skip to main content

SciTech ConnectSciTech Connect

Title: Phase locking of multi-helicity neoclassical tearing modes in tokamak plasmas

The attractive “hybrid” tokamak scenario combines comparatively high q{sub 95} operation with improved confinement compared with the conventional H{sub 98,y2} scaling law. Somewhat unusually, hybrid discharges often exhibit multiple neoclassical tearing modes (NTMs) possessing different mode numbers. The various NTMs are eventually observed to phase lock to one another, giving rise to a significant flattening, or even an inversion, of the core toroidal plasma rotation profile. This behavior is highly undesirable because the loss of core plasma rotation is known to have a deleterious effect on plasma stability. This paper presents a simple, single-fluid, cylindrical model of the phase locking of two NTMs with different poloidal and toroidal mode numbers in a tokamak plasma. Such locking takes place via a combination of nonlinear three-wave coupling and conventional toroidal coupling. In accordance with experimental observations, the model predicts that there is a bifurcation to a phase-locked state when the frequency mismatch between the modes is reduced to one half of its original value. In further accordance, the phase-locked state is characterized by the permanent alignment of one of the X-points of NTM island chains on the outboard mid-plane of the plasma, and a modified toroidal angular velocity profile, interior to themore » outermost coupled rational surface, which is such that the core rotation is flattened, or even inverted.« less
Authors:
 [1]
  1. Institute for Fusion Studies, University of Texas at Austin, Austin, Texas 78712 (United States)
Publication Date:
OSTI Identifier:
22408322
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 22; Journal Issue: 4; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ANGULAR VELOCITY; BIFURCATION; COMPARATIVE EVALUATIONS; CYLINDRICAL CONFIGURATION; HELICITY; MODE RATIONAL SURFACES; NONLINEAR PROBLEMS; PHASE STABILITY; PLASMA; PLASMA CONFINEMENT; ROTATING PLASMA; SCALING LAWS; TEARING INSTABILITY; TOKAMAK DEVICES