Resistive wall mode dynamics and active feedback control in DIII-D

Garofalo, A. M.; Chu, M. S.; Fredrickson, E.; Gryaznevich, M.; Jensen, T. H.; Johnson, L. C.; Haye, R.J. La; Navratil, G.A.; Okabayashi, M.; Scoville, J.T.; Strait, E. J.; Turnbull, A. D.; Team, Diii-D

doi:10.1088/0029-5515/41/9/305

Cited by 107 publications

(97 citation statements)

References 17 publications

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“…Using the RWM ''smart shell'' feedback with radial-field sensors 11,12 does not significantly affect the RWM onset relative to the observations in discharges without feedback. This suggests that at the values of ␤ N reached in these experiments, the growth rate of the RWM that becomes unstable at low rotation is beyond the stabilization capability of the ''smart shell'' feedback algorithm, given the present hardware system.…”

Section: Dynamic Symmetrization Of the Magnetic-field Geometrymentioning

confidence: 81%

“…[8][9][10] Direct stabilization by feedback has been demonstrated only for small increments in ␤ N above the no-wall limit, i.e., the limit calculated in absence of a conducting wall and plasma rotation, ␤ N no-wall . 11,12 Rotational stabilization has been shown to allow ␤ N significantly above ␤ N no-wall , 13 but the duration of the stabilization period is always limited by the plasma rotation slowing down whenever ␤ exceeds the no-wall limit. 14,15 The RWM becomes unstable once the plasma rotation has dropped below a critical value.…”

Section: Introductionmentioning

confidence: 99%

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Sustained rotational stabilization of DIII-D plasmas above the no-wall beta limit

et al. 2002

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Sustained stabilization of the n=1 kink mode by plasma rotation at beta approaching twice the stability limit calculated without a wall has been achieved in DIII-D by a combination of error field reduction and sufficient rotation drive. Previous experiments have transiently exceeded the no-wall beta limit. However, demonstration of sustained rotational stabilization has remained elusive because the rotation has been found to decay whenever the plasma is wall stabilized. Recent theory [Boozer, Phys. Rev. Lett. 86, 5059 (2001)] predicts a resonant response to error fields in a plasma approaching marginal stability to a low-n kink mode. Enhancement of magnetic nonaxisymmetry in the plasma leads to strong damping of the toroidal rotation, precisely in the high-beta regime where it is needed for stabilization. This resonant response, or “error field amplification” is demonstrated in DIII-D experiments: applied n=1 radial fields cause enhanced plasma response and strong rotation damping at beta above the no wall limit but have little effect at lower beta. The discovery of an error field amplification has led to sustained operation above the no-wall limit through improved magnetic field symmetrization using an external coil set. The required symmetrization is determined both by optimizing the external currents with respect to the plasma rotation and by use of feedback to detect and minimize the plasma response to nonaxisymmetric fields as beta increases. Ideal stability analysis and rotation braking experiments at different beta values show that beta is maintained 50% higher than the no wall stability limit for durations greater than 1 s, and approaches beta twice the no-wall limit in several cases, with steady-state rotation levels. The results suggest that improved magnetic-field symmetry could allow plasmas to be maintained well above no-wall beta limit for as long as sufficient torque is provided.

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Section: Dynamic Symmetrization Of the Magnetic-field Geometrymentioning

confidence: 81%

Section: Introductionmentioning

confidence: 99%

Sustained rotational stabilization of DIII-D plasmas above the no-wall beta limit

et al. 2002

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“…The theoretically predicted "amplification" of magnetic field asymmetries by the resonant response of a marginally stable RWM [7] has been directly observed in DIII-D experiments [15]. was used to apply a pulsed n = l radial magnetic field perturbation.…”

Section: Stabilization By Plasma Rotationmentioning

confidence: 99%

Resistive wall stabilization of high-beta plasmas in DIII–D

Strait

Bialek

Bogatu³

et al. 2003

Nucl. Fusion

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116

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Abstract. Recent DIII-D experiments show that ideal kink modes can be stabilized at high beta by a resistive wall, with sufficient plasma rotation. However, the resonant response by a marginally stable resistive wall mode to static magnetic field asymmetries can lead to strong damping of the rotation. Careful reduction of such asymmetries has allowed plasmas with beta well above the ideal MHD nowall limit, and approaching the ideal-wall limit, to be sustained for durations exceeding one second. Feedback control can improve plasma stability by direct stabilization of the resistive wall mode or by reducing magnetic field asymmetry. Assisted by plasma rotation, direct feedback control of resistive wall modes with growth rates more than 5 times faster than the characteristic wall time has been observed. These results open a new regime of tokamak operation above the free-boundary stability limit, accessible by a combination of plasma rotation and feedback control.

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“…The role of sheared rotation in the quenching of turbulence with subsequent improvement in confinement [1][2][3] is well known, as well as the stabilizing effect of toroidal rotation on pressure limiting resistive wall modes [4]. The lower rotation expected as a consequence of lower torque and larger inertia may thus be detrimental for ITER performance.…”

Section: Introductionmentioning

confidence: 99%

Perturbative studies of toroidal momentum transport using neutral beam injection modulation in the Joint European Torus: Experimental results, analysis methodology, and first principles modeling

Mantica

Tala²,

Ferreira³

et al. 2010

Physics of Plasmas

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ABSTRACT.Perturbative experiments have been carried out in the JET tokamak in order to identify the diffusive and convective components of toroidal momentum transport. The torque source was modulated either by modulating tangential neutral beam power or by modulating in anti-phase tangential and normal beams to produce a torque perturbation in the absence of a power perturbation. The resulting periodic perturbation in the toroidal rotation velocity was modelled using time dependent transport simulations in order to extract empirical profiles of momentum diffusivity and pinch. Details of the experimental technique, data analysis and modelling are provided. The momentum diffusivity in the core region (0.2<ρ<0.8) was found to be close to the ion heat diffusivity ( χ φ / χ i~0 .7-1.7) and a significant inward momentum convection term, up to 20m/s, was found, leading to an effective momentum diffusivity significantly lower than the ion heat diffusivity (

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Resistive wall mode dynamics and active feedback control in DIII-D

Cited by 107 publications

References 17 publications

Sustained rotational stabilization of DIII-D plasmas above the no-wall beta limit

Sustained rotational stabilization of DIII-D plasmas above the no-wall beta limit

Resistive wall stabilization of high-beta plasmas in DIII–D

Perturbative studies of toroidal momentum transport using neutral beam injection modulation in the Joint European Torus: Experimental results, analysis methodology, and first principles modeling

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