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Title: Collisionless microinstabilities in stellarators. I. Analytical theory of trapped-particle modes

This is the first in a series of papers about collisionless, electrostatic micro-instabilities in stellarators, with an emphasis on trapped-particle modes. It is found that, in so-called maximum-J configurations, trapped-particle instabilities are absent in large regions of parameter space. Quasi-isodynamic stellarators have this property (approximately), and the theory predicts that trapped electrons are stabilizing to all eigenmodes with frequencies below the electron bounce frequency. The physical reason is that the bounce-averaged curvature is favorable for all orbits, and that trapped electrons precess in the direction opposite to that in which drift waves propagate, thus precluding wave-particle resonance. These considerations only depend on the electrostatic energy balance and are independent of all geometric properties of the magnetic field other than the maximum-J condition. However, if the aspect ratio is large and the instability phase velocity differs greatly from the electron and ion thermal speeds, it is possible to derive a variational form for the frequency showing that stability prevails in a yet larger part of parameter space than what follows from the energy argument. Collisionless trapped-electron modes should therefore be more stable in quasi-isodynamic stellarators than in tokamaks.
Authors:
; ;  [1] ;  [2]
  1. Max-Planck-Institut für Plasmaphysik, EURATOM Association, 17491 Greifswald (Germany)
  2. (Germany)
Publication Date:
OSTI Identifier:
22218344
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 20; Journal Issue: 12; Other Information: (c) 2013 Euratom; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; APPROXIMATIONS; ASPECT RATIO; DRIFT INSTABILITY; MAGNETIC FIELDS; PHASE VELOCITY; PLASMA; PLASMA SIMULATION; STELLARATORS; TOKAMAK DEVICES; TRAPPED ELECTRONS; TRAPPED-PARTICLE INSTABILITY; TRAPPING; VARIATIONAL METHODS; WAVE PROPAGATION