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Title: Influence of magnetic shear on impurity transport

Abstract

The magnetic shear dependence of impurity transport in tokamaks is studied using a quasilinear fluid model for ion temperature gradient (ITG) and trapped electron (TE) mode driven turbulence in the collisionless limit and the results are compared with nonlinear gyrokinetic results using GYRO [J. Candy and R. E. Waltz, J. Comput. Phys 186, 545 (2003)]. It is shown that the impurity transport is sensitive to the magnetic shear, in particular for weak, negative, and large positive shear where a strong reduction of the effective impurity diffusivity is obtained. The fluid and gyrokinetic results are in qualitative agreement, with the gyrokinetic diffusivities typically a factor 2 larger than the fluid diffusivities. The steady state impurity profiles in source-free plasmas are found to be considerably less peaked than the electron density profiles for moderate shear. Comparisons between anomalous and neoclassical transport predictions are performed for ITER-like profiles [R. Aymar, P. Barabaschi, and Y. Shimomura, Plasma Phys. Controlled Fusion 44, 519 (2002)].

Authors:
; ; ; ;  [1];  [2];  [3]
  1. Department of Radio and Space Science, Chalmers University of Technology and Euratom-VR Association, Goeteborg (Sweden)
  2. (United States)
  3. (Sweden)
Publication Date:
OSTI Identifier:
20974973
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2730491; (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; COMPARATIVE EVALUATIONS; ELECTRON DENSITY; ELECTRON TEMPERATURE; ION TEMPERATURE; ITER TOKAMAK; NEOCLASSICAL TRANSPORT THEORY; NONLINEAR PROBLEMS; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA IMPURITIES; PLASMA INSTABILITY; SHEAR; TEMPERATURE GRADIENTS; TRAPPED ELECTRONS

Citation Formats

Nordman, H., Fueloep, T., Candy, J., Strand, P., Weiland, J., General Atomics, P.O. Box 85608, San Diego, California 92186-5608, and Department of Radio and Space Science, Chalmers University of Technology and Euratom-VR Association, Goeteborg. Influence of magnetic shear on impurity transport. United States: N. p., 2007. Web. doi:10.1063/1.2730491.
Nordman, H., Fueloep, T., Candy, J., Strand, P., Weiland, J., General Atomics, P.O. Box 85608, San Diego, California 92186-5608, & Department of Radio and Space Science, Chalmers University of Technology and Euratom-VR Association, Goeteborg. Influence of magnetic shear on impurity transport. United States. doi:10.1063/1.2730491.
Nordman, H., Fueloep, T., Candy, J., Strand, P., Weiland, J., General Atomics, P.O. Box 85608, San Diego, California 92186-5608, and Department of Radio and Space Science, Chalmers University of Technology and Euratom-VR Association, Goeteborg. Tue . "Influence of magnetic shear on impurity transport". United States. doi:10.1063/1.2730491.
@article{osti_20974973,
title = {Influence of magnetic shear on impurity transport},
author = {Nordman, H. and Fueloep, T. and Candy, J. and Strand, P. and Weiland, J. and General Atomics, P.O. Box 85608, San Diego, California 92186-5608 and Department of Radio and Space Science, Chalmers University of Technology and Euratom-VR Association, Goeteborg},
abstractNote = {The magnetic shear dependence of impurity transport in tokamaks is studied using a quasilinear fluid model for ion temperature gradient (ITG) and trapped electron (TE) mode driven turbulence in the collisionless limit and the results are compared with nonlinear gyrokinetic results using GYRO [J. Candy and R. E. Waltz, J. Comput. Phys 186, 545 (2003)]. It is shown that the impurity transport is sensitive to the magnetic shear, in particular for weak, negative, and large positive shear where a strong reduction of the effective impurity diffusivity is obtained. The fluid and gyrokinetic results are in qualitative agreement, with the gyrokinetic diffusivities typically a factor 2 larger than the fluid diffusivities. The steady state impurity profiles in source-free plasmas are found to be considerably less peaked than the electron density profiles for moderate shear. Comparisons between anomalous and neoclassical transport predictions are performed for ITER-like profiles [R. Aymar, P. Barabaschi, and Y. Shimomura, Plasma Phys. Controlled Fusion 44, 519 (2002)].},
doi = {10.1063/1.2730491},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • One of the scientific success stories of fusion research over the past decade is the development of the E{times}B shear stabilization model to explain the formation of transport barriers in magnetic confinement devices. This model was originally developed to explain the transport barrier formed at the plasma edge in tokamaks after the L (low) to H (high) transition. This concept has the universality needed to explain the edge transport barriers seen in limiter and divertor tokamaks, stellarators, and mirror machines. More recently, this model has been applied to explain the further confinement improvement from H (high) mode to VH (verymore » high) mode seen in some tokamaks, where the edge transport barrier becomes wider. Most recently, this paradigm has been applied to the core transport barriers formed in plasmas with negative or low magnetic shear in the plasma core. These examples of confinement improvement are of considerable physical interest; it is not often that a system self-organizes to a higher energy state with reduced turbulence and transport when an additional source of free energy is applied to it. The transport decrease that is associated with E{times}B velocity shear effects also has significant practical consequences for fusion research. The fundamental physics involved in transport reduction is the effect of E{times}B shear on the growth, radial extent, and phase correlation of turbulent eddies in the plasma. The same fundamental transport reduction process can be operational in various portions of the plasma because there are a number of ways to change the radial electric field E{sub r}. An important theme in this area is the synergistic effect of E{times}B velocity shear and magnetic shear. Although the E{times}B velocity shear appears to have an effect on broader classes of microturbulence, magnetic shear can mitigate some potentially harmful effects of E{times}B velocity shear and facilitate turbulence stabilization. (Abstract Truncated)« less
  • Impurity transport in tokamak core plasmas is investigated with a three-dimensional fluid global code. It is shown that, in the presence of an internal transport barrier (ITB) created by a reversed magnetic shear configuration, one can obtain a reversal of the impurity pinch velocity which can change from the inward direction to the outward direction. This scenario is favorable for expelling impurities from the central region and decontaminating the core plasma. The mechanism of pinch reversal is attributed to a change of direction of the curvature pinch and to a modification of the dominant underlying instability caused by a changemore » of the gradient of the ion temperature and consequently of the ITB formation.« less
  • Development of the E x B shear stabilization model to explain the formation of transport barriers in magnetic confinement devices is a major achievement of fusion research. This concept has the universality needed to explain the H-mode edge transport barriers seen in limiter and divertor tokamaks, stellarators, and mirror machines; the broader edge transport barrier seen in VH-mode plasmas; and the core transport barriers formed in tokamaks with low or negative magnetic shear. These examples of confinement improvement are of considerable physical interest; it is not often that a system self-organizes to reduce transport when an additional source of freemore » energy is applied to it. The transport decrease associated with E x B velocity shear is also of great practical benefit to fusion research. The fundamental physics involved in transport reduction is the effect of E x B shear on the growth, radial extent, and phase correlation of turbulent eddies in the plasma. The same basic transport reduction process can be operational in various portions of the plasma because there are a number of ways to change the radial electric field E{sub r}. An important theme in this area is the synergistic effect of E x B velocity shear and magnetic shear. Although the E x B velocity shear appears to have an effect on broader classes of microturbulence, magnetic shear can mitigate some potentially harmful effects of E x B velocity shear and facilitate turbulence stabilization. The experimental results on DIII-D and other devices are generally consistent with the basic theoretical models.« less
  • A nonlocal theory of the electrostatic parallel velocity shear instability in a three-dimensional slab with a uniformly sheared magnetic field has been developed. It is shown that in the limit of a weak parallel velocity gradient, the linear growth rate can be increased depending upon the direction of the magnetic shear (s) with respect to the radial curvature of the parallel velocity profile (d{sup 2}v{sub {parallel}}/dx{sup 2}). When these parameters have the same sign, the growth rate can actually be stronger than in the limit of no magnetic shear. In this limit of increased instability, the eigenmode is broadened, thusmore » producing enhanced transport. This effect should be observable when the scale length of the curvature is of order {approx}L{sub s}{rho}{sub s}. For strong parallel velocity gradients that are more typical of flows in tokamaks, the effect of the varying Doppler shift becomes more prominent on the stability of the mode, the net result being that the sensitivity of the growth rates on the sign of the magnetic shear becomes insignificant. This effect, however, is effectively offset when a finite density gradient is included. When the density scale length is of order the scale length of v{sub {parallel}}, the growth rate is moderately reduced, but becomes dependent again upon the sign of the magnetic shear.« less
  • Observations of radiation from iron and from argon used as a test gas indicate that co-injection inhibits impurity accumulation in the interior of ISX-B discharges, but counter-injection enhances accumulation. These results agree qualitatively with recent theoretical calculations.