Progress in the development of integrated advanced ST plasma scenarios in NSTX (Ono et al 2000 Nucl. Fusion 40 557) is reported. Recent high-performance plasmas in NSTX following lithium coating of the plasma facing surfaces have achieved higher elongation and lower internal inductance than previously. Analysis of the thermal confinement in these lithiumized discharges shows a stronger plasma current and weaker toroidal field dependence than in previous ST confinement scaling studies; the ITER-98(y, 2) scaling expression describes these scenarios reasonably well. Analysis during periods free of MHD activity has shown that the reconstructed current profile can be understood as the sum of pressure driven, inductive and neutral beam driven currents, without requiring any anomalous fast-ion transport. Non-inductive fractions of 65–70%, and βP > 2, have been achieved at lower plasma current. Some of these low-inductance discharges have a significantly reduced no-wall βN limit, and often have βN at or near the with-wall limit. Coupled m/n = 1/1 + 2/1 kink/tearing modes can limit the sustained β values when rapidly growing ideal modes are avoided. A βN controller has been commissioned and utilized in sustaining high-performance plasmas. ‘Snowflake’ divertors compatible with high-performance plasmas have been developed. Scenarios with significantly larger aspect ratios have also been developed, in support of next-step ST devices. Overall, these NSTX plasmas have many characteristics required for next-step ST devices.

Kinetic theory and experimental observations of a special class of energetic particle driven instabilities called here beta-induced Alfvén-acoustic eigenmodes ͑BAAEs͒ are reported confirming, previous results ͓N. N. Gorelenkov et al., Plasma Phys. Controlled Fusion 49, B371 ͑2007͔͒. The kinetic theory is based on the ballooning dispersion relation where the drift frequency effects are retained. BAAE gaps are recovered in kinetic theory. It is shown that the observed certain low-frequency instabilities on DIII-D ͓J. L. Luxon, Nucl. Fusion 42, 614 ͑2002͔͒ and National Spherical Torus Experiment ͓M. Ono, S. M. Kaye, Y.-K. M. Peng et al., Nucl. Fusion 40, 557 ͑2000͔͒ are consistent with their identification as BAAEs. BAAEs deteriorate the fast ion confinement in DIII-D and can have a similar effect in next-step fusion plasmas, especially if excited together with multiple global toroidicity-induced shear Alfvén eigenmode instabilities.BAAEs can also be used to diagnose safety factor profiles, a technique known as magnetohydrodynamic spectroscopy.

We report the observation of a correlation between shear Alfvén eigenmode activity and electron transport in plasma regimes where the electron temperature gradient is flat, and thus the drive for temperature gradient microinstabilities is absent. Plasmas having rapid central electron transport show intense, broadband global Alfvén eigenmode (GAE) activity in the 0.5-1.1 MHz range, while plasmas with low transport are essentially GAE-free. The first theoretical assessment of a GAE-electron transport connection indicates that overlapping modes can resonantly couple to the bulk thermal electrons and induce their stochastic diffusion.

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.