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Title: Novel design methods for magnetic flux loops in the National Compact Stellarator Experiment

Abstract

Magnetic pickup loops on the vacuum vessel (VV) can provide an abundance of equilibrium information for stellarators. A substantial effort has gone into designing flux loops for the National Compact Stellarator Experiment (NCSX) [Zarnstorff et al., Plasma Phys. Controlled Fusion 43, A237 (2001)], a three-field period quasi-axisymmetric stellarator under construction at the Princeton Plasma Physics Laboratory. The design philosophy, to measure all of the magnetic field distributions normal to the VV that can be measured, has necessitated the development of singular value decomposition algorithms for identifying efficient loop locations. Fields are expected to be predominantly stellarator symmetric (SS)--the symmetry of the machine design--with toroidal mode numbers per torus (n) equal to a multiple of 3 and possessing reflection symmetry in a period. However, plasma instabilities and coil imperfections will generate non-SS fields that must also be diagnosed. The measured symmetric fields will yield important information on the plasma current and pressure profile as well as on the plasma shape. All fields that obey the design symmetries could be measured by placing flux loops in a single half-period of the VV, but accurate resolution of nonsymmetric modes, quantified by the condition number of a matrix, requires repositioning loops to equivalent locationsmore » on the full torus. A subarray of loops located along the inside wall of the vertically elongated cross section was designed to detect n=3, m=5 or 6 resonant field perturbations that can cause important islands. Additional subarrays included are continuous in the toroidal and poloidal directions. Loops are also placed at symmetry points of the VV to obtain maximal sensitivity to asymmetric perturbations. Combining results from various calculations which have made extensive use of a database of 2500 free-boundary VMEC equilibria, has led to the choice of 225 flux loops for NCSX, of which 151 have distinct shapes.« less

Authors:
; ; ; ;  [1];  [2];  [2];  [2];  [2]
  1. Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
20975042
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2472368; (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; ALGORITHMS; ASYMMETRY; AXIAL SYMMETRY; DESIGN; DISTURBANCES; ELECTRIC CURRENTS; EQUILIBRIUM; INFORMATION; MAGNETIC FIELDS; MAGNETIC FLUX; PLASMA; PLASMA CONFINEMENT; PLASMA DIAGNOSTICS; PLASMA INSTABILITY; PLASMA PRESSURE; STELLARATORS

Citation Formats

Pomphrey, N., Lazarus, E., Zarnstorff, M., Boozer, A., Brooks, A., Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543, Columbia University, New York, New York 10027, and Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543. Novel design methods for magnetic flux loops in the National Compact Stellarator Experiment. United States: N. p., 2007. Web. doi:10.1063/1.2472368.
Pomphrey, N., Lazarus, E., Zarnstorff, M., Boozer, A., Brooks, A., Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543, Columbia University, New York, New York 10027, & Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543. Novel design methods for magnetic flux loops in the National Compact Stellarator Experiment. United States. doi:10.1063/1.2472368.
Pomphrey, N., Lazarus, E., Zarnstorff, M., Boozer, A., Brooks, A., Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543, Columbia University, New York, New York 10027, and Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543. Tue . "Novel design methods for magnetic flux loops in the National Compact Stellarator Experiment". United States. doi:10.1063/1.2472368.
@article{osti_20975042,
title = {Novel design methods for magnetic flux loops in the National Compact Stellarator Experiment},
author = {Pomphrey, N. and Lazarus, E. and Zarnstorff, M. and Boozer, A. and Brooks, A. and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 and Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543 and Columbia University, New York, New York 10027 and Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543},
abstractNote = {Magnetic pickup loops on the vacuum vessel (VV) can provide an abundance of equilibrium information for stellarators. A substantial effort has gone into designing flux loops for the National Compact Stellarator Experiment (NCSX) [Zarnstorff et al., Plasma Phys. Controlled Fusion 43, A237 (2001)], a three-field period quasi-axisymmetric stellarator under construction at the Princeton Plasma Physics Laboratory. The design philosophy, to measure all of the magnetic field distributions normal to the VV that can be measured, has necessitated the development of singular value decomposition algorithms for identifying efficient loop locations. Fields are expected to be predominantly stellarator symmetric (SS)--the symmetry of the machine design--with toroidal mode numbers per torus (n) equal to a multiple of 3 and possessing reflection symmetry in a period. However, plasma instabilities and coil imperfections will generate non-SS fields that must also be diagnosed. The measured symmetric fields will yield important information on the plasma current and pressure profile as well as on the plasma shape. All fields that obey the design symmetries could be measured by placing flux loops in a single half-period of the VV, but accurate resolution of nonsymmetric modes, quantified by the condition number of a matrix, requires repositioning loops to equivalent locations on the full torus. A subarray of loops located along the inside wall of the vertically elongated cross section was designed to detect n=3, m=5 or 6 resonant field perturbations that can cause important islands. Additional subarrays included are continuous in the toroidal and poloidal directions. Loops are also placed at symmetry points of the VV to obtain maximal sensitivity to asymmetric perturbations. Combining results from various calculations which have made extensive use of a database of 2500 free-boundary VMEC equilibria, has led to the choice of 225 flux loops for NCSX, of which 151 have distinct shapes.},
doi = {10.1063/1.2472368},
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}
}
  • The National Compact Stellarator Experiment will have a complete set of magnetic diagnostics to constrain equilibrium reconstructions and for plasma control. The flux loops lying on the exterior surface of the vacuum vessel and the flux loops cowound with the field coils must be installed during machine construction because they will later be inaccessible. Designs and installation techniques for these diagnostics are described.
  • The National Compact Stellarator Experiment (NCSX) is a three-field period compact stellarator presently in the construction phase at Princeton, NJ. The design parameters of the device are major radius R=1.4m, average minor radius <a> = 0.32m, 1.2 {le} toroidal field (B{sub t}) {le} 1.7 T, and auxiliary input power up to 12 MW with neutral beams and radio-frequency heating. The NCSX average aspect ratio <R/a> of 4.4 lies well below present stellarator experiments and designs, enabling the investigation of high {beta} physics in a compact stellarator geometry. Also the NCSX design choice for a quasi-axisymmetric configuration aims toward the achievementmore » of tokamak-like transport. In this paper, we report on the magnetic field line tracing calculations used to evaluate conceptual plasma facing component (PFC) designs. In contrast to tokamaks, axisymmetric target plates are not required to intercept the majority of the heat flux in stellarators, owing to the nature of the 3-D magnetic field footprint. The divertor plate design investigated in this study covers approximately one half of the toroidal extent in each period. Typical Poincare plots in Figure 1 illustrate the plasma cross-section at several toroidal angles for a computed NCSX high-beta equilibrium. The plates used for these calculations are centered in each period about the elongated cross-section shown in Figure 1a, extending to +/- {pi}/6 in each direction. Two methods for tracing the edge field line topology were used in this study. The first entails use of the VMEC/MFBE-2001 packages, whereas the second entails use of the PIES code with a post-processor by Michael Drevlak; the same field line integration routine was used to evaluate the equilibria for this comparison. Both inputs were generated based on the {beta}=4%, =iota=0.5 equilibrium computed from the final NCSX coil set. We first compare these two methods for a specific plate geometry, and conclude with a comparison of the strike characteristics for two different target plate poloidal lengths using the latter method. The details of the magnetic topology differ when computed with VMEC/MFBE as compared with an iterated PIES solution. This difference is illustrated in Figure 2. The presence of islands in the PIES solution effectively reduces the radius of the last closed magnetic surface (LCMS) by about 8 cm. As expected, this difference in the edge topology translates to a difference in field line terminations.« less
  • A new experiment, the National Compact Stellarator Experiment (NCSX) [Monticello {ital et al.} {open_quotes}Physics Consideration for the Design of NCSX,{close_quotes} {ital Proceedings of 25th EPS Conference on Controlled Fusion and Plasma Physics, Prague, 1998} (European Physical Society, Petit-Lancy), paper 1.187], hopes to overcome the deleterious ripple transport usually associated with stellarators by creating a quasi-axisymmetric configuration. A quasi-axisymmetric configuration is one in which the Fourier spectrum of the magnetic field strength in so-called Boozer coordinates is dominated by the toroidal angle averaged (n=0) components. In this article the concept of pseudosymmetry is used to improve ripple transport in a four-periodmore » variant of NCSX. By definition, pseudosymmetric magnetic configurations have no locally trapped particles. To obtain a pseudosymmetric configuration, different target functions are considered. It is found that a target function equal to the area of ripple of the magnetic field magnitude along the field line is very effective in reducing the neoclassical transport coefficient. {copyright} {ital 1999 American Institute of Physics.}« less