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epr_2013_cth_panel1.pdf2013-03-05 12:16:49Stephen Knowlton
epr_2013_cth_panel2.pdf2013-03-05 12:14:47Stephen Knowlton

The Compact Toroidal Hybrid stellarator/tokamak: an overview of research and plans

Author: Stephen F. Knowlton
Requested Type: Poster Only
Submitted: 2012-12-06 13:04:45

Co-authors: M. Archmiller, M. Cianciosa, G. Hartwell, J. Hanson, J. Hebert, J. Herfindal, S. Knowlton, X. Ma, D. Maurer, M. Pandya, P. Traverso

Contact Info:
Auburn University
206 Allison Laboratory
Auburn, Alabama   36849

Abstract Text:
The Compact Toroidal Hybrid (CTH), one of the few operating stellarator experiments in the US, provides information on fully 3D magnetic configurations that could lead to a robustly stable fusion power platform. Unusual for present stellarators, CTH (B0 ≤ 0.7 T, R0 = 0.75 m, a ≤ 0.26 m) routinely operates with a substantial ohmically-driven plasma current Ip ≤ 75 kA to explore disruption avoidance in tokamak-like configurations. Results for density-driven disruptions in hybrid discharges show that the strict dependence of the disruptive Greenwald density limit [1] on the plasma current no longer holds when the external rotational transform provided by the stellarator coils is raised. The benefits of externally-applied transform on the passive elimination of low-q disruptions and reducing vertical displacement events in shaped plasmas is also evident in CTH studies.

These studies benefit from 3D equilibrium analysis and improved diagnostics. Measurement of the central electron temperature is estimated from the soft X-ray Bremsstrahlung spectrum, and a new 2-color soft X-ray diode array has been installed for temporal Te profile measurements. More accurate Te profile data will become available from a new Thomson scattering system now under construction. Density profiles are measured with a 3-chord mm-wave interferometer. Magnetic sensors and soft X-ray chordal intensity measurements are used in the V3FIT three-dimensional reconstruction code [2] to determine the time-dependent flux surfaces and rotational transform profiles, and most profile information from diagnostics is fitted to flux surfaces from V3FIT. MHD disruption precursors are analyzed with SVD techniques.
Ongoing studies will (1) further our understanding of disruption avoidance and (2) continue the testing and improvement of 3D reconstruction with V3FIT. To make a firmer connection of these studies to the known disruption characteristics of tokamaks, we propose extending the operation of CTH to the tokamak extreme of zero external rotational transform (the minimum edge vacuum transform attainable at present is ιVAC(a) = 0.04). Conceptual design efforts to revise the coil configuration of CTH to provide a clearer transition to the tokamak regime are under consideration.

Supported by USDOE Grant No. DE-FG02-00ER54610
[1] M.Greenwald, Plasma Phys. Control. Fusion 44 (2002) R27
[2] J. Hanson et al., Nucl. Fusion 49 (2009) 075031

Characterization: 1.3,1.4


University of Texas

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR2013)
February 12-15, 2013
Fort Worth, Texas

EPR 2013