Kinetic damping of low n ballooning modes

Abstract

Summary form only given, as follows. The stability problem of the ballooning mode in tokamaks has been revisited with a fully kinetic shooting code which exploits the Gaussian-Hermite quadrature method for efficient velocity space integration. As recently reported, the ion temperature gradient (/spl eta//sub i/) destabilizes a non-ideal MHD ballooning mode in the MHD second stability regime. Kinetic resonance is not essential for this mode because the two-fluid approximation is able to recover it at least qualitatively. Strong stabilizing influence of the ion temperature gradient on long wavelength, low n ballooning modes has subsequently been found. Modes in the range k/sub /spl theta//spl rho// 0.3. As is well known, in ideal MHD, high n assumption renders more complete energy minimization and high n modes are expected to be more unstable. The present investigation based on kinetic analysis indicates that only high n ballooning modes can be excited in realistic discharges with finite ion temperature gradients. The damping of low n ballooning mode is a kinetic effect due to the ion magnetic drift resonance which is enhanced by the ion temperature gradient. The two-fluid approximation is unable to reveal stabilizing influence of /spl eta//sub i/. The predicted absence of low n ballooning mode, also confirmed in semi-local kinetic analysis, implies that as the MHD ballooning limit is approached, no catastrophic phenomena (e.g., disruption) should occur although enhancement in the anomalous transport is expected.