slightly increased in the near-separatrix region, and is reduced in the region ψN < 0.95 in the with-lithium case. The profile shows a broadening of the region with low diffusivity with lithium, while the minimum value within the steep-gradient region is comparable in the two cases. The linear microstability properties of the edge plasma without and with lithium have been analysed. At the pedestal top microtearing modes are unstable without lithium. These are stabilized by the stronger density gradient with lithium, becoming TEM-like with growth rates reduced and comparable to E × B shearing rates. In the region ψN > 0.95, both the pre- and with-lithium cases are calculated to be unstable to ETG modes, with higher growth rates with lithium. Both cases are also found to lie near the onset for kinetic ballooning modes, but in the second-stable region where growth rates decrease with increasing pressure gradient.

Fast-ion transport induced by Alfvén eigenmodes ͑AEs͒ is studied in beam-heated plasmas on the National Spherical Torus Experiment ͓Ono et al., Nucl. Fusion 40, 557 ͑2000͔͒ through space, time, and energy resolved measurements of the fast-ion population. Fast-ion losses associated with multiple toroidicity-induced AEs ͑TAEs͒, which interact nonlinearly and terminate in avalanches, are characterized. A depletion of the energy range Ͼ20 keV, leading to sudden drops of up to 40% in the neutron rate over 1 ms, is observed over a broad spatial range. It is shown that avalanches lead to a relaxation of the fast-ion profile, which in turn reduces the drive for the instabilities. The measured radial eigenmode structure and frequency of TAEs are compared with the predictions from a linear magnetohydrodynamics stability code. The partial disagreement suggests that nonlinearities may compromise a direct comparison between experiment and linear theory.