ELECTROMAGNETIC ION-ACOUSTIC BALLOONING MODE IN TOKAMAKS

It is shown that in toroidal geometry, extremely long-wavelength modes with the ion finite Larmor radius parameter (k perpendicular to rho)(2) similar or equal to O(10(-3)) and mode frequency of the order of the ion acoustic transit frequency can be strongly destabilized by a finite beta (the ratio of plasma to magnetic pressure) well below the MHD ballooning limit. Ion Landau resonance is suppressed by the ion magnetic drift for modes propagating in the electron diamagnetic drift. The quasilinear electron and ion thermal fluxes both increase with beta. In the strong turbulence regime, a Bohm-type thermal diffusivity corrected for finite beta, chi alpha (r/R)(2)(cT/eB)q(2) beta, emerges.