Drift ballooning and ITG instabilities in tokamak plasmas

Doctoral thesis, 2004

A major aim in fusion research is to understand, predict and control confined plasma instability in terms of equilibrium, stability and transport. Substantial progress has been made in recent years in understanding stability and transport in terms of MHD instability and drift wave instabilities. The Ballooning and the Ion Temperature Gradient (ITG) modes play important roles in explaining MHD and drift-ballooning instabilities. In this work the collisionless electromagnetic ballooning mode and the toroidally induced ITG mode have been studied by means of a two-fluid model. Eigenvalue equations have been derived analytically and solved numerically. Using the ballooning mode formalism the model investigates the effects of density, temperature and magnetic inhomogeneities, comparison with gyrofluid and gyrokinetic models, comparing reverse and normal shear, finite beta and elongation effects. The stabilizing effects of arbitrary Ln/LB (characteristic length scales of density and magnetic field inhomogeneities) indicate that perpendicular compressibility strongly reduces growth rates and the mode completely stabilizes for Ln/LB ∼ 1 . The model also reproduces two stability boundaries in Ln/LB. When compared with the gyrofluid and gyrokinetic models, the fluid model reproduces the gyrokinetic results qualitatively. However, close to marginal stability only the gyrofluid model reproduces the kinetic eigenvalues quantitatively. Finite beta effects on the ITG mode indicate that the electromagnetic effects are important for high performance discharges whereas the electrostatic approximation is usually sufficient for L-mode equilibria. Experimental relevance of the strong beta dependence on the stabilization of the ITG mode is also shown using data from JET databases. Finite beta and negative shear effects on ITG mode show strong stabilization, improvement in the second stability threshold and reduction in growthrates. With elongation and finite beta effects in the flat density regime the ITG mode slightly destabilizes (large εn), whereas in the peak density profiles (small εn) the mode stabilizes. The stabilization of the ITG mode with large beta and elongation, is expected to reduce anomalous ITG-mode transport.