KINETIC EFFECTS ON IDEAL BALLOONING STABILITY OF THE TJ-II HELIAC DEVICE

Kinetic corrections to a previous ideal MHD ballooning stability study of the TJ-II heliac standard configuration are numerically calculated using a criterion derived from the gyrokinetic Maxwell equations for modes with perpendicular wavelengths on the scale of the ion gyroradiu s in a general magnetic geometry. In this criterion, finite ion gyrora dius and trapped particle effects are retained at first order. The exi stence of temperature gradients and different ion and electron tempera tures are accounted for. The relevant kinetic regime for TJ-II is foun d to be the intermediate frequency regime, with omega(bi), omega(ti) < gamma < omega(be), omega(te) across all the relevant region whenever electron cyclotron resonance heating (ECRH) plasmas are considered. Th erefore, only trapped electrons are included in the calculations. The threshold poloidal wavenumber m(u) for diamagnetic stabilization of un stable ballooning modes is obtained, and a reduction of about 30% of t he instability region radial width has been found. The trapped electro n contribution to the growth rate is shown to be destabilizing for all modes with poloidal wavenumber below this threshold and is evaluated perturbatively, shoeing that an important part of it is due to those e lectrons trapped in the ripple of the magnetic field.