Effects of limiter biasing on the ATF torsatron

T. Uckan, S. C. Aceto, L. R. Baylor, J. D. Bell, T. S. Bigelow, A. C. England, J. H. Harris, R. C. Isler, T. C. Jernigan, J. F. Lyon, C. H. Ma, P. K. Mioduszewski, M. Murakami, D. A. Rasmussen, J. B. Wilgen, J. J. Zielinski

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4 Scopus citations

Abstract

Positive limiter biasing on the currentless ATF torsatron produces a significant increase in the particle confinement with no improvement in the energy confinement. Experiments have been carried out in 1-T plasmas with ∼400 kW of ECH. Two rail limiters located at the last closed flux surface (LCFS), one at the top and one at the bottom of the device, are biased at positive and negative potentials with respect to the vessel. When the limiters are positively biased at up to 300 V, the density increases sharply to the ECH cutoff value. At the same time, the Hα radiation drops, indicating that the particle confinement improves. When the density is kept constant, the Hα radiation is further reduced and there is almost no change of plasma stored energy. Under these conditions, the density profiles become peaked and the electric field becomes outward-pointing outside the LCFS and more negative inside the LCFS. In contrast, negative biasing yields some reduction of the density and stored energy at constant gas feed, and the plasma potential profile remains the same. Biasing has almost no effect on the intrinsic impurity levels in the plasma.

Original languageEnglish
Pages (from-to)308-311
Number of pages4
JournalJournal of Nuclear Materials
Volume196-198
Issue numberC
DOIs
StatePublished - Dec 1 1992

Funding

Biasing experiments on tokamaks have been very successful in improving the global confinement parameters (to H-mode-like values) by setting up a radial electric field at the plasma edge \[1,2\]. Experiments on the B-3 stellarator \[3\] also showed that radial electric field induced by edge biasing resulted in rapid plasma rotation and improved particle confinement. These experiments have been extended to the current-free Advanced Toroidal Facility \[4\] (ATF) for further study and characterization of the effects of an electric field on plasma confinement. The ATF has a torsatron configuration with l = 2, twelve field periods (M = 12), a major radius R 0 = 2.1 m, and an average plasma radius a = 0.27 m. The current-free magnetic configuration of ATF, which is produced by external means, has moderate shear; the rotational transform (~/2rr = l/q, where q is the safety factor) at the last closed flux surface (LCFS) is ~/2ar = 1, which is about a factor of 3 higher than the central value. Initial biasing experiments have been carried out in plasmas with electron cyclotron heating (ECH). ECH plasmas are created at a magnetic field B = 0.95 T using a 53 GHz gyrotron source with heating power up to PECH ~ 400 kW. In these ECH plasmas, a representative line-averaged * Research sponsored by the Office of Fusion Energy, US Department of Energy, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.

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