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Title: A model for the advantage of early electron cyclotron current drive in the suppression of neoclassical tearing modes

Abstract

An analytic model for the advantage of the early application of electron cyclotron current drive (ECCD) in the suppression of neoclassical tearing modes (NTMs) is presented. The improved performance of early ECCD is attributed to the second (smaller) saturation island width, which appears for sufficiently small (relative to the ECCD deposition width) critical island widths, in the strongly nonlinear growth rate profile. The operational range for the advantage of early ECCD is obtained, and it is shown that it is favored by broad deposition profiles. The preliminary experimental results in ASDEX Upgrade [H. Zohm et al., Nucl. Fusion 41, 197 (2001)] are consistent with the present model.

Authors:
; ;  [1];  [2]
  1. National Technical University of Athens, Association Euratom-Hellenic Republic (Greece)
  2. (Germany)
Publication Date:
OSTI Identifier:
20974934
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 4; Other Information: DOI: 10.1063/1.2720372; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ASDEX TOKAMAK; DEPOSITION; ECR CURRENT DRIVE; INHIBITION; NEOCLASSICAL TRANSPORT THEORY; NONLINEAR PROBLEMS; PLASMA; PLASMA CONFINEMENT; TEARING INSTABILITY

Citation Formats

Lazaros, Avrilios, Maraschek, Marc, Zohm, Hartmut, and Max Planck Institute for Plasma Physics, Euratom Association, Garching Germany. A model for the advantage of early electron cyclotron current drive in the suppression of neoclassical tearing modes. United States: N. p., 2007. Web. doi:10.1063/1.2720372.
Lazaros, Avrilios, Maraschek, Marc, Zohm, Hartmut, & Max Planck Institute for Plasma Physics, Euratom Association, Garching Germany. A model for the advantage of early electron cyclotron current drive in the suppression of neoclassical tearing modes. United States. doi:10.1063/1.2720372.
Lazaros, Avrilios, Maraschek, Marc, Zohm, Hartmut, and Max Planck Institute for Plasma Physics, Euratom Association, Garching Germany. Sun . "A model for the advantage of early electron cyclotron current drive in the suppression of neoclassical tearing modes". United States. doi:10.1063/1.2720372.
@article{osti_20974934,
title = {A model for the advantage of early electron cyclotron current drive in the suppression of neoclassical tearing modes},
author = {Lazaros, Avrilios and Maraschek, Marc and Zohm, Hartmut and Max Planck Institute for Plasma Physics, Euratom Association, Garching Germany},
abstractNote = {An analytic model for the advantage of the early application of electron cyclotron current drive (ECCD) in the suppression of neoclassical tearing modes (NTMs) is presented. The improved performance of early ECCD is attributed to the second (smaller) saturation island width, which appears for sufficiently small (relative to the ECCD deposition width) critical island widths, in the strongly nonlinear growth rate profile. The operational range for the advantage of early ECCD is obtained, and it is shown that it is favored by broad deposition profiles. The preliminary experimental results in ASDEX Upgrade [H. Zohm et al., Nucl. Fusion 41, 197 (2001)] are consistent with the present model.},
doi = {10.1063/1.2720372},
journal = {Physics of Plasmas},
number = 4,
volume = 14,
place = {United States},
year = {Sun Apr 15 00:00:00 EDT 2007},
month = {Sun Apr 15 00:00:00 EDT 2007}
}
  • In the generalized Rutherford equation describing the nonlinear evolution of the width of the magnetic island associated with a neoclassical tearing mode, the effect of localized current drive is represented by a term {Delta}{sub CD}{sup Prime }. We investigate oscillations in {Delta}{sub CD}{sup Prime} originating from the rotation of the island through the electron cyclotron power deposition region and their dependence on the collisional time scale on which the driven current is generated, the rotation period, the island size, and the power deposition width. Furthermore, their consequences for the island growth or the stabilization are analyzed. This work shows thatmore » the net result of the oscillations in {Delta}{sub CD}{sup Prime} is a slight increase in the stabilizing effect of electron cyclotron current drive and consequently, a reduction in the minimum power requirement to fully suppress an island.« less
  • The generalized Rutherford equation for the neoclassical tearing mode is studied. New analytical expressions for the nonlinear stability criterion, the seed island width, and the saturated island width are derived. These are especially useful when the saturated island width is small. A formalism for calculating the current needed to stabilize the mode is established by adding an externally driven current. Inserting the reference parameters of the International Thermonuclear Experimental Reactor (ITER) [ITER-JCT and Home Teams, Plasma Phys. Controlled Fusion {bold 37}, A19 (1995)], a value of 160 kA to be driven by Electron Cyclotron Current Drive (ECCD) in order tomore » completely stabilize an m=2 mode is found, well within the capabilities of the ITER ECCD system. If higher currents can be driven, the local {beta}{sub p} at the resonant surface can be increased significantly. {copyright} {ital 1997 American Institute of Physics.}« less
  • Neoclassical tearing modes are found to limit the achievable beta in many high performance discharges in DIII-D. Electron cyclotron current drive within the magnetic islands formed as the tearing mode grows has been proposed as a means of stabilizing these modes or reducing their amplitude, thereby increasing the beta limit by a factor around 1.5. Some experimental success has been obtained previously on Asdex-U. Here we examine the parameter range in DIII-D in which this effect can best be studied. (c) 1999 American Institute of Physics.
  • It is well known that electron cyclotron current drive (ECCD) around the o-point of magnetic island along the plasma current direction can stabilize neoclassical tearing modes (NTMs) in tokamak devices. The effects of the radial misalignment between the island and the driven current, the phase misalignment, and the on-duty ratio for modulated current drive on NTM stabilization are studied numerically in this paper. A small radial misalignment is found to significantly decrease the stabilizing effect. When a sufficiently large phase misalignment occurs for the modulated ECCD, the stabilization effect is also reduced a lot. The optimal on-duty ratio of modulatedmore » ECCD to stabilize NTMs is found to be in the range of 60%–70%. A larger on-duty ratio than 50% could also mitigate the effect of phase misalignment if it is not too large. There is no benefit from modulation if the phase misalignment is larger than a threshold.« less