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Title: Surface currents associated with external kink modes in tokamak plasmas during a major disruption

Authors:
ORCiD logo [1];  [2]
  1. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA
  2. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA
Publication Date:
Sponsoring Org.:
USDOE
OSTI Identifier:
1405216
Grant/Contract Number:
DEAC02-76CH03073
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 24; Journal Issue: 10; Related Information: CHORUS Timestamp: 2017-11-06 17:20:59; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics
Country of Publication:
United States
Language:
English

Citation Formats

Ng, C. S., and Bhattacharjee, A. Surface currents associated with external kink modes in tokamak plasmas during a major disruption. United States: N. p., 2017. Web. doi:10.1063/1.4995264.
Ng, C. S., & Bhattacharjee, A. Surface currents associated with external kink modes in tokamak plasmas during a major disruption. United States. doi:10.1063/1.4995264.
Ng, C. S., and Bhattacharjee, A. 2017. "Surface currents associated with external kink modes in tokamak plasmas during a major disruption". United States. doi:10.1063/1.4995264.
@article{osti_1405216,
title = {Surface currents associated with external kink modes in tokamak plasmas during a major disruption},
author = {Ng, C. S. and Bhattacharjee, A.},
abstractNote = {},
doi = {10.1063/1.4995264},
journal = {Physics of Plasmas},
number = 10,
volume = 24,
place = {United States},
year = 2017,
month =
}

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on October 27, 2018
Publisher's Accepted Manuscript

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  • The fast termination phase of a vertical displacement event (VDE) in a tokamak is modeled as a sequence of shrinking equilibria, where the core current profile remains constant so that the safety-factor at the axis, q{sub axis}, remains fixed and the q{sub edge} systematically decreases. At some point, the n = 1 kink mode is destabilized. Kink modes distort the magnetic field lines outside the plasma, and surface currents are required to nullify the normal component of the B-field at the plasma boundary and maintain equilibrium at finite pressure. If the plasma touches a conductor, the current can be transferredmore » to the conductor, and may be measurable by the halo current monitors. This report describes a practical method to model the plasma as it evolves during a VDE, and determine the surface currents, needed to maintain equilibrium. The main results are that the onset conditions for the disruption are that the growth-rate of the n = 1 kink exceeds half the Alfven time and the associated surface current needed to maintain equilibrium exceeds one half of the core plasma current. This occurs when q{sub edge} drops below a low integer, usually 2. Application to NSTX provides favorable comparison with non-axisymmetric halo-current measurements. The model is also applied to ITER and shows that the 2/1 mode is projected to be the most likely cause of the final disruption.« less
  • The investigations of major disruptions in the TVD and DAMAVAND tokamaks showed that, in the rapid phase of disruption, accelerated ({approx}1 keV) ions and charge-exchange neutrals are generated near the rational magnetic surfaces; this is accompanied by the bursts of line emission from light impurities (C, O). In the present paper, an analysis is made of the bursts of the CV triplet emission (2271-2278 A) observed over all of the viewing chords in high-current discharges and also of a decrease in the spectral line emission below its initial (predisruption) level both in the plasma core and at the plasma edgemore » in low-current discharges. The data from measurements of the spatial and temporal parameters of the CV line emission from the central and peripheral plasma regions in the rapid phase of disruption in the DAMAVAND tokamak are compared to the results from model calculations of the kinetics of the charge-state distribution of carbon impurity ions (during the disruption, their kinetics is governed by the increase in the effective recombination rate). A key result of the kinetic model is an increase in the effective rate of charge exchange of impurity ions by two orders of magnitude. Numerical simulations show that the dispersion of the charge-state distribution increases substantially; this is attributed to the rapid phase of disruption being dominated by the recombination of impurity ions through charge exchange with neutrals rather than by the anomalous transport. In this case, carbon impurities in the plasma are transported to the region of increased radiative losses on a time scale of 50 {mu}s.« less
  • An analytical theory of the resistive-wall instability of external kink modes in a tokamak allowing for toroidal effects is developed. It is assumed that each mode is a set of a number of poloidal harmonics, one of which is the main harmonic while the others are side-band ones. It is allowed for that some of the side-band harmonics can possess singular points inside the plasma. In describing the interaction of each of these types of harmonics with the remaining ones, the local and nonlocal ballooning effects are taken into account. In addition, the magnetic well/hill related to each singular harmonicmore » is allowed for. A set of approximate (model) equations are derived, which allow for the separate study of the nonlocal ballooning effects and the local effects typical for the Mercier modes (joint action of the magnetic well and local ballooning effects). A general expression for the growth rate, which consists of cylindrical and toroidal parts, is found. The cylindrical part of the growth rate for the case of a parabolic distribution of the longitudinal current is calculated. The contribution to the toroidal part of the growth rate due to both nonsingular and singular side-band harmonics is obtained. It is shown that, in both cases, the side-band harmonics play a destabilizing role and make the instability more sensitive to the value of the plasma pressure, as is observed in experiments. The case when the singular point of the main harmonic is sufficiently close to the plasma boundary is discussed. It is shown that, in this case, the instability can be sensitive to the presence of a magnetic well and to the local ballooning effects related to this harmonic.« less
  • An analytical theory of stabilization of external kink modes in a tokamak with rotating plasma is developed, which is of interest in connection with experiments on the DIII-D tokamak demonstrating such a stabilization. It is assumed that, in addition to the main poloidal harmonic, the mode includes one or more side-band poloidal harmonics with singular points lying inside the plasma. Near these singular points, plasma inertia and related toroidal effects, the compressible part of plasma pressure and longitudinal viscosity, are allowed for. These effects are described kinetically taking into account the toroidal trapping of the resonant ions, which is essentialmore » if the toroidal velocity is small compared to the ion thermal velocity. Thereby, the theory presented includes both ion Landau damping and its weakening due to toroidal trapping. Near the singular points high-beta effects, which result in the finiteness of the Mercier index {ital s}, are allowed for. It is shown that the influence of plasma rotation on the external kink modes is most significant in the case of {ital s}{lt}0, i.e., when the development of the instability in a non-rotating plasma is most highly favored. In this case, the plasma rotation plays a stabilizing role, even when the ion Landau damping is neglected. The analysis presented also confirms the hypothesis of Bondeson and Ward on the stabilizing effect of ion Landau damping if this damping is not too small.« less
  • The effect of rigid toroidal rotation on the stability of a tokamak to external kink modes is examined. For simplicity a surface current model is assumed. For a high-[beta] tokamak it is shown that to leading order the equations governing stability in the presence of rotation are identical to those for a static plasma with [beta] replaced by [beta]+[ital S], where [beta] is the plasma beta and [ital S] is a parameter which provides a measure of the rotation rate. For a circular cross-section tokamak the critical beta for stability to external kink modes in the presence of rigid toroidalmore » rotation is given, to leading order in the inverse aspect ratio [epsilon], by [beta]=0.21[epsilon][minus][ital S]. For an elliptical cross-section tokamak the largest critical beta is obtained for a vertical elongation of 2.2 and is given, to leading order in [epsilon], by [beta]=0.37[epsilon][minus][ital S]. The lower limit on the kink safety factor [ital q]* or the Mercier [ital q] increases with increase in [ital S]. Quantitative estimates of this increase can be obtained from the stability boundaries for the static problem.« less