Pressure- and q-profile effects on ideal infernal modes in tokamaks with an extended region of low magnetic shear
2011 (English)In: 38th EPS Conference on Plasma Physics 2011 (EPS 2011): Europhysics Conference Abstracts, 2011, 1664-1667 p.Conference paper (Refereed)
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. . 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. , 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
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
Fusion, Plasma and Space Physics
IdentifiersURN: 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