Neoclassical tearing modes and their control

La Haye, R. J.

doi:doi:10.1063/1.2180747

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Foreword: Papers from the 47th Annual Meeting of the APS Division of Plasma Physics, Denver, Colorado, 2005

Physics of zonal flows

Zonal flows, which means azimuthally symmetric band-like shear flows, are ubiquitous phenomena in nature and the laboratory. It is now widely recognized that zonal flows are a key constituent in virtually all cases and regimes of drift wave turbulence, indeed, so much so that this classic problem is now frequently referred to as “drift wave-zonal flow turbulence.” In this review, new viewpoints and unifying concepts are presented, which facilitate understanding of zonal flow physics, via theory...

A principal pressure limit in tokamaks is set by the onset of neoclassical tearing modes (NTMs), which are destabilized and maintained by helical perturbations to the pressure-gradient driven “bootstrap” current. The resulting magnetic islands break up the magnetic surfaces that confine the plasma. The NTM is linearly stable but nonlinearly unstable, and generally requires a “seed” to destabilize a metastable state. In the past decade, NTM physics has been studied and its effects identified as performance degrading in many tokamaks. The validation of NTM physics, suppressing the NTMs, and/or avoiding them altogether are areas of active study and considerable progress. Recent joint experiments give new insight into the underlying physics, seeding, and threshold scaling of NTMs. The physics scales toward increased NTM susceptibility in ITER, underlying the importance of both further study and development of control strategies. These strategies include regulation of “sawteeth” to reduce seeding, using static “bumpy” magnetic fields to interfere with the perturbed bootstrap current, and/or applying precisely located microwave power current drive at an island to stabilize (or avoid destabilization of) the NTM. Sustained stable operation without the highly deleterious m = 2, n = 1 island has been achieved at a pressure consistent with the no-wall n = 1 ideal kink limit, by using electron cyclotron current drive at the q = 2 rational surface, which is found by real-time accurate equilibrium reconstruction. This improved understanding of NTM physics and stabilization strategies will allow design of NTM control methods for future burning-plasma experiments like ITER.

Article Outline

INTRODUCTION
TEARING MODE PHYSICS
1. Classical tearing mode and magnetic islands
2. Deleterious effects of tearing modes
NEOCLASSICAL TEARING MODE PHYSICS
1. Bootstrap current
2. Helically perturbed bootstrap current
3. Observations of neoclassical tearing modes
4. Transport threshold for NTMs
5. Polarization threshold for NTMs
6. Other small island effects on NTMs
7. Curvature effects on NTMs
8. Marginal and onset conditions for NTMs
9. Seeding of NTMs
“WIND TUNNEL” STUDIES OF NTM BETA LIMITS
CONTROL OF NTMs
1. Sawteeth control, frequently interrupted regime-NTMs and externally applied static helical field
2. Radio frequency (rf) current drive (CD) to increase classical tearing stability
3. rf CD to replace the missing bootstrap current
4. Stabilization of NTMs with lower hybrid current drive (LHCD)
5. Stabilization of NTMs with electron cyclotron current drive (ECCD)
6. ECCD stabilization of NTMs on ITER
SUMMARY

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KEYWORDS and PACS

Keywords

tearing instability, Tokamak devices, plasma toroidal confinement, plasma pressure, plasma transport processes, plasma nonlinear processes, metastable states, sawtooth instability

PACS

52.35.Py

Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Fa

Tokamaks, spherical tokamaks
52.55.Wq

Current drive; helicity injection
52.25.Fi

Transport properties
52.35.Mw

Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.2180747

PUBLICATION DATA

ISSN

1070-664X (print)

1089-7674 (online)

Publisher

$abstract.society.value is a Member of CrossRef

American Institute of Physics

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