Impact of ideal MHD stability limits on high-beta hybrid operation

The Asdex Upgrade Team, The Diii-D Team, P. Piovesan, V. Igochine, F. Turco, D.A. Ryan, M.R. Cianciosa, Y.Q. Liu, L. Marrelli, D. Terranova, R.S. Wilcox, A. Wingen, C. Angioni, A. Bock, C. Chrystal, I. Classen, M. Dunne, N.M. Ferraro, R. Fischer, A. GudeC.T. Holcomb, A. Lebschy, T.C. Luce, M. Maraschek, R. McDermott, T. Odstrčil, C. Paz-Soldan, M. Reich, M. Sertoli, W. Suttrop, N.Z. Taylor, M. Weiland, M. Willensdorfer

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Abstract

The hybrid scenario is a candidate for stationary high-fusion gain tokamak operation in ITER and DEMO. To obtain such performance, the energy confinement and the normalized pressure βN must be maximized, which requires operating near or above ideal MHD no-wall limits. New experimental findings show how these limits can affect hybrid operation. Even if hybrids are mainly limited by tearing modes, proximity to the no-wall limit leads to 3D field amplification that affects plasma profiles, e.g. rotation braking is observed in ASDEX Upgrade throughout the plasma and peaks in the core. As a result, even the small ASDEX Upgrade error fields are amplified and their effects become visible. To quantify such effects, ASDEX Upgrade measured the response to 3D fields applied by 8�2 non-axisymmetric coils as βN approaches the no-wall limit. The full n = 1 response profile and poloidal structure were measured by a suite of diagnostics and compared with linear MHD simulations, revealing a characteristic feature of hybrids: the n = 1 response is due to a global, marginally-stable n = 1 kink characterized by a large m = 1, n = 1 core harmonic due to qmin being just above 1. A helical core distortion of a few cm forms and affects various core quantities, including plasma rotation, electron and ion temperature, and intrinsic W density. In similar experiments, DIII-D also measured the effect of this helical core on the internal current profile, providing information useful to understanding of the physics of magnetic flux pumping, i.e. anomalous current redistribution by MHD modes that keeps qmin>1. Thanks to flux pumping, a broad current profile is maintained in DIII-D even with large on-axis current drive, enabling fully non-inductive operation at high βN up to 3.5-4.
Original languageEnglish
Article number014027
Pages (from-to)-
JournalPlasma Physics and Controlled Fusion
Volume59
Issue number1
DOIs
Publication statusPublished - 1 Jan 2017
Externally publishedYes

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All Science Journal Classification (ASJC) codes

  • Nuclear Energy and Engineering
  • Condensed Matter Physics

Cite this

Asdex Upgrade Team, T., Diii-D Team, T., Piovesan, P., Igochine, V., Turco, F., Ryan, D. A., Cianciosa, M. R., Liu, Y. Q., Marrelli, L., Terranova, D., Wilcox, R. S., Wingen, A., Angioni, C., Bock, A., Chrystal, C., Classen, I., Dunne, M., Ferraro, N. M., Fischer, R., ... Willensdorfer, M. (2017). Impact of ideal MHD stability limits on high-beta hybrid operation. Plasma Physics and Controlled Fusion, 59(1), -. [014027]. https://doi.org/10.1088/0741-3335/59/1/014027