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Pressure- and q-profile effects on ideal infernal modes in tokamaks with an extended region of low magnetic shear
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
2011 (English)In: 38th EPS Conference on Plasma Physics 2011 (EPS 2011): Europhysics Conference Abstracts, 2011, 1664-1667 p.Conference paper (Refereed)
Abstract [en]

An extended region of low magnetic shear is a common feature of the plasma in many tokamak experiments. Such a region can involve a substantial part of the entire plasma volume, for instance in "hybrid scenario" plasmas characterized by a nearly constant safety factor q 1 in the plasma core, and also in equilibria with similar profiles of q that are often developing in spherical tokamaks. Regions around rational surfaces where q is locally flattened could occur in the presence of an island, or as a result of partial reconnection. Such a flattening of q deteriorates the ideal stability of the plasma due to the reduced stabilising effect of field-line bending. A class of global instabilities that can grow in such regions are the "infernal" modes first discussed by Manickam et al. [1]. In the present work we study both q- profile and pressure profile effects on these kind of modes using an analytical formulation of the ideal MHD stability equations for a toroidal plasma with large aspect ratio (LAR). In the next section we present a system of equations useful for analyzing the ideal stability of LAR tokamaks including a region of low magnetic shear between an inner radius rIN and an outer radius r0uT (and finite magnetic shear outside this region). This system is thereafter used to investigate combined q- and pressure profile effects on the infernal modes in low-shear equilibria with q 1 in the core region, in particular the m = n > 1 modes. We consider first a family of profiles of similar nature as those used in Ref. [1], and thereafter look at the stability of a somewhat similar plasma, having low shear in the vicinity of the q = 1 rational surface and q0 < 1 at the magnetic axis. It will be shown that, under certain circumstances, a broad spectrum of infernal n > 1 instabilities can exist in such equilibria, and a possible connection between such modes and the sawtooth instability is pointed out.

Place, publisher, year, edition, pages
2011. 1664-1667 p.
, 38th EPS Conference on Plasma Physics 2011, EPS 2011 - Europhysics Conference Abstracts, 35 2
Keyword [en]
Analytical formulation, Broad spectrum, Core region, Global instability, Large aspect ratio, Magnetic axis, Magnetic shear, MHD stability, Partial reconnection, Plasma core, Plasma volume, Pressure profiles, Q profiles, Sawtooth instability, Spherical Tokamak, System of equations, Tokamak experiment, Toroidal plasmas, Aspect ratio, Magnetism, Magnetohydrodynamics, Plasma stability, Safety factor, Shear flow, Tokamak devices, Magnetoplasma
National Category
Fusion, Plasma and Space Physics
URN: urn:nbn:se:uu:diva-186847ISBN: 9781618395931OAI: oai:DiVA.org:uu-186847DiVA: diva2:574820
38th EPS Conference on Plasma Physics 2011, EPS 2011, 27 June 2011 through 1 July 2011, Strasbourg, France
Available from: 2012-12-06 Created: 2012-11-29 Last updated: 2012-12-06Bibliographically approved

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Wahlberg, Christer
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Department of Physics and Astronomy
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