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Electron-cyclotron heated plasmas in the Compact Toroidal Hybrid

Author: Stephen F. Knowlton
Requested Type: Poster Only
Submitted: 2006-12-19 10:42:26

Co-authors: G. Hartwell, J. Hanson, J. Munoz, J. Peterson, J. Shields, A. Stevenson

Contact Info:
Auburn University
206 Allison Laboratory
Auburn, AL   36830

Abstract Text:
Electron-cyclotron heated plasmas of up to 0.5 second duration are produced in the Compact Toroidal Hybrid (CTH). CTH is a low-aspect ratio five-field period torsatron with a continuously-wound l = 2 helical coil (R = 0.75 m, a ≤ 0.2 m, B ≤ 0.7 T). The vacuum rotational transform is variable from i(a) = 0,19 to 0.5 with a set of auxiliary toroidal field coils. RF power at 18 GHz is provided by a 15 kW klystron, with most experiments performed with fundamental resonance heating. Plasma densities of up to the cutoff density 4 x 1018 m-3 are inferred from single-chord 4mm interferometry. Ohmic plasma currents, intended primarily for three-dimensional equilibrium and stability studies, are driven with a solenoidal transformer. Initial experiments have succeeded in generating several kA over a time interval of 50 msec with a simultaneous rise in the line-averaged density. Ongoing experiments are aimed at increasing the level of driven current to strongly modify the vacuum equilibrium fields to enable a test of reconstruction of the 3-D magnetic equilibrium with magnetic diagnostics.

Some flux surfaces in non-axisymmetric configurations such as stellarators may exhibit static magnetic islands and stochastic field line behavior due to the influence of small coil winding errors and background fields. Extensive field-mapping has been performed in CTH to assess the small deviations of the measured field structure from the design configuration with the goal of improving the model of the actual coils sets. An inversion process has been developed to assess from the measured magnetic axis location which coil set is most likely responsible for the observed deviations. Field mapping at several toroidal locations to distinguish n = 0 errors from higher toroidal mode number shifts has enabled a small discrepancies in the poloidal field coils to be identified and largely corrected. Furthermore, magnetic islands observed at the i = 1/3 rational surface are reduced with a set of 15 error-correction coils. Further work will extend to identifying and controlling static magnetic islands in plasmas.

Supported by US DOE Grant DE-FG02-00ER54610

Characterization: E3


University of Maryland

Innovative Confinement Concepts Workshop
February 12-14, 2007
College Park, Maryland

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