Gyrokinetic analysis of linear microinstabilities for the stellarator Wendelstein 7-X

A linear collisionless gyrokinetic investigation of ion temperature gradient (ITG) modes-considering both adiabatic and full electron dynamics-and trapped electron modes (TEMs) is presented for the stellarator Wendelstein 7-X (W7-X) [G. Grieger , Plasma Physics and Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 525]. The study of ITG modes reveals that in W7-X, microinstabilities of distinct character coexist. The effect of changes in the density gradient and temperature ratio is discussed. Substantial differences with respect to the axisymmetric geometry appear in W7-X, concerning the relative separation of regions with a large fraction of helically trapped particles and those of pronounced bad curvature. For both ITG modes and TEMs, the dependence of their linear growth rates on the background gradients is studied along with their parallel mode structure. (c) 2007 American Institute of Physics.