Modeling of diamagnetic stabilization of ideal magnetohydrodynamic instabilities associated with the transport barrier

A new code, MISHKA-D (Drift MHD), has been developed as an extension of the ideal magnetohydrodynamics (MHD) code MISHKA-1 in order to investigate the finite gyroradius stabilizing effect of ion diamagnetic drift frequency, omega (*i), on linear ideal MHD eigenmodes in tokamaks in general toroidal geometry. The MISHKA-D code gives a self-consistent computation of both stable and unstable eigenmodes with eigenvalues \gamma\congruent to omega (*i) in plasmas with strong radial variation in the ion diamagnetic frequency. Test results of the MISHKA-D code show good agreement with the analytically obtained omega*(i) spectrum and stability limits of the internal kink mode, n/m=1/1, used as a benchmark case. Finite-n ballooning and low-n kink (peeling) modes in the edge transport barrier just inside the separatrix are studied for high confinement mode (H-mode) plasmas with the omega (*i) effect included. The ion diamagnetic stabilization of the ballooning modes is found to be most effective for narrow edge pedestals. For low enough plasma density the omega (*i) stabilization can lead to a second zone of ballooning stability, in which all the ballooning modes are stable for any value of the pressure gradient. For internal transport barriers typical of the Joint European Torus [JET, P. H. Rebut , Proceedings of the 10th International Conference, Plasma Physics and Controlled Nuclear Fusion, London (International Atomic Energy Agency, Vienna, 1985), Vol. I, p. 11] optimized shear discharges, the stabilizing influence of ion diamagnetic frequency on the n=1 global pressure driven disruptive mode is studied. A strong radial variation of omega (*i) is found to significantly decrease the stabilizing omega (*i) effect on the n=1 mode, in comparison with the case of constant omega (*i) estimated at the foot of the internal transport barrier. (C) 2001 American Institute of Physics.