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Title: Magnetic control of magnetohydrodynamic instabilities in tokamaks

Abstract

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{sup −3} to 10{sup −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,more » 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.« less

Authors:
 [1]
  1. General Atomics, PO Box 85608, San Diego, California 92186-5608 (United States)
Publication Date:
OSTI Identifier:
22408049
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 22; Journal Issue: 2; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; AXIAL SYMMETRY; CONTROL; COUPLING; HIGH-BETA PLASMA; ITER TOKAMAK; KINK INSTABILITY; MAGNETIC FIELDS; MAGNETIC ISLANDS; MAGNETOHYDRODYNAMICS; PLASMA PRESSURE; RESONANCE; STABILITY; STABILIZATION; TEARING INSTABILITY

Citation Formats

Strait, E. J. Magnetic control of magnetohydrodynamic instabilities in tokamaks. United States: N. p., 2015. Web. doi:10.1063/1.4902126.
Strait, E. J. Magnetic control of magnetohydrodynamic instabilities in tokamaks. United States. doi:10.1063/1.4902126.
Strait, E. J. 2015. "Magnetic control of magnetohydrodynamic instabilities in tokamaks". United States. doi:10.1063/1.4902126.
@article{osti_22408049,
title = {Magnetic control of magnetohydrodynamic instabilities in tokamaks},
author = {Strait, E. J.},
abstractNote = {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{sup −3} to 10{sup −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.},
doi = {10.1063/1.4902126},
journal = {Physics of Plasmas},
number = 2,
volume = 22,
place = {United States},
year = 2015,
month = 2
}
  • Cited by 14
  • 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 responsemore » 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. Furthermore, 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.« less
  • The resonant excitations of high-[ital n] magnetohydrodynamic instabilities by the energetic ions/alpha particles in tokamaks are theoretically analyzed. Here, [ital n] is the toroidal mode number. The magnetohydrodynamic eigenmodes, typically, consist of two-scale structures; one corresponds to the singular ( inertial'') region and the other the regular (ideal) region. Due to the finite-size orbits, the energetic particle contributions in the singular region are suppressed. Analytical dispersion relations can be derived via the asymptotic matching analysis. The dispersion relations have the generic form of the fishbone'' dispersion relation [Phys. Rev. Lett. [bold 52], 1122 (1984)] and demonstrate, in particular, the existencemore » of two types of modes; that is, the discrete gap mode and the energetic-particle continuum mode. Specific expressions are given for both the kinetic ballooning modes and the toroidal Alfven modes.« less
  • The ideal magnetohydrodynamic instability of a symmetric q = 1 magnetic island is studied by using an initial value method for a pressureless cylindrical tokamak plasma. It is shown that the island is unstable to a half-coalescence motion which destroys the up--down symmetry of the magnetic configuration. The mode characteristics are qualitatively similar to those predicted by Bussac et al. (Phys. Lett. 105A, 51 (1984)).