Safety factor profile shape and the ideal magnetohydrodynamic stability of n = 1 internal modes

P. Detragiache

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The ideal magnetohydrodynamic (MHD) stability of n = 1 internal modes in an axisymmetric toroidal configuration is investigated for plasma equilibria with rather general profiles of the safety factor q using an analytical approach (Bussac et al 1975 Phys. Rev. Lett. 35 1638). We find that instability strongly depends on the q radial profile and that, characteristically, it occurs for values of the relevant poloidal beta parameter βp1,crit in the range of 0.1 to 0.2. These values are considerably lower than the often quoted value βp1,crit = 0.3, obtained for a parabolic q-profile in the limit 1 - q0 ≪ 1. In general, q-profiles with a large central zone of low magnetic shear and an outer zone of high magnetic shear have worse stability properties than profiles with a smooth shear variation across the whole plasma column. Effects due to shaping of the plasma boundary are also considered. Ellipticity is destabilizing, while triangularity is stabilizing. The overall effect of shaping depends on the value of the magnetic shear inside the q = 1 surface, and low values are required for the beneficial effect of triangularity to prevail over that of ellipticity. © 1998 IOP Publishing Ltd.
Original languageEnglish
Pages (from-to)1501 - 1512
Number of pages12
JournalPlasma Physics and Controlled Fusion
Issue number8
Publication statusPublished - 1998
Externally publishedYes


All Science Journal Classification (ASJC) codes

  • Physics and Astronomy(all)
  • Condensed Matter Physics
  • Nuclear and High Energy Physics

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