MHD phenomena in reversed shear discharges on ASDEX Upgrade

Günter, S.; Schade, S.; Maraschek, M.; Pinches, S. D.; Strumberger, E.; Wolf, R. C.; Yu, Q.

doi:10.1088/0029-5515/40/8/311

Cited by 67 publications

(90 citation statements)

References 27 publications

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“…Up to now the potentially beneficial effect of the H-mode transition, improving the MHD stability due to a broadening of the pressure profile, has not been achieved in ASDEX Upgrade. The pressure, corresponding to the temperature profiles shown in figure 30 for the later time, is already close to the theoretical pressure limit without a conducting wall (Günter et al 2000a). At β N ≈ 1.7 this finally causes the disruptive termination of the discharge, when the H-mode transition occurs with q 95 in the vicinity of 4.…”

Section: Tftrsupporting

confidence: 57%

Internal transport barriers in tokamak plasmas*

Wolf

2002

Plasma Phys. Control. Fusion

323

310

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Internal transport barriers in tokamak plasmas are explored in order to improve confinement and stability beyond the reference scenario, used for the ITER extrapolation, and to achieve higher bootstrap current fractions as an essential part of non-inductive current drive. Internal transport barriers are produced by modifications of the current profile using external heating and current drive effects, often combined with partial freezing of the initial skin current profile. Thus, formerly inaccessible ion temperatures and Q eq DT values have been (transiently) achieved. The present paper reviews the state of the art of these techniques and their effects on plasma transport in view of optimizing the confinement properties. Implications and limits for possible steady state operations and extrapolation to burning plasmas are discussed.

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Section: Tftrsupporting

confidence: 57%

Internal transport barriers in tokamak plasmas*

Wolf

2002

Plasma Phys. Control. Fusion

323

310

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“…On ASDEX Upgrade, a DTM occurring during the current ramp-up in strongly reversed shear scenarios led to a transient loss of the ITB. This could be avoided by additional ECR heating, which, according to modelling, prevents DTM formation due to increased conductivity [260].…”

Section: Mhd Stability In Plasmas With Strong Negative Magnetic Shearmentioning

confidence: 99%

Chapter 3: MHD stability, operational limits and disruptions

Hender¹,

Wesley²,

Bialek³

et al. 2007

Nucl. Fusion

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1,000

947

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Progress in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document Nucl. Fusion 39 2137-2664, is reviewed. Recent theoretical and experimental research has made important advances in both understanding and control of MHD stability in tokamak plasmas. Sawteeth are anticipated in the ITER baseline ELMy H-mode scenario, but the tools exist to avoid or control them through localized current drive or fast ion generation. Active control of other MHD instabilities will most likely be also required in ITER. Extrapolation from existing experiments indicates that stabilization of neoclassical tearing modes by highly localized feedback-controlled current drive should be possible in ITER. Resistive wall modes are a key issue for S128 Chapter 3: MHD stability, operational limits and disruptions advanced scenarios, but again, existing experiments indicate that these modes can be stabilized by a combination of plasma rotation and direct feedback control with non-axisymmetric coils. Reduction of error fields is a requirement for avoiding non-rotating magnetic island formation and for maintaining plasma rotation to help stabilize resistive wall modes. Recent experiments have shown the feasibility of reducing error fields to an acceptable level by means of non-axisymmetric coils, possibly controlled by feedback. The MHD stability limits associated with advanced scenarios are becoming well understood theoretically, and can be extended by tailoring of the pressure and current density profiles as well as by other techniques mentioned here. There have been significant advances also in the control of disruptions, most notably by injection of massive quantities of gas, leading to reduced halo current fractions and a larger fraction of the total thermal and magnetic energy dissipated by radiation. These advances in disruption control are supported by the development of means to predict impending disruption, most notably using neural networks. In addition to these advances in means to control or ameliorate the consequences of MHD instabilities, there has been significant progress in improving physics understanding and modelling. This progress has been in areas including the mechanisms governing NTM growth and seeding, in understanding the damping controlling RWM stability and in modelling RWM feedback schemes. For disruptions there has been continued progress on the instability mechanisms that underlie various classes of disruption, on the detailed modelling of halo currents and forces and in refining predictions of quench rates and disruption power loads. Overall the studies reviewed in this chapter demonstrate that MHD instabilities can be controlled, avoided or ameliorated to the extent that they should not compromise ITER operation, though they will necessarily impose a range of constraints.

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“…17,18 Thus, a dynamic interaction between DTMs and shear flows is expected. 5 In this Letter, we describe in cylindrical geometry how a broad spectrum of linear DTMs is stabilized by increasing velocity shear and show that, above a certain threshold, fast growing electromagnetic KH-like modes with dominant high mode numbers m ϳ 10 appear. The linear instability of both DTMs and KH-like modes is analyzed and first nonlinear simulation results are presented for a case with finite resistivity.…”

mentioning

confidence: 90%

Large-mode-number magnetohydrodynamic instability driven by sheared flows in a tokamak plasma with reversed central shear

Bierwage

Günter

2007

Physics of Plasmas

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The effect of a narrow sub-Alfvénic shear flow layer near the minimum qmin of the tokamak safety factor profile in a configuration with reversed central shear is analyzed. Sufficiently strong velocity shear gives rise to a broad spectrum of fast growing Kelvin-Helmholtz (KH)-like ideal magnetohydrodynamic modes with dominant mode numbers m,n∼10. Nonlinear simulations with finite resistivity show magnetic reconnection near ripples caused by KH-like vortices, the formation of turbulent structures, and a flattening of the flow profile. The KH modes are compared to double tearing modes that dominate at lower shearing rates. The possible application of these results in tokamaks with internal transport barrier is discussed.

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MHD phenomena in reversed shear discharges on ASDEX Upgrade

Cited by 67 publications

References 27 publications

Internal transport barriers in tokamak plasmas*

Internal transport barriers in tokamak plasmas*

Chapter 3: MHD stability, operational limits and disruptions

Large-mode-number magnetohydrodynamic instability driven by sheared flows in a tokamak plasma with reversed central shear

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