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.

Ämnesord

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

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

Nyckelord

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

Publikations- och innehållstyp

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