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Disruption avoidance by application of 3D stellarator equilibrium fields to tokamak discharges

Author: David A. Maurer
Requested Type: Consider for Invited
Submitted: 2012-12-06 12:57:41

Co-authors: David Maurer, Matthew Archmiller, Gregory Hartwell, James Hanson, Jeffrey Herfindal, Stephen Knowlton, Xinxing Ma, Mihir Pandya

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

Abstract Text:
The avoidance and mitigation of major disruptions remains a critical challenge for ITER and future burning plasma tokamaks. While it is generally understood that even high-beta stellarators are likely to be immune from disruptions associated with an unstable toroidal current profile, it may prove that the addition of modest external rotational transform to tokamak discharges expand the disruption-free operating regime of tokamaks, and potentially reduce the severity of disruptions when they do occur [1]. Experiments targeting the avoidance of several classes of disruptions in hybrid stellarator/tokamak configurations are performed in the Compact Toroidal Hybrid (CTH), a five-field period torsatron with auxiliary toroidal field coils to vary the externally-applied transform. CTH also has an ohmic heating system to provide a toroidal current in stellarator plasma generated by ECRH (B0 ≤ 0.7 T, R0 = 0.75 m, a ≤ 0.26 m, Ip ≤ 75 kA). These studies follow up work on early stellarators and innovative hybrids [2-4].

Deliberate current-driven disruptions generated in CTH discharges result from excessive density, low safety factor, and uncontrolled vertical displacement. While the density limit at the lowest edge vacuum transform (ιVAC(a) = 0.04) presently achievable in CTH is found to be in accord with the empirical Greenwald limit [5], the experimental densities exceed this limit by up to a factor of three as the vacuum transform is raised to ιVAC(a) = 0.25. Density-limit disruptions exhibit similar MHD precursors as in tokamaks. Low-q disruptions near ι(a) = 0.5 are also observed in CTH when the vacuum transform is low, but no longer take place when the vacuum transform is raised above ιVAC(a) > 0.055, even though the edge transform rise above the value of ½. Passive suppression of vertical displacement events (VDE) in elongated current-carrying plasmas is also observed with the addition of external transform. The level of external transform required to suppress vertical motion is in agreement with simple analytic estimates of marginal stability [6].

Supported by USDOE Grant No. DE-FG02-00ER54610
[1] A. H. Boozer, Phys. Plasmas 16 (2009) 058102
[2] H. Ikezi and K. F. Schwarzenegger, Phys. Fluids 22 (1979) 2010
[3] W VII-A Team, Nucl. Fusion, 20, 1093 (1980)
[4] J. Fujita, S. Itoh, K, Kadota, et al., in Plasma Phys. and Control. Fusion Res.(Proc. 8th Int. Conf., Brussels,1980) Vol. 1, 209 IAEA Vienna (1981)
[5] M. Greenwald, Plasma Phys. Control. Fusion 44 (2002) R27
[6] G.Y. Fu, Phys. Plasmas 7 (2000) 1079

Characterization: 1.3,1.4

Comments:
If not given a talk please place with the other Auburn poster submission. Thank you.