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![(a) Spectrogram of Mirnov coil [black-1, red-2, green-3, blue-4, cyan-5, magenta-6]. (b) Voltage for neutral beam sources “A,” “B,” and “C.” (c) Neutron rate (black) and total injected beam power (red). (d) Plasma current.](profile/Shigeyuki-Kubota/publication/228363493/figure/fig2/AS:317536236261394@1452717946677/a-Spectrogram-of-Mirnov-coil-black-1-red-2-green-3-blue-4-cyan-5-magenta-6-b.png)
(a) Spectrogram of Mirnov coil [black-1, red-2, green-3, blue-4, cyan-5, magenta-6]. (b) Voltage for neutral beam sources “A,” “B,” and “C.” (c) Neutron rate (black) and total injected beam power (red). (d) Plasma current.
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Experiments on the National Spherical Torus Experiment M. Ono et al., Nucl. Fusion 40, 557 2000 found strong bursts of toroidal Alfvén eigenmode TAE activity correlated with abrupt drops in the neutron rate. A fairly complete data set offers the opportunity to benchmark the NOVA C. codes in the low aspect ratio tokamak ST geometry. The internal str...
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Context 1
... target plasma has a "flat-top" plasma current of 800 kA Fig. 1d with a nominal toroidal field of 4.5 kG. Neutral beams were used to heat the plasma during the cur- rent ramp and to provide a measurement of the initial q-profile with the motional Stark effect MSE diagnostic 101 Figs. 1b and 1c. At the current flat-top time 0.175 s, the beam power was reduced below the thresh- old needed to excite ...Context 2
... kV for the 90 kV source used during the current ramp. The fast-ion population built up during the current ramp was then allowed to relax for 65 ms. At 0.24 s an additional source at 65 kV was added to increase the fast-ion beta above the threshold for exciting TAE. TAE on- set shortly before 0.25 s and the first strong TAE burst occurs at 0.258 s Fig. 1a causing a transient drop in the neu- tron rate Fig. 1c, black curve. As will be described below, the q-profile is still strongly evolving, reaching q min 1 around 0.3 s, at which time a fishbonelike EPM triggered a core kink mode, suppressing the TAE activity, possibly through redistribution of core fast ions. At 0.3 s the 90 kV source ...Context 3
... fast-ion population built up during the current ramp was then allowed to relax for 65 ms. At 0.24 s an additional source at 65 kV was added to increase the fast-ion beta above the threshold for exciting TAE. TAE on- set shortly before 0.25 s and the first strong TAE burst occurs at 0.258 s Fig. 1a causing a transient drop in the neu- tron rate Fig. 1c, black curve. As will be described below, the q-profile is still strongly evolving, reaching q min 1 around 0.3 s, at which time a fishbonelike EPM triggered a core kink mode, suppressing the TAE activity, possibly through redistribution of core fast ions. At 0.3 s the 90 kV source was substituted for the two lower voltage sources to ...Context 4
... primary diagnostics for detection of instabilities in the NSTX plasma are arrays of external magnetic pick-up loops, Mirnov coils. External arrays of Mirnov coils are used to measure the frequency spectrum and toroidal wavelengths of the individual TAE instabilities. In Fig. 1a, and Fig. 4a in more detail, are overlaid spectrograms of the even-n and odd-n magnetic fluctuations. The colors indicate the to- roidal mode numbers according to the code: black: n = 1, red: n = 2, green: n = 3, blue: n = 4, cyan: n = 5, and magenta: n = 6. The spectrogram covers the time range from 0.25 to 0.3 s, roughly from the ...Context 5
... neu- tron rates no longer match the measured neutron rates. If TAEs were causing anomalous fast-ion losses not modeled in TRANSP, a higher D-recycling fraction would be needed to match the measured neutron rate. The fast-ion distribution was calculated in TRANSP, and an example of the distribution versus pitch-angle and energy is shown in Fig. 10. This distribution is taken at the radius near the minimum in q and has a peak at full energy with a pitch, V / V 0.75. The width of the peak broadens as the ions slow down due to pitch-angle scattering. Nearer the magnetic axis the peak at full energy in pitch is 0.5. The dashed line in the figure shows where V V Alfvén note that ions ...Context 6
... after about 0.3 s in the plasma shot being discussed here. TRANSP may be used to calculate the current relaxation in the period from 0.17 to 0.3 s, but the q-profile evolution from a similar shot with MSE data during the time of interest was used for the following analysis. A comparison of the q-profiles at 0.32 s from the two shots is shown in Fig. 11 red and black curves and the q-profiles at roughly the beginning of TAE activity blue and at the time of the ava- lanche being studied in detail here ...Context 7
... the equal arc and Boozer coordinate sys- tems. The default 151 point radial grid can be doubled to 301 points for higher radial resolution calculations. It is found that changing the coordinate system results in a different set of eigenmodes, most likely because the numerical interaction of the modes with the Alfvén continuum is changed. From Fig. 12 it should also be noted that, while NSTX is "low" aspect ratio and the TAE "gap" should be correspondingly wide, the region near the axis remains, of course, "high" aspect ratio and the gap can be quite small in this region, depending sensitively on how close q0 is to rational. Thus, even in this low-aspect ratio geometry, many of ...Context 8
... fast-ion transport… Phys. Plasmas 16, 122505 2009 sis at this time will be focused on the dominant, n = 3, TAE, comparing results from calculations done in Boozer coordi- nates, equal arc coordinates, and double-resolution equal arc coordinates. The equal arc coordinate gap structure essen- tially the same in all coordinate systems is shown in Fig. ...Context 9
... frequencies for the eigenmodes shown in Fig. 12 are indicated for each eigenmode in the upper left. The number in parentheses after some frequencies indicates the number of "degenerate" eigenmodes found at that frequency modes with nearly the same frequency and structure, for which only one solution is shown. An example of mode degeneracy, which illustrates the numerical issues ...Context 10
... the upper left. The number in parentheses after some frequencies indicates the number of "degenerate" eigenmodes found at that frequency modes with nearly the same frequency and structure, for which only one solution is shown. An example of mode degeneracy, which illustrates the numerical issues arising from the continuum interaction, is shown in Fig. 13. The mode at 79.4 kHz Fig. 12, second column is accompanied by simi- lar solutions at 79.6 and 80.1 kHz, as shown in an arc across the top Fig. 13. The linear eigenmodes are scaled to match amplitudes in the outer part of the plasma and overlaid at the bottom of the figure. For the modes at 79.6 and 80.1 kHz, the mode amplitude inside ...Context 11
... parentheses after some frequencies indicates the number of "degenerate" eigenmodes found at that frequency modes with nearly the same frequency and structure, for which only one solution is shown. An example of mode degeneracy, which illustrates the numerical issues arising from the continuum interaction, is shown in Fig. 13. The mode at 79.4 kHz Fig. 12, second column is accompanied by simi- lar solutions at 79.6 and 80.1 kHz, as shown in an arc across the top Fig. 13. The linear eigenmodes are scaled to match amplitudes in the outer part of the plasma and overlaid at the bottom of the figure. For the modes at 79.6 and 80.1 kHz, the mode amplitude inside the radius of continuum ...Context 12
... nearly the same frequency and structure, for which only one solution is shown. An example of mode degeneracy, which illustrates the numerical issues arising from the continuum interaction, is shown in Fig. 13. The mode at 79.4 kHz Fig. 12, second column is accompanied by simi- lar solutions at 79.6 and 80.1 kHz, as shown in an arc across the top Fig. 13. The linear eigenmodes are scaled to match amplitudes in the outer part of the plasma and overlaid at the bottom of the figure. For the modes at 79.6 and 80.1 kHz, the mode amplitude inside the radius of continuum interaction is clipped to help illustrate that the outer eigenmode structures are nearly identical for these three modes; ...Context 13
... eigenmode interactions with the continuum not only introduce numerical uncertainties in the calculation of the eigenmode structure, they also introduce a real and important damping term for the TAE. The equilibrium q-profile at the time of the fourth TAE burst analyzed in Fig. 12 happened to have q01.53 on axis; close to the q0 = 1.5 which would have closed the gap on axis. Thus, as the q-profile evolves through the shot, it would be expected that the gap on axis opens and closes, affecting the threshold for exciting the TAE. ...Context 15
... TAE onsets shortly after the current ramp ends, so the internal plasma current profile is still evolving. From the onset time of the TAE until the last of the strong bursts, the q-profile evolves, with q02 at the start of TAE activity and q01.5 by the last strong TAE burst Fig. 14. The shaded regions of Fig. 14b indicate where the n = 2 red, n=3 green, and n = 4 blue gaps should be nearly closed on axis. While there is perhaps some correlation for the ampli- tude of the n = 4 modes, the q-profile evolution appears to have little effect on the n = 2 and n = 3 ...Context 16
... TAE onsets shortly after the current ramp ends, so the internal plasma current profile is still evolving. From the onset time of the TAE until the last of the strong bursts, the q-profile evolves, with q02 at the start of TAE activity and q01.5 by the last strong TAE burst Fig. 14. The shaded regions of Fig. 14b indicate where the n = 2 red, n=3 green, and n = 4 blue gaps should be nearly closed on axis. While there is perhaps some correlation for the ampli- tude of the n = 4 modes, the q-profile evolution appears to have little effect on the n = 2 and n = 3 ...Context 17
... simulations were done for equilibria roughly span- ning the range in q-profiles from the first strong TAE burst at 0.257 s to the fourth burst at 0.285 s, not quite the full range in q-profiles between the blue and green curves in Fig. 11. For these simulations, all that was varied was the q-profile in the center of the plasma Fig. 15. As indicated in the figure, the reference q-profile was scaled by a constant factor, then offset to keep qaconstant, for example, q b r = 0.990 q ref r + 0.16 the green ...Context 18
... simulations were done for equilibria roughly span- ning the range in q-profiles from the first strong TAE burst at 0.257 s to the fourth burst at 0.285 s, not quite the full range in q-profiles between the blue and green curves in Fig. 11. For these simulations, all that was varied was the q-profile in the center of the plasma Fig. 15. As indicated in the figure, the reference q-profile was scaled by a constant factor, then offset to keep qaconstant, for example, q b r = 0.990 q ref r + 0.16 the green ...Context 19
... n = 3 continua, as calculated in NOVA, are shown in Fig. 16. Notice that the gap is nearly closed on axis for the reference q-profile black, but opens substantially for the "earlier" q-profiles red, green and blue. As before, not all eigenmodes could be "tracked" through the equilibrium changes. The modes clearly in the gap fared best, with the mode at 69.4 kHz in the reference case black ...Context 20
... kHz closes the n = 3 TAE gap. A version of NOVA was created Time (s) a) n=1, n=2, n=3, n=4, n=5 r/a 0.0 0.7 0.2 0.5 reference q ref q ref (r) x 0.995 + 0.08 q ref (r) x 0.990 + 0.16 q ref (r) x 0.985 + 0. which includes some of the physical effects of sheared rotation. 119 The continuum for the n = 3 gap including the sheared rotation is shown in Fig. 17. The five eigenmode solutions that were found for this gap structure are shown on the right. Each of them has strong interactions with the continuum. The stronger interaction with the continuum results in greater variation of radial structure than for the cases without the Doppler correction. The strength of the interaction with the ...Context 21
... NSTX plasmas cf. Crocker et al., Ref. 118, Figs. 10 and 11. The estimated GAM frequency at q min , 100 kHz, is comparable to even the Doppler-shifted TAE frequency, so rsAE would be expected to be suppressed. 117 The evolution of the amplitude, frequency and mode structure of the n = 3 TAE shows no sensitivity to the evolution of q min e.g., Fig. 14, as would be expected for rsAE. Thus, while these modes might be considered to be a hybrid mixture of coupled TAE and rsAE, the TAE characteristics are ...Context 22
... NOVA code calculates ideal poloidal harmonic con- tributions to the flux surface displacement for the eigen- modes. The displacement is used to calculate the perturbed density profile on the outboard midplane Fig. 18. The phase shifts resulting from the density perturbations are then calcu- lated for a range of reflectometer frequencies. The perturbed density in Fig. 18 is shown for calculations using just the mode displacement blue, and also for the mode displace- ment together with compressional corrections red. The pro- file shape is not ...Context 23
... ideal poloidal harmonic con- tributions to the flux surface displacement for the eigen- modes. The displacement is used to calculate the perturbed density profile on the outboard midplane Fig. 18. The phase shifts resulting from the density perturbations are then calcu- lated for a range of reflectometer frequencies. The perturbed density in Fig. 18 is shown for calculations using just the mode displacement blue, and also for the mode displace- ment together with compressional corrections red. The pro- file shape is not significantly changed by the inclusion of the compressional terms, but the mode amplitude needed to match the reflectometer data is increased by a factor of ...Context 24
... examples of n = 3 eigenmodes from NOVA simula- tions are compared with the reflectometer data in Fig. 19. The solid blue curve is the simulated reflectometer response "synthetic reflectometer diagnostic" and the five experi- mental points from the reflectometer array are shown in red. The experimental data represent an average of ten measure- ments of the radial mode structure during the final burst. In Figs. 19a and 19b two of the ...Context 25
... s. The dominant mode is the n = 3 mode, described above. There is an n = 6 mode, but it appears to be a har- monic of the n = 3, so its role in the fast-ion transport is not clear. NOVA found multiple solutions for both the n = 2 and n = 4 modes for the Doppler-corrected equilibrium. The best fits to the reflectometer array data are shown in Fig. 21. The signal-to-noise level for the outer points of the n = 2 modes is not that good, and the disagreement with the outermost point is not of concern. Phase information is indicated by positive and negative displacements, as the NOVA eigenmodes and the experimental data typically showed either no radial phase shifts as in Fig. 19 or ...Context 26
... data are shown in Fig. 21. The signal-to-noise level for the outer points of the n = 2 modes is not that good, and the disagreement with the outermost point is not of concern. Phase information is indicated by positive and negative displacements, as the NOVA eigenmodes and the experimental data typically showed either no radial phase shifts as in Fig. 19 or 180° phase jumps for some of the other modes. The n = 4 mode has a phase inversion, which is reproduced for the NOVA eigenmode. These eigenmodes are also found for the continuum calculated including sheared- Doppler ...Similar publications
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This Letter presents the first observation on the interplay between nonlocal transport and neoclassical tearing modes (NTMs) during transient nonlocal heat transport events in the HL-2A tokamak. The nonlocality is triggered by edge cooling and large-scale, inward propagating avalanches. These lead to a locally enhanced pressure gradient at the q =...
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Experiments on the National Spherical Torus Experiment [M. Ono, et al., Nucl. Fusion 40 (2000) 557 ] have found strong bursts of Toroidal Alfven Eigenmode (TAE) activity correlated with abrupt drops in the neutron rate. A fairly complete data set offers the opportunity to benchmark the NOVA [C. Z. Cheng, Phys. Reports 211, 1-51 (1992)] and ORBIT [R...
Citations
... They are needed to sustain the high temperatures required for nuclear fusion to take place, but can also drive instabilities in the plasma [1][2][3][4][5][6]. Fast ions can themselves be transported away from the plasma core by magnetohydrodynamic modes [7][8][9][10][11][12], thus rendering the study of fast ions vital in the development of magnetic confinement fusion devices. ...
... The regularisation matrix L can be related to the prior covariance matrix of f in a Bayesian perspective. Since the prior covariance matrix is positive semi-definite, we can decompose it as C −1 f = λ 2 L T L, such that finding the MAP estimate of f is equivalent to finding the solution to (11). To make the dependence of the prior covariance matrix on λ explicit, we will from now on denote it by C f λ . ...
Tomographic reconstructions of a 3D fast-ion constants-of-motion phase-space distribution function are computed by inverting synthetic signals based on projected velocities of the fast ions along the diagnostic lines of sight. The fast-ion distribution functions are parameterised by three constants of motion, the kinetic energy, the magnetic moment and the toroidal canonical angular momentum. The reconstructions are computed using both zeroth-order and firstorder Tikhonov regularisation expressed in terms of Bayesian inference to allow uncertainty quantification. In addition to this, a discontinuity appears to be present in the solution across the trapped passing boundary surface in the three-dimensional phase space due to a singularity in the Jacobian of the transformation from position and velocity space to phase space. A method to allow for this apparent discontinuity while simultaneously penalising large gradients in the solution is demonstrated. Finally, we use our new methods to optimise the diagnostic performance of a set of six fans of sightlines by finding where the detectors contribute most complementary diagnostic information for the future COMPASS-Upgrade tokamak.
... The confinement of alpha particles, or more generally of fast ions, is essential, as they sustain the high temperature in the fusion plasma in future burning plasmas. The fast ions can experience large transport away from the plasma core [1][2][3][4][5][6], but they can also themselves drive plasma instabilities, deteriorating plasma confinement [7][8][9][10][11][12]. Therefore, it is vital to understand the distribution of the fast ions in order to understand their behavior. ...
The fast-ion phase-space distribution function in axisymmetric tokamak plasmas is completely described by the three constants of motion: energy, magnetic moment and toroidal canonical angular momentum. In this work, the observable regions of constants-of-motion phase-space, given a diagnostic setup, are identified and explained using projected velocities of the fast ions along the diagnostic lines-of-sight as a proxy for several fast-ion diagnostics, such as fast-ion D α spectroscopy, collective Thomson scattering, neutron emission spectroscopy and gamma-ray spectroscopy. The observable region in constants-of-motion space is given by a position condition and a velocity condition, and the diagnostic sensitivity is given by a gyro-orbit and a drift-orbit weighting. As a practical example, 3D orbit weight functions quantifying the diagnostic sensitivity to each point in phase-space are computed and investigated for the future COMPASS-Upgrade and MAST-Upgrade tokamaks.
... Comparison of the observed mode frequencies with shear Alfvén spectra suggests that the observed instabilities may be classified into the GAE, located in the relatively inner region of the plasma, or the TAE with m = 1 + 2, located at an r/a 99 of 0.7. The instabilities may be classified into one kind of Alfvén eigenmode avalanche [105,106]. ...
Studies of energetic particle transport due to energetic-particle-driven Alfvénic instability have progressed using neutron and energetic particle diagnostics in Large Helical Device deuterium plasmas. Alfvénic instability excited by injecting an intensive neutral beam was observed by a magnetic probe and a far-infrared laser interferometer. The interferometer showed Alfvénic instability composed of three modes that existed from the core to the edge of the plasma. A comparison between the observed frequency and shear Alfvén spectra suggested that the mode activity was most likely classified as an Alfvénic avalanche. A neutron fluctuation detector and a fast ion loss detector indicated that Alfvénic instability induced transport and loss of co-going transit energetic ions. The dependence of the drop rate of the neutron signal on the Alfvénic instability amplitude showed that significant transport occurred. Significant transport might be induced by the large amplitude and radially extended multiple modes, as well as a large deviation of the energetic ion orbit from the flux surface.
... with long range frequency chirping and has been observed for a variety of modes in experiments [1][2][3][4][5]. References [6][7][8][9][10][11] show a correlation between wave-particle resonant interactions and fast ion loss and redistribution. ...
... We do that by relating the EPs frequencies to Pθ c and Pξ c using equations (11a), (11b) and (10). Given (10), H 0 is known as the particle energy (E), and Pξ c and PΩ are also known quantities and can be evaluated using (9b) and (9c), respectively, for ξ c and Ω being ignorable coordinates in H 0 of (8). Hence, equation (10) can be inverted to write ...
We present a procedure to examine energetic particle phase-space during long range frequency chirping phenomena in tokamak plasmas. To apply the proposed method, we have performed self-consistent simulations using the MEGA code and analyzed the simulation data. We demonstrate a travelling wave in phase-space and that there exist specific slices of phase-space on which the resonant particles lie throughout the wave evolution. For non-linear evolution of an n=6 toroidicity-induced Alfven eigenmode (TAE), our results reveal the formation of coherent phase-space structures (holes/clumps) after coarse-graining of the distribution function. These structures cause a convective transport in phase-space which implies a radial drift of the resonant particles. We also demonstrate that the rate of frequency chirping increases with the TAE damping rate. Our observations of the TAE behaviour and the corresponding phase-space dynamics are consistent with the Berk-Breizman (BB) theory.
... Its occurrence is ubiquitous not only in nature [1,2], e.g., familiar magnetic reconnection avalanche event-solar flare [3], but also in magnetic confinement fusion devices [4][5][6][7]. In magnetic confinement fusion experiments, avalanche is thought to be a representative example of the synergistic interaction between multiple modes leading to nonlinear enhancement in energetic-ion transport and loss [8][9][10][11]. Avalanche is a highly nonlinear event and the wave-particle resonant interaction and wave-wave nonlinear coupling coexist in this event. This Page 1 of 16 AUTHOR SUBMITTED MANUSCRIPT -NF-104946.R1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A c c e p t e d M a n u s c r i p t event may detrimentally poses a significant obstacle to the successful achievement of optimized and high performance plasmas in reactor scale devices such as International Thermonuclear Experiment Reactor ...
Large burst activity, identified as toroidal Alfv\'{e}n eigenmode (TAE) avalanche, occurs frequently in neutral-beam heated plasmas in National Spherical Torus Experiment (NSTX). Based on the typical experimental observation of TAE avalanche on NSTX, a self-consistent nonlinear multiple wave-number ($k_{\parallel}\simeq n/R$, where $n$ toroidal mode-number and $R$ major radius) simulation associated with TAE avalanches is performed using the experimental parameters and profiles before the occurrence of TAE avalanche as the M3D-K input. The wave-wave nonlinear coupling among different modes and the resonant interaction between different modes and energetic-ions during TAE avalanches are identified in the nonlinear multiple wave-number simulations. The resonance overlap during the TAE avalanche is clearly observed in the simulation. It is found that the effective wave-wave coupling and a sufficiently strong drive are two important ingredients for the onset of TAE avalanches. TAE avalanche is considered to be a strongly nonlinear process and it is always accompanied by the simultaneous rapid frequency-chirping and large amplitude bursting of multiple modes and significant energetic-ion losses. The experimental phenomenon is observed on NSTX and is qualitatively reproduced by the simulation results in this work. These findings indicate that the onset of avalanche is triggered by nonlinearity of the system, and are also conducive to understanding the underlying mechanism of avalanche transport of energetic particles in the future burning plasmas, such as ITER.
... On LAPD the avalanche events were associated with unstable drift-Alfvén waves, and exhibited both radial and poloidal dynamics [6]. In addition, the avalanche transport of energeticparticles (EPs) was proposed on theory and investigated on experiments in MCF plasmas [7][8][9][10]. Effects of EP-driven instabilities on the heat transport of bulk plasmas, including electron and ion energy channels, were explored in some devices [11][12][13][14][15]. On the other hand, non-linear mode couplings (NMCs) can produce different scale structures, determine their excitation/saturation/damping, transfer wave energy among different scale structures, and finally, affect the plasma confinement/transport. ...
... In general, spherical tokamaks exhibit Alfv enic wave chirping more prominently than conventional tokamaks, 15 which has been explained in terms of the reduced relative microturbulence at the ion scale in spherical machines. 16 Alfv enic avalanches, during which a significant fraction of the fast ions is typically lost, 17 are often preceded by a sequence of chirping events. In this work, we examine unstable Alfv en modes with damping a significant fraction of the drive, using the guiding center code Orbit 18 and a delta f formalism to reproduce the chirping behavior. ...
Modulation of mode amplitude and frequency of TAE modes, observed experimentally and referred to as chirping, is investigated using a guiding center code and a δf formalism. Chirping is observed as the development in time of Fourier sidebands that move above and below the nominal mode frequency. Subsequent doubling of the sidebands is also sometimes observed. Equilibria with conventional positive magnetic shear are used, as well as NSTX reversed shear cases. The onset of chirping can be triggered by a sudden increase in mode damping, as can occur by the mode contacting the continuum.
... A distinctive feature of Alfvénic wave behavior in STs [27] as compared to conventional tokamaks is that chirping and avalanches are customary in the former and infrequent in the latter. The ubiquitous Alfvénic chirping in NSTX is observed to be a precursor of the phase locking of high-intensity modes of several toroidal mode numbers, known as avalanche [28,29]. During the avalanche, the escape of a substantial fraction of fast ions occurs, which can typically reach up to 40%. ...
Rare Alfv\'enic wave transitions between fixed-frequency and chirping phases are identified in NSTX, where Alfv\'enic waves are normally observed to exhibit either chirping or avalanching responses. For those transitions, we apply a criterion [Duarte et al, Nucl. Fusion 57, 054001 (2017)] to predict the nature of fast ion redistribution in tokamaks to be in the convective or diffusive nonlinear regimes. For NSTX discharges in which the transition is not accompanied by changes in the beam deposited power or modifications in the injected radiofrequency power, it has been found that the anomalous fast ion transport is a likely mediator of the bifurcation between the fixed-frequency mode behavior and rapid chirping. For a quantitative assessment, global gyrokinetic simulations of the effects of electrostatic ion temperature gradient turbulence and trapped electron mode turbulence on chirping were pursued using the GTS code. The investigation is extended by means of predictive studies of the probable spectral behavior of Alfv\'enic eigenmodes for baseline ITER cases consisting of elmy, advanced and hybrid scenarios. It has been observed that most modes are found to be borderline between the steady and the chirping phases.
... A distinctive feature of Alfvénic wave behavior in STs [27] as compared to conventional tokamaks is that chirping and avalanches are customary in the former and infrequent in the latter. The ubiquitous Alfvénic chirping in NSTX is observed to be a precursor of the phase locking of high-intensity modes of several toroidal mode numbers, known as avalanche [28,29]. During the avalanche, the escape of a substantial fraction of fast ions occurs, which can typically reach up to 40%. ...
Rare Alfvénic wave transitions between fixed-frequency and chirping phases are identified in NSTX, where Alfvénic waves are normally observed to exhibit either chirping or avalanching responses. For those transitions, we apply a criterion [Duarte et al, Nucl. Fusion 57, 054001 (2017)] to predict the nature of fast ion redistribution in tokamaks to be in the convective or diffusive non-linear regimes. For NSTX discharges in which the transition is not accompanied by changes in the beam deposited power or modifications in the injected radiofrequency power, it has been found that the anomalous fast ion transport is a likely mediator of the bifurcation between the fixed-frequency mode behavior and rapid chirping. For a quantitative assessment, global gyrokinetic simulations of the effects of electrostatic ion temperature gradient turbulence and trapped electron mode turbulence on chirping were pursued using the GTS code. The investigation is extended by means of predictive studies of the probable spectral behavior of Alfvénic eigenmodes for baseline ITER cases consisting of elmy, advanced and hybrid scenarios. It has been observed that most modes are found to be borderline between the steady and the chirping phases.
... In order to characterize the mode being observed in the experiment, NSTX reflectometer measurements are compared to the mode structures computed by NOVA, employing a similar procedure as the one used in Refs. [6,24]. In DIII-D, similar identification is performed using Electron Cyclotron Emission (ECE) [25]. ...
A general criterion is proposed and found to successfully predict the emergence of chirping oscillations of unstable Alfv\'enic 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.
