BEHAVIOR OF MAGNETIC ISLANDS IN 3D MHD EQUILIBRIA OF HELICAL DEVICES

摘要

Magnetic island formation in 3D finite-β equilibria in the H-1 Heliac is studied by using the HINT code. It is found that the size of a dangerous island should increase with β but that destruction of the equilibrium at low β is avoided because the rotational transform evolves to exclude the rational surface concerned. At higher β there is evidence of near-resonant flux surface deformations which may lead to an equilibrium limit. A reconnected equilibrium at still higher β exhibits a double island structure which is similar to homoclinic phase portraits which have been observed after separatrix reconnection in Hamiltonian systems. The physical mechanisms of island formation in finite-β helical equilibria have been investigated. The HINT code predicts that the global effect of the Pfirsch-Schliiter currents can lead to self-healing of magnetic islands independent of whether or not the plasma is stable to resistive interchange modes. This result has been compared with the predictions of a boundary-layer analysis which has been extended to consider configurations with islands in the vacuum magnetic field.