Global magnetohydrodynamic instabilities in the L-2M stellarator
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Global Magnetohydrodynamic Instabilities in the L-2M Stellarator M. I. Mikhailova, S. V. Shchepetovb, C. Nührenbergc, and J. Nührenbergc a
b
National Research Centre Kurchatov Institute, pl. Akademika Kurchatova 1, Moscow, 123182 Russia Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia c Max-Planck-Institut für Plasmaphysik, Wendelsteinstrasse 1, Greifswald, D-17491 Germany e-mail: [email protected], [email protected] Received May 21, 2015
Abstract―Analysis of global magnetohydrodynamic (MHD) instabilities in the L-2M stellarator (Prokhorov General Physics Institute, Russian Academy of Sciences) is presented. The properties of free-boundary equilibria states are outlined, the stability conditions for small-scale modes are briefly discussed, and the number of trapped particles is estimated. All the magnetic configurations under study are stable against ballooning modes. It is shown that global ideal internal MHD modes can be found reliably only in Mercier unstable plasmas. In plasma that is stable with respect to the Mercier criterion, global unstable modes that are localized in the vicinity of the free plasma boundary and are not associated with any rational magnetic surface inside the plasma (the so-called peeling modes) can be found. The radial structure of all perturbations under study is almost entirely determined by the poloidal coupling of harmonics. The results of calculations are compared with the available experimental data. DOI: 10.1134/S1063780X15120107
1. INTRODUCTION The L-2M device (Prokhorov General Physics Institute, Russian Academy of Sciences) is a classical stellarator with a large magnetic shear and plane geometrical axis. The total number of helical field periods is N = 14, the multipolarity is l0 = 2, and the major radius is R0 = 100 cm [1]. The three-dimensional vacuum magnetic surfaces can easily be parameterized by means of a single-valued transformation. The average radius of the magnetic surface increases linearly from a ≡ 0 on the magnetic axis to a = ap = 11.5 cm at the plasma boundary (further, we will use the dimensionless variable x = a/ap). The rotational transform μ* created by external windings in vacuum is μ*(x = 0) = 0.18 on the magnetic axis and μ*(x = 1) = 0.78 at the plasma boundary. The early experiments were carried out at 〈β〉 ≤ 0.25%, where 〈β〉 is the volume-averaged ratio of the plasma pressure to the magnetic field pressure. At present, the experiments are being performed with a more powerful system of electron cyclotron heating [2]. Therefore, according to, e.g., the wellknown ISS-04 stellarator scaling [3], the 〈β〉 value can reach 0.4% at an average density of ne = 2.9 × 1013 cm–3 and a heating power of P = 900 kW. In the present work, the stability of global modes in zero net current plasma equilibria (i.e., those, in which the total current flowing through the cross section of each magnetic surface is zero) is studied using the
CAS3D numerical code [4]. Thus, the only source of instability is the pla
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