Pressure- and q-profile effects on ideal infernal modes in tokamaks with an extended region of low magnetic shear

• 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.

Subject headings

NATURVETENSKAP  -- Fysik -- Fusion, plasma och rymdfysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Fusion, Plasma and Space Physics (hsv//eng)

Keyword

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

Publication and Content Type

ref (subject category)

kon (subject category)