Full Toroidal Geometry Research Articles

Resistive ballooning stability of ASDEX equilibria

Experiments on the maximum attainable beta values of ASDEX discharges show that the limits for this parameter lie in the range of theoretical predictions. In a previous publication a theoretical identification of the corresponding instabilities has been attempted. No significant correlation with global plasma instabilities could be found, but the two-dimensional MHD equilibria calculated on the basis of ASDEX experimental parameters turned out to be close to the marginal ideal ballooning limit. In the work presented here, the previous investigation on resistive ballooning modes is extended. Separatrix bounded as well as limiter controlled plasma equilibria are considered. Because of the small aspect ratio of ASDEX (A ≅ 4) all equilibrium as well as stability calculations are performed in full toroidal geometry. After the formulation of a system of four equations describing the resistive evolution of velocity and magnetic fields in the high-m stability limit in co-ordinate invariant form and its Fourier approximation in the neighbourhood of a localization field line, the resulting quasimode equations are solved, applying methods of finite-element discretization. Complex growth rates γ are found, with a positive real part for values of the toroidal mode number n below 100. Calculated values of Re {γ} ≤ 10−3/τA S−1 (with τA being the Alfvén time) are small and therefore in agreement with the experimentally observed non-disruptive behaviour at the βp limit. Thus we believe that the characteristic signatures which govern ASDEX high βp discharges can be explained by resistive ballooning modes.

Finite beta effects on tearing modes in the tokamak

Details are given of a numerical study of finite-beta tearing modes in the tokamak. The linear compressible resistive MHD equations are solved in full toroidal geometry with no ordering assumptions. The results show a strong stabilizing effect as beta is increased, arising from the average curvature within the resistive layer. This stabilizing effect is particularly pronounced for high conductivity temperatures and small aspect ratios.

Observations of effects of banana trapping on neutral-beam-injected ions in PLT

The horizontally scanning charge-exchange analyser on the Princeton Large Torus (PLT) was used to measure fast co-moving ions during counter-injection of neutral beams. The large flux of charge-exchange neutrals observed in the near-parallel direction under these circumstances exhibited a sharp cut-off as the analyser was scanned to a fully parallel aiming angle. At a fixed particle energy, the cut-off angle showed a strong dependence on the plasma current, which can be predicted by using finite-banana-width tokamak orbit theory. Monte-Carlo calculations using a Fokker-Planck formalism and full toroidal geometry give substantive agreement with the experimental data, indicating that banana-trapping effects on neutral-beam-injected ions were directly observed.