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Title: Shear Flow Generation in Stellarators - Configurational Variations

Abstract

Plasma momentum transport within magnetic surfaces plays a fundamental role in a number of toroidal plasma physics issues, such as turbulence suppression, impurity transport, bootstrap current generation and the shielding of resonant magnetic error field perturbations. Stellarators provide opportunities for improved understanding of plasma flow effects because (a) new forms of quasi-symmetry (e.g. helical, poloidal) can be produced that differ significantly from the tokamak and (b) symmetry-breaking effects (always present to some degree) reduce the close coupling between parallel and cross-field transport characteristics of symmetric systems. External control coils can also be used to further enhance or suppress such effects. A method has been developed to evaluate the variation of neoclassical self-generated plasma flows in stellarators both within and across magnetic surfaces. This introduces a new dimension into both the optimization of stellarators and to the improved understanding of the existing confinement database. Application of this model to a range of configurations indicates that flow directionality and shearing rates are significantly influenced by the magnetic structure. In addition, it is demonstrated that flows in stellarators are sensitive to profile effects and the presence of external momentum sources, such as neutral beams.

Authors:
 [1];  [1];  [2];  [1];  [1]
  1. ORNL
  2. University of Wisconsin, Madison
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
931736
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Nuclear Fusion; Journal Volume: 47
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BOOTSTRAP CURRENT; CONFINEMENT; DIMENSIONS; MAGNETIC SURFACES; OPTIMIZATION; PHYSICS; PLASMA; SHEAR; SHIELDING; STELLARATORS; TRANSPORT; TURBULENCE; stellarator; plasma flow; plasma transport

Citation Formats

Spong, Donald A, Harris, Jeffrey H, Ware, A. S., Hirshman, Steven Paul, and Berry, Lee A. Shear Flow Generation in Stellarators - Configurational Variations. United States: N. p., 2007. Web. doi:10.1088/0029-5515/47/7/013.
Spong, Donald A, Harris, Jeffrey H, Ware, A. S., Hirshman, Steven Paul, & Berry, Lee A. Shear Flow Generation in Stellarators - Configurational Variations. United States. doi:10.1088/0029-5515/47/7/013.
Spong, Donald A, Harris, Jeffrey H, Ware, A. S., Hirshman, Steven Paul, and Berry, Lee A. Mon . "Shear Flow Generation in Stellarators - Configurational Variations". United States. doi:10.1088/0029-5515/47/7/013.
@article{osti_931736,
title = {Shear Flow Generation in Stellarators - Configurational Variations},
author = {Spong, Donald A and Harris, Jeffrey H and Ware, A. S. and Hirshman, Steven Paul and Berry, Lee A},
abstractNote = {Plasma momentum transport within magnetic surfaces plays a fundamental role in a number of toroidal plasma physics issues, such as turbulence suppression, impurity transport, bootstrap current generation and the shielding of resonant magnetic error field perturbations. Stellarators provide opportunities for improved understanding of plasma flow effects because (a) new forms of quasi-symmetry (e.g. helical, poloidal) can be produced that differ significantly from the tokamak and (b) symmetry-breaking effects (always present to some degree) reduce the close coupling between parallel and cross-field transport characteristics of symmetric systems. External control coils can also be used to further enhance or suppress such effects. A method has been developed to evaluate the variation of neoclassical self-generated plasma flows in stellarators both within and across magnetic surfaces. This introduces a new dimension into both the optimization of stellarators and to the improved understanding of the existing confinement database. Application of this model to a range of configurations indicates that flow directionality and shearing rates are significantly influenced by the magnetic structure. In addition, it is demonstrated that flows in stellarators are sensitive to profile effects and the presence of external momentum sources, such as neutral beams.},
doi = {10.1088/0029-5515/47/7/013},
journal = {Nuclear Fusion},
number = ,
volume = 47,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • The shear Alfven spectrum in three-dimensional configurations, such as stellarators and rippled tokamaks, is more densely populated due to the larger number of mode couplings caused by the variation in the magnetic field in the toroidal dimension. This implies more significant computational requirements that can rapidly become prohibitive as more resolution is requested. Alfven eigenfrequencies and mode structures are a primary point of contact between theory and experiment. A new algorithm based on the Jacobi-Davidson method is developed here and applied for a reduced magnetohydrodynamics model to several stellarator configurations. This technique focuses on finding a subset of eigenmodes clusteredmore » about a specified input frequency. This approach can be especially useful in modeling experimental observations, where the mode frequency can generally be measured with good accuracy and several different simultaneous frequency lines may be of interest. For cases considered in this paper, it can be a factor of 10{sup 2}-10{sup 3} times faster than more conventional methods.« less
  • The generation of shear flow driven by a large amplitude drift wave, represented by the Hasegawa--Mima--Charney (HMC) equation, has been investigated. It is shown that, besides a finite amplitude threshold for the shear flow instability to occur, there is also a necessary condition on the aspect ratio of the large amplitude drift wave, due to conservation of the average potential vorticity, that needs to be satisfied. A comprehensive comparison of the shear-flow instability criterion for the HMC equation and the incompressible, invisicid, hydrodynamic equation has been undertaken. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.
  • At the Nevada Terawatt Facility we investigated the generation of a sheared plasma flow using conical wire arrays with an additional wire located on the axis of the pinch. The additional center wire generates axial current carrying plasma that serves as a target for the plasma accelerated from the outer wires, generating a sheared plasma flow which leads to the growth of the Kelvin-Helmholtz instability. These experiments were conducted on Zebra, a 2 TW pulse power device capable of delivering a 1 MA current in 100 ns. This paper will focus on the implosion dynamics that lead to shear flowmore » and the development of the Kelvin Helmholtz instability.« less
  • Suppression of resistive [ital g]-mode turbulence by background shear flow generated from a small external flow source and amplified by the fluctuation-induced Reynolds stress is demonstrated and analyzed. The model leads to a paradigm for the low-to-high (L--H) confinement mode transition. To demonstrate the L--H transition model, single-helicity nonlinear fluid simulations using the vorticity equation for the electrostatic potential, the pressure fluctuation equation, and the background poloidal flow equation are used in the sheared slab configuration. The relative efficiency of the external flow and the Reynolds stress for producing shear flow depends on the poloidal flow damping parameter [nu], whichmore » is given by neoclassical theory. For large [nu], the external flow is a dominant contribution to the total background poloidal shear flow and its strength predicted by the neoclassical theory is not enough to suppress the turbulence significantly. In contrast, for small [nu], it is shown that the fluctuations drive a Reynolds stress that becomes large and suddenly, at some critical point in time, shear flow much larger than the external flow is generated and leads to an abrupt, order unity reduction of the turbulent transport just like that of the L--H transition in tokamak experiments. It is also found that, even in the case of no external flow, the shear flow generation due to the Reynolds stress occurs through the nonlinear interaction of the resistive [ital g] modes and reduces the transport. To supplement the numerical solutions, the Landau equation for the mode amplitude of the resistive [ital g] mode is derived, taking into account the fluctuation-induced shear flow and the opposite action of the Reynolds stress in the resistive [ital g] turbulence compared with the classical shear flow Kelvin--Helmholtz (KH) driven turbulence is analyzed.« less