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Experiments in the DIII-D tokamak show that many overlapping small-amplitude Alfvén eigenmodes (AEs) cause fast-ion transport to sharply increase above a critical threshold in beam power, leading to fast-ion density profile resilience and reduced fusion performance. The threshold is above the AE linear stability limit and varies between diagnostics that are sensitive to different parts of fast-ion phase-space. Comparison with theoretical analysis using the nova and orbit codes shows that, for the neutral particle diagnostic, the threshold corresponds to the onset of stochastic particle orbits due to wave-particle resonances with AEs in the measured region of phase space. The bulk fast-ion distribution and instability behavior was manipulated through variations in beam deposition geometry, and no significant differences in the onset threshold outside of measurement uncertainties were found, in agreement with the theoretical stochastic threshold analysis. Simulations using the 'kick model' produce beam ion density gradients consistent with the empirically measured radial critical gradient and highlight the importance of including the energy and pitch dependence of the fast-ion distribution function in critical gradient models. The addition of electron cyclotron heating changes the types of AEs present in the experiment, comparatively increasing the measured fast-ion density and radial gradient. These studies provide the basis for understanding how to avoid AE transport that can undesirably redistribute current and cause fast-ion losses, and the measurements are being used to validate AE-induced transport models that use the critical gradient paradigm, giving greater confidence when applied to ITER.

Experiments on the DIII-D tokamak have identified how multiple simultaneous Alfvén eigenmodes (AEs) lead to overlapping wave-particle resonances and stochastic fast-ion transport in fusion grade plasmas [C. S. Collins et al., Phys. Rev. Lett. 116, 095001 (2016)]. The behavior results in a sudden increase in fast-ion transport at a threshold that is well above the linear stability threshold for Alfvén instability. A novel beam modulation technique [W. W. Heidbrink et al., Nucl. Fusion 56, 112011 (2016)], in conjunction with an array of fast-ion diagnostics, probes the transport by measuring the fast-ion flux in different phase-space volumes. Well above the threshold, simulations that utilize the measured mode amplitudes and structures predict a hollow fast-ion profile that resembles the profile measured by fast-ion Dα spectroscopy; the modelling also successfully reproduces the temporal response of neutral-particle signals to beam modulation. The use of different modulated sources probes the details of phase-space transport by populating different regions in phase space and by altering the amplitude of the AEs. Both effects modulate the phase-space flows.

We show that the degradation of fast-ion confinement in steady-state DIII-D discharges is quantitatively consistent with predictions based on the effects of multiple unstable Alfvén eigenmodes on beam-ion transport. Simulation and experiment show that increasing the radius where the magnetic safety factor has its minimum is effective in minimizing beam-ion transport. This is favorable for achieving high performance steady-state operation in DIII-D and future reactors. A comparison between the experiments and a critical gradient model, in which only equilibrium profiles were used to predict the most unstable modes, show that in a number of cases this model reproduces the measured neutron rate well.

A general criterion is proposed and found to successfully predict the emergence of chirping oscillations of unstable Alfvénic eigenmodes in tokamak plasma experiments. The model includes realistic eigenfunction structure, detailed phase-space dependences of the instability drive, stochastic scattering and the Coulomb drag. The stochastic scattering combines the effects of collisional pitch angle scattering and micro-turbulence spatial diffusion. The latter mechanism is essential to accurately identify the transition between the fixed-frequency mode behavior and rapid chirping in tokamaks and to resolve the disparity with respect to chirping observation in spherical and conventional tokamaks.

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.

Analysis of neutron and fast-ion Dα data from the DIII-D tokamak shows that Alfvén eigenmode activity degrades fast-ion confinement in many high βN, high qmin, steady-state scenario discharges. (βN is the normalized plasma pressure and qmin is the minimum value of the plasma safety factor.) Fast-ion diagnostics that are sensitive to the co-passing population exhibit the largest reduction relative to classical predictions. The increased fast-ion transport in discharges with strong AE activity accounts for the previously observed reduction in global confinement with increasing qmin; however, not all high qmin discharges show appreciable degradation. Two relatively simple empirical quantities provide convenient monitors of these effects: (1) an 'AE amplitude' signal based on interferometer measurements and (2) the ratio of the neutron rate to a zero-dimensional classical prediction. © 2014 IOP Publishing Ltd.

The application of static magnetic field perturbations to a tokamak plasma is observed to alter the dynamics of high-frequency bursting Alfvén modes that are driven unstable by energetic ions. In response to perturbations with an amplitude of δB/B∼0.01 at the plasma boundary, the mode amplitude is reduced, the bursting frequency is increased, and the frequency chirp is smaller. For modes of weaker bursting character, the magnetic perturbation induces a temporary transition to a saturated continuous mode. Calculations of the perturbed distribution function indicate that the 3D perturbation affects the orbits of fast ions that resonate with the bursting modes. The experimental evidence represents an important demonstration of the possibility of controlling fast-ion instabilities through "phase-space engineering" of the fast-ion distribution function, by means of externally applied perturbation fields.

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.

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.

A second fast ion D-alpha (FIDA) installation is planned at NSTX to complement the present perpendicular viewing FIDA diagnostics. Following the present diagnostic scheme, the new diagnostic will consist of two instruments: a spectroscopic diagnostic that measures fast ion spectra and profiles at 16 radial points with 5-10 ms resolution and a system that uses a band pass filter and photomultiplier to measure changes in FIDA light with 50 kHz sampling rate. The new pair of FIDA instruments will view the heating beams tangentially. The viewing geometry minimizes spectral contamination by beam emission or edge sources of background emission. The improved velocity-space resolution will provide detailed information about neutral-beam current drive and about fast ion acceleration and transport by injected radio frequency waves and plasma instabilities.

Combined neutral beam injection and high-harmonic fast-wave (HHFW) heating accelerate deuterium fast ions in the National Spherical Torus Experiment (NSTX). With 1.1 MW of HHFW power, the neutron emission rate is about three times larger than in the comparison discharge without HHFW heating. Acceleration of fast ions above the beam injection energy is evident on an E||B type neutral particle analyzer (NPA), a 4-chord solid state neutral particle analyzer (SSNPA) array and a 16-channel fast-ion D-alpha (FIDA) diagnostic. The accelerated fast ions observed by the NPA and SSNPA diagnostics mainly come from passive charge exchange reactions at the edge due to the NPA/SSNPA localization in phase space. The spatial profile of accelerated fast ions that is measured by the FIDA diagnostic is much broader than in conventional tokamaks because of the multiple resonance layers and large orbits in NSTX. The fast-ion distribution function calculated by the CQL3D Fokker-Planck code differs from the measured spatial profile, presumably because the current version of CQL3D uses a zero-banana-width model. In addition, compressional Alfven eigenmode activity is stronger during the HHFW heating and it may affect the fast-ion spatial profile. © 2010 IOP Publishing Ltd.

Combined neutral beam injection and fast wave heating at cyclotron harmonics accelerate deuterium fast ions in the National Spherical Torus Experiment (NSTX) and in the DIII-D tokamak. Acceleration above the injected energy is evident in fast-ion D-alpha (FIDA) and volume-average neutron data. The FIDA diagnostic measures spatial profiles of the accelerated fast ions. In DIII-D, the acceleration is at a 4th or 5th cyclotron harmonic; the maximum enhancement in the high-energy FIDA signal is 8-10 cm beyond the resonance layer. In NSTX, acceleration is observed at five harmonics (7-11) simultaneously; overall, the profile of accelerated fast ions is much broader than in DIII-D. The energy distribution predicted by the CQL3D Fokker-Planck code agrees fairly well with measurements in DIII-D. However, the predicted profiles differ from experiment, presumably because the current version of CQL3D uses a zero-banana-width model. © 2009 American Institute of Physics.

A new diagnostic, aimed at energy-resolved measurements of the spatial and temporal dynamics of fast ions in NSTX plasmas, is described. It is based on active charge-exchange recombination spectroscopy. The fast-ion signal is inferred from light emitted in the wavelength range of the D(alpha) line by fast ions recombining with an injected neutral beam. Two complementary systems are operational. The first system, based on a spectrometer coupled to a charge coupled device detector, has 16 channels with space, time, and energy resolution of 5 cm, 10 ms, and 10 keV, respectively. The second system monitors the energy-integrated fast-ion signal on time scales of approximately 20 micros at three different radii. Signals are measured by a multianode photomultiplier tube. For both systems, each channel includes two paired views, intercepting and missing the neutral beam for a direct subtraction of the background signal not associated with fast ions. Examples of signals from the 2008 NSTX run are presented.

A specific experimental apparatus consisting of an ion source and a detector for the investigation of the interaction between suprathermal ions and drift-wave turbulence is developed on the toroidal plasma experiment. Due to the low plasma temperature (∼5 eV), a spatially localized, small-size ion source (∼4 cm) mounted inside the vacuum vessel with relatively low ion energy (∼100 eV-1 keV) can be used. The source consists of an aluminosilicate Li-6 ion emitter (6 mm diameter, 10-30 μA current) installed on a two-dimensional (2D) poloidally moving system. The location, energy, and current density profile of the ion beam will be measured using a 2D movable gridded energy analyzer. © 2006 American Institute of Physics.