Requirements for active resistive wall mode (RWM) feedback control

Externally applied, non-axisymmetric magnetic fields form the basis of several relatively simple and direct methods to control magnetohydrodynamic (MHD) instabilities in a tokamak, and most present and planned tokamaks now include a set of non-axisymmetric control coils for application of fields with low toroidal mode numbers. Non-axisymmetric applied fields are routinely used to compensate small asymmetries (δB/B∼10−3 to 10−4) of the nominally axisymmetric field, which otherwise can lead to instabilities through braking of plasma rotation and through direct stimulus of tearing modes or kink modes. This compensation may be feedback-controlled, based on the magnetic response of the plasma to the external fields. Non-axisymmetric fields are used for direct magnetic stabilization of the resistive wall mode—a kink instability with a growth rate slow enough that feedback control is practical. Saturated magnetic islands are also manipulated directly with non-axisymmetric fields, in order to unlock them from the wall and spin them to aid stabilization, or position them for suppression by localized current drive. Several recent scientific advances form the foundation of these developments in the control of instabilities. Most fundamental is the understanding that stable kink modes play a crucial role in the coupling of non-axisymmetric fields to the plasma, determining which field configurations couple most strongly, how the coupling depends on plasma conditions, and whether external asymmetries are amplified by the plasma. A major advance for the physics of high-beta plasmas (β = plasma pressure/magnetic field pressure) has been the understanding that drift-kinetic resonances can stabilize the resistive wall mode at pressures well above the ideal-MHD stability limit, but also that such discharges can be very sensitive to external asymmetries. The common physics of stable kink modes has brought significant unification to the topics of static error fields at low beta and resistive wall modes at high beta. These and other scientific advances, and their application to control of MHD instabilities, will be reviewed with emphasis on the most recent results and their applicability to ITER.

Section: B Modeling Of Rwm Feedback Controlmentioning

confidence: 84%

Magnetic control of magnetohydrodynamic instabilities in tokamaks

Strait

2014

“…This type of excitation with low-frequency oscillatory was attributable to the low gain operation with proportional gain as was observed in other current-driven-RWM experiments with low gain operation. 15,37 …”

Section: Response To Applied N ¼ 1 External Fieldmentioning

confidence: 99%

Off-axis fishbone-like instability and excitation of resistive wall modes in JT-60U and DIII-D

Okabayashi¹,

Matsunaga²,

deGrassie³

et al. 2011

An energetic-particle (EP)-driven "off-axis-fishbone-like mode (OFM)" often triggers a resistive wall mode (RWM) in JT-60U and DIII-D devices, preventing long-duration high-b N discharges. In these experiments, the EPs are energetic ions (70-85 keV) injected by neutral beams to produce high-pressure plasmas. EP-driven bursting events reduce the EP density and the plasma rotation simultaneously. These changes are significant in high-b N low-rotation plasmas, where the RWM stability is predicted to be strongly influenced by the EP precession drift resonance and by the plasma rotation near the q ¼ 2 surface (kinetic effects). Analysis of these effects on stability with a self-consistent perturbation to the mode structure using the MARS-K code showed that the impact of EP losses and rotation drop is sufficient to destabilize the RWM in low-rotation plasmas, when the plasma rotation normalized by Alfvén frequency is only a few tenths of a percent near the q ¼ 2 surface. The OFM characteristics are very similar in JT-60U and DIII-D, including nonlinear mode evolution. The modes grow initially like a classical fishbone, and then the mode structure becomes strongly distorted. The dynamic response of the OFM to an applied n ¼ 1 external field indicates that the mode retains its external kink character. These comparative studies suggest that an energetic particle-driven "off-axis-fishbone-like mode" is a new EP-driven branch of the external kink mode in wall-stabilized plasmas, analogous to the relationship of the classical fishbone branch to the internal kink mode.

“…These features seem to confirm the experimental ECE measurements for the internal structure. 10 In addition, Figs. 6͑a͒ and 6͑b͒ show that the current driven kink mode causes maximal plasma displacement near the top and bottom regions of the plasma cross section.…”

Section: B Internal Mode Structurementioning

confidence: 96%

“…The DIII-D experiments 10 show a strong interaction between the direct mode control and the error field ͑EF͒ correction. Since this interaction also occurs ͑probably in a more pronounced form͒ in the pressure driven RWM control, understanding this issue is of crucial importance for designing the feedback systems in future fusion devices.…”

Section: Introductionmentioning

confidence: 96%

“…8 In order to gain a "clean" understanding of the RWM behavior under the feedback control in tokamak conditions, specific, current driven RWMs were formed and feedback controlled in DIII-D, at low plasma pressure and slow plasma rotation. 10 Under these conditions, the kinetic effects on the mode are minimized. From the control point of view, there is probably no critical difference between the current and the pressure driven RWMs.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Resonant field amplification with feedback-stabilized regime in current driven resistive wall mode

Liu

Chu

et al. 2010

The stability and resonant field response of current driven resistive wall modes are numerically studied for DIII-D ͓J. L. Luxon, Nucl. Fusion 42, 614 ͑2002͔͒ low pressure plasmas. The resonant field response of the feedback-stabilized resistive wall mode is investigated both analytically and numerically, and compared with the response from intrinsically stable or marginally stable modes. The modeling qualitatively reproduces the experimental results. Furthermore, based on some recent results and on the indirect numerical evidence in this work, it is suggested that the mode stability behavior observed in DIII-D experiments is due to the kink-peeling mode stabilization by the separatrix geometry. The phase inversion radius of the computed plasma displacement does not generally coincide with the radial locations of rational surfaces, also supporting experimental observations.