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Observations on the National Spherical Torus Experiment (NSTX) indicate that externally applied non-axisymmetric magnetic perturbations (MP) can reduce the amplitude of toroidal Alfvén eigenmodes (TAE) and global Alfvén eigenmodes (GAE) in response to pulsed n = 3 non-resonant fields. From full-orbit following Monte Carlo simulations with the one- and two-fluid resistive MHD plasma response to the magnetic perturbation included, it was found that in response to MP pulses the fast-ion losses increased and the fast-ion drive for the GAEs was reduced. The MP did not affect the fast-ion drive for the TAEs significantly but the Alfvén continuum at the plasma edge was found to be altered due to the toroidal symmetry breaking which leads to coupling of different toroidal harmonics. The TAE gap was reduced at the edge creating enhanced continuum damping of the global TAEs, which is consistent with the observations. The results suggest that optimized non-axisymmetric MP might be exploited to control and mitigate Alfvén instabilities by tailoring the fast-ion distribution function and/or continuum structure.

Chirping toroidicity-induced Alfvén eigenmodes (TAEs) are destabilized during neutral beam injection on the National Spherical Torus Experiment (NSTX (Ono M. et al 2000 Nucl. Fusion 40 557)) by super-Alfvénic ions with velocities up to five times larger than the Alfvén velocity. TAEs exhibit repeated bursts in amplitude and down-chirps in frequency. Larger bursts, so-called TAE avalanches, are eventually observed and correlate with a loss of fast ions up to 30% over ∼1ms. Frequency, amplitude and radial structure of TAEs are characterized via magnetic pickup coils and a multi-channel reflectometer system. The modes have a broad radial structure, which appears to be unaffected by the large frequency and amplitude variations. However, the large mode amplitude does impact the modes' dynamics by favouring the coupling among different modes. In addition, the coupling involves kink-like modes and can therefore degrade the thermal plasma confinement. In spite of the non-linear regime characterizing the TAE dynamics, the measured properties are found to be in reasonable agreement with solutions from the ideal MHD code NOVA. © 2012 IAEA, Vienna.

In the last two experimental campaigns, the low aspect ratio NSTX has explored physics issues critical to both toroidal confinement physics and ITER. Experiments have made extensive use of lithium coatings for wall conditioning, correction of non-axisymmetric field errors and control of n = 1 resistive wall modes (RWMs) to produce high-performance neutral-beam heated discharges extending to 1.7 s in duration with non-inductive current fractions up to 0.7. The RWM control coils have been used to trigger repetitive ELMs with high reliability, and they have also contributed to an improved understanding of both neoclassical tearing mode and RWM stabilization physics, including the interplay between rotation and kinetic effects on stability. High harmonic fast wave (HHFW) heating has produced plasmas with central electron temperatures exceeding 6 keV. The HHFW heating was used to show that there was a 20-40% higher power threshold for the L-H transition for helium than for deuterium plasmas. A new diagnostic showed a depletion of the fast-ion density profile over a broad spatial region as a result of toroidicity-induced Alfvén eigenmodes (TAEs) and energetic-particle modes (EPMs) bursts. In addition, it was observed that other modes (e.g. global Alfvén eigenmodes) can trigger TAE and EPM bursts, suggesting that fast ions are redistributed by high-frequency AEs. The momentum pinch velocity determined by a perturbative technique decreased as the collisionality was reduced, although the pinch to diffusion ratio, Vpinch/χ, remained approximately constant. The mechanisms of deuterium retention by graphite and lithium-coated graphite plasma-facing components have been investigated. To reduce divertor heat flux, a novel divertor configuration, the 'snowflake' divertor, was tested in NSTX and many beneficial aspects were found. A reduction in the required central solenoid flux has been realized in NSTX when discharges initiated by coaxial helicity injection were ramped in current using induction. The resulting plasmas have characteristics needed to meet the objectives of the non-inductive start-up and ramp-up program of NSTX. © 2011 IAEA, Vienna.

The National Spherical Torus Experiment (NSTX, (Ono et al 2000 Nucl. Fusion 40 557)) routinely operates with neutral beam injection as the primary system for heating and current drive. The resulting fast ion population is super-Alfvénic, with velocities 1 < vfast/v Alfven < 5. This provides a strong drive for toroidicity-induced Alfvén eigenmodes (TAEs). As the discharge evolves, the fast ion population builds up and TAEs exhibit increasing bursts in amplitude and down-chirps in frequency, which eventually lead to a so-called TAE avalanche. Avalanches cause large (≲30%) fast ion losses over ∼1 ms, as inferred from the neutron rate. The increased fast ion losses correlate with a stronger activity in the TAE band. In addition, it is shown that a n = 1 mode with frequency well below the TAE gap appears in the Fourier spectrum of magnetic fluctuations as a result of non-linear mode coupling between TAEs during avalanche events. The non-linear coupling between modes, which leads to enhanced fast ion transport during avalanches, is investigated. © 2011 IAEA, Vienna.

The National Spherical Torus Experiment (NSTX, (Ono et al 2000 Nucl. Fusion 40 557)) routinely operates with neutral beam injection as the primary system for heating and current drive. The resulting fast ion population is super-Alfvénic, with velocities 1 < vfast/v Alfven < 5. This provides a strong drive for toroidicity-induced Alfvén eigenmodes (TAEs). As the discharge evolves, the fast ion population builds up and TAEs exhibit increasing bursts in amplitude and down-chirps in frequency, which eventually lead to a so-called TAE avalanche. Avalanches cause large (≲30%) fast ion losses over ∼1 ms, as inferred from the neutron rate. The increased fast ion losses correlate with a stronger activity in the TAE band. In addition, it is shown that a n = 1 mode with frequency well below the TAE gap appears in the Fourier spectrum of magnetic fluctuations as a result of non-linear mode coupling between TAEs during avalanche events. The non-linear coupling between modes, which leads to enhanced fast ion transport during avalanches, is investigated. © 2011 IAEA, Vienna.

The effects of a sheared toroidal rotation on the dynamics of bursting toroidicity-induced Alfv́n eigenmodes are investigated in neutral beam heated plasmas on the National Spherical Torus Experiment (NSTX) [M. Ono, Nucl. Fusion 40, 557 (2000)]. The modes have a global character, extending over most of the minor radius. A toroidal rotation shear layer is measured at the location of maximum drive for the modes. Contrary to results from other devices, no clear evidence of decorrelation of the modes by the sheared rotation is found. Instead, experiments with simultaneous neutral beam and radio-frequency auxiliary heating show a strong correlation between the dynamics of the modes and the instability drive. It is argued that kinetic effects involving changes in the mode drive and damping mechanisms other than rotation shear, such as continuum damping, are mostly responsible for the bursting dynamics of the modes on NSTX. © 2010 American Institute of Physics.

The effects of a sheared toroidal rotation on the dynamics of bursting toroidicity-induced Alfv́n eigenmodes are investigated in neutral beam heated plasmas on the National Spherical Torus Experiment (NSTX) [M. Ono, Nucl. Fusion 40, 557 (2000)]. The modes have a global character, extending over most of the minor radius. A toroidal rotation shear layer is measured at the location of maximum drive for the modes. Contrary to results from other devices, no clear evidence of decorrelation of the modes by the sheared rotation is found. Instead, experiments with simultaneous neutral beam and radio-frequency auxiliary heating show a strong correlation between the dynamics of the modes and the instability drive. It is argued that kinetic effects involving changes in the mode drive and damping mechanisms other than rotation shear, such as continuum damping, are mostly responsible for the bursting dynamics of the modes on NSTX. © 2010 American Institute of Physics.

Kinetic theory and experimental observations of a special class of energetic particle driven instabilities called here beta-induced Alfv́n-acoustic eigenmodes (BAAEs) are reported confirming, previous results [N. N. Gorelenkov, Plasma Phys. Controlled Fusion 49, B371 (2007)]. 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 (2002)] and National Spherical Torus Experiment [M. Ono, S. M. Kaye, Y.-K. M. Peng, Nucl. Fusion 40, 557 (2000)] 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 Alfv́n eigenmode instabilities. BAAEs can also be used to diagnose safety factor profiles, a technique known as magnetohydrodynamic spectroscopy. © 2009 American Institute of Physics.

Alfv́n cascade (AC) modes are observed in the National Spherical Torus Experiment [M. Ono, Nucl. Fusion 40, 557 (2000)] reversed shear plasmas over a wide range (up to ∼25% on axis, or ∼11% at minimum q) of Β (ratio of kinetic pressure to magnetic pressure). At low Β, the AC mode spectrum shows characteristics similar to conventional tokamaks. At higher Β, distinct Β and ∇Β effects are observed in the spectrum, including a significant reduction in the relative size of the frequency sweep and a toroidal mode number dependence in the minimum mode frequency. AC mode structure is obtained using reflectometry. Fast-ion loss associated with AC mode activity is observed. AC mode polarization at the plasma edge is consistent with expectation. Magnetohydrodynamic (MHD) spectroscopy is shown to be usable to determine qmin at both low Β and high Β. Observed AC mode structure and frequency are found to be consistent with calculations for the same plasma conditions and geometry using the linear, ideal MHD hybrid kinetic code NOVA-K [C. Z. Cheng, Phys. Rep. 211, 1 (1992)]. © 2008 American Institute of Physics.

Alfvén cascade modes have been found in low density, low β plasmas on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. An extension of the theory of cascade modes which includes the coupling to geodesic acoustic modes [Breizman et al., Phys. Plasmas 12, 112506 (2005)] is shown to imply their absence for typical spherical tokamak ratios of electron thermal to magnetic energy, β. A scan in electron β confirmed a threshold for suppression of cascade modes in good agreement with theoretical predictions. © 2007 American Institute of Physics.

The National Spherical Torus Experiment (NSTX) has made considerable progress in advancing the scientific understanding of high performance long-pulse plasmas needed for future spherical torus (ST) devices and ITER. Plasma durations up to 1.6 s (five current redistribution times) have been achieved at plasma currents of 0.7 MA with non-inductive current fractions above 65% while simultaneously achieving βT and βN values of 17% and 5.7 (%m T MA-1), respectively. A newly available motional Stark effect diagnostic has enabled validation of current-drive sources and improved the understanding of NSTX 'hybrid'-like scenarios. In MHD research, ex-vessel radial field coils have been utilized to infer and correct intrinsic EFs, provide rotation control and actively stabilize the n ≤ 1 resistive wall mode at ITER-relevant low plasma rotation values. In transport and turbulence research, the low aspect ratio and a wide range of achievable β in the NSTX provide unique data for confinement scaling studies, and a new microwave scattering diagnostic is being used to investigate turbulent density fluctuations with wavenumbers extending from ion to electron gyro-scales. In energetic particle research, cyclic neutron rate drops have been associated with the destabilization of multiple large toroidal Alfven eigenmodes (TAEs) analogous to the 'sea-of-TAE' modes predicted for ITER, and three-wave coupling processes have been observed for the first time. In boundary physics research, advanced shape control has enabled studies of the role of magnetic balance in H-mode access and edge localized mode stability. Peak divertor heat flux has been reduced by a factor of 5 using an H-mode-compatible radiative divertor, and lithium conditioning has demonstrated particle pumping and results in improved thermal confinement. Finally, non-solenoidal plasma start-up experiments have achieved plasma currents of 160 kA on closed magnetic flux surfaces utilizing coaxial helicity injection. © 2007 IAEA.

The spectrum of compressional Alfvén eigenmodes (CAE) is analysed and shown to be discrete in tokamaks with low aspect ratio, such as the National Spherical Torus Experiment (NSTX), as well as in conventional tokamaks, such as DIII-D. The study is focused on recent similarity experiments on NSTX and DIII-D in which sub-cyclotron frequency instabilities of CAEs were observed at similar plasma conditions (W.W. Heidbrink et al 2006 Nucl. Fusion 46 324). The global ideal MHD code NOVA recovers the main properties of these modes predicted by theory and observed in both devices. The discrete spectrum of CAEs is characterized by three quantum mode numbers for each eigenmode, (M, S and n), where M, S and n are poloidal, radial and toroidal mode numbers, respectively. The expected mode frequency splitting corresponding to each of these mode numbers seems to be observed in experiments and is consistent with our numerical analysis. The polarization of the observed magnetic field oscillations in NSTX was measured and is also consistent with the numerical analysis, which helps to identify them as CAE activity. CAE mode structure was obtained and shown to be localized in both radial and poloidal directions with typical radial localization toward the plasma edge and poloidal localization at the low field side of the plasma cross section. © 2006 IAEA, Vienna.

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 Alfván 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 120 kHz, the toroidal Alfván eigenmodes (TAE) with a frequency range of 50 kHz to 200 kHz, and the compressional and global Alfván eigenmodes (CAE and GAE, respectively) 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 21 and can have a toroidal mode number n>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. © 2006 American Institute of Physics.