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icc_2011_paper_cth.pdf2011-08-26 15:48:37Stephen Knowlton

Disruption avoidance in current-carrying tokamak/stellarator discharges in CTH

Author: Stephen F. Knowlton
Requested Type: Poster Only
Submitted: 2011-06-10 16:54:49

Co-authors: J.D. Hanson, G. Hartwell, M. C. Miller, B. A. Stevenson

Contact Info:
Auburn University
206 Allison Laboratory
Auburn, AL   36849
USA

Abstract Text:
The prediction and mitigation of major disruptions of toroidal plasmas continues to be of crucial interest to fusion research because the control of disruptions remains a serious concern for the reliable operation of ITER. Stellarators can provide inherently steady-state plasma sustainment, and are also attractive because disruptions of stellarator discharges rarely occur, and limiting behavior is typically more benign than in tokamaks. The behavior of disruptions in stellarators is nonetheless of interest in helical configurations with tokamak-like levels of bootstrap current, e.g. finite-beta stellarators and quasi-axisymmetric devices. Moreover, it is conceivable that an innovative stellarator-tokamak hybrid configuration with flexible three-dimensional field shaping could resolve some of the key challenges of tokamak confinement, i.e. robust disruption avoidance and sustainment of the rotational transform [1].
A fundamental study of 3D equilibrium and the suppression of disruptions is underway on the Compact Toroidal Hybrid (CTH) experiment (R0 = 0.75 m, <a> ≤ 0.2 m, B0 ≤ 0.7 T, line-averaged electron density = 0.2 – 3 x 1019 m-3). CTH is a flexible heliotron capable of operating with significant ohmic current. At B0 = 0.6 T, the edge stellarator (vacuum field) rotational transform is variable from ι_st (a) = 0.02 to 0.4 in plasmas generated by electron-cyclotron resonant heating at 14 and 17.65 GHz. Plasma currents up to 65 kA are driven with an ohmic transformer to generate net edge transforms as high as ι(a) = 0.6 in sawtoothing discharges (ι(0) ~ 1) without disruption. Major disruptions characterized by one or more large negative loop voltage spikes during the current quench preceded by growing resonant MHD activity can be triggered at low vacuum transforms. However, complete disruptions are not observed in discharges with higher vacuum transform ι_st(a) ≥ 0.1, although partial current collapses may still take place on an resistive time-scale. Transient MHD oscillations also occur in non-disrupting discharges.
The 3D MHD equilibria of current-carrying CTH discharges are deduced with the V3FIT code using external magnetic diagnostics. Insertable Hall probes to measure the internal poloidal field and soft X-ray tomography are also applied to the reconstruction process . Efforts are underway to model disruptive CTH discharges with NIMROD with the help of the ICC PSI Center at the University of Washington.

[1] A. H. Boozer, Phys. Plasmas 16, 058102 (2009)
[2.] J. D. Hanson et al, Nucl. Fusion 49 075031 (2009).


This work was supported by the U.S. Department of Energy under grant DE-FG02-00ER54610.

Characterization: A4,A5

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University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011