Electron cyclotron heating can drastically alter reversed shear Alfvén eigenmode activity in DIII-D through finite pressure effects

Zeeland, M. A. Van; Heidbrink, W. W.; Sharapov, S. E.; Spong, D. A.; Cappa, Á.; Chen, Xi; Collins, C.; García-Muñoz, M.; Gorelenkov, N. N.; Kramer, G. J.; Lauber, P.; Zhang, Lin; Petty, C. C.

doi:10.1088/0029-5515/56/11/112007

Cited by 52 publications

(72 citation statements)

References 62 publications

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“…Higher central electron temperature causes a delay in the current penetration to the axis, increasing the central safety factor profile inward from ρ qmin . The local increase in electron temperature and safety factor alters the AE continuum, resulting in a TAE-dominant spectrum, as described in [31]. A comparison of the time-averaged AE amplitude versus beam power for all of the beam power scans discussed in this paper is shown in figure 2( f ).…”

Section: Critical Gradient Experiments At Diii-dmentioning

confidence: 87%

Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

et al. 2017

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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.

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Section: Critical Gradient Experiments At Diii-dmentioning

confidence: 87%

Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

et al. 2017

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“…When α increases, the RSAEs associated with the TAE gap move away from the Alfvén continuum into the TAE gap as was studied experimentally in [19] while the RSAEs associated with the EAE, NAE (Noncircular triangularity-induced AE), and higher order gaps emerge from the Alfvén continuum as is shown in figure 4 for an EAE-RSAE.…”

Section: Alfvén Eigenmode Suppression Techniquesmentioning

confidence: 87%

Improving fast-ion confinement in high-performance discharges by suppressing Alfvén eigenmodes

Kramer¹,

Podestà²,

Holcomb³

et al. 2017

Nucl. Fusion

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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.

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“…It is an ideal tool for controlling MHD instabilities since it can provide highly localized power deposition and toroidal current with a known location and good controllability, which significantly changes electron temperature and/or rotational transform (safety factor). Stabilization/Destabilization of AEs by ECH has been reported on the DIII-D tokamak for the first time [10][11][12]. In high beta plasmas of the DIII-D tokamak, reversed shear AEs (RSAEs) were stabilized with localized ECH which was deposited at off-axis minimum in safety factor q min .…”

Section: Introductionmentioning

confidence: 98%

“…In high beta plasmas of the DIII-D tokamak, reversed shear AEs (RSAEs) were stabilized with localized ECH which was deposited at off-axis minimum in safety factor q min . These observations can be explained by that the RSAE minimum frequency increases and overcomes the TAE frequency due to an increase in local electron temperature and its gradient by ECH [12]. Effect of ECH on AEs was also experimentally studied in the TJ-II stellarator/heliotron (S/H) device [13].…”

Section: Introductionmentioning

confidence: 99%

Suppression of fast-ion-driven MHD instabilities by ECH/ECCD on Heliotron J

et al. 2017

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Experiments of suppressing fast-ion-driven MHD instabilities such as energetic particle modes (EPMs) and global Alfvén eigenmodes (GAEs) have been made by using a second harmonic X-mode electron cyclotron heating (ECH) and current drive (ECCD) in the helical-axis heliotron device, Heliotron J. ECCD experiments show that the GAE destabilized by fast ions of neutral beam injection (NBI) with the observed frequency around 140 kHz are fully stabilized, and the EPMs with the observed frequency around 90 kHz are suppressed when the EC-driven plasma current flowing in the counter direction reaches approximately 0.7 kA. The low magnetic shear under the vacuum condition is modified into positive magnetic shear when counter-ECCD is applied, and the amplitude of GAEs and EPMs decreases with an increase of the EC-driven plasma current. These results indicate that magnetic shear is a key role in controlling GAEs as well as EPMs. The comparison of the calculation of shear Alfvén spectra with experimental results shows that the increasing continuum damping rate with an increase in local magnetic shear by EC-driven current is important for both EPMs and GAEs. Moreover, the increase in plasma current lead to the inward movement of GAEs. This effect would also contribute to suppression of GAEs because the continuum damping rate increases more and more toward core. Steady ECH is also found experimentally to be effective to control the amplitude of both GAEs and EPMs. The amplitude of EPMs, and especially for GAEs decreases with an increase in the ECH power under fixed density conditions.

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Electron cyclotron heating can drastically alter reversed shear Alfvén eigenmode activity in DIII-D through finite pressure effects

Cited by 52 publications

References 62 publications

Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

Improving fast-ion confinement in high-performance discharges by suppressing Alfvén eigenmodes

Suppression of fast-ion-driven MHD instabilities by ECH/ECCD on Heliotron J

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