On the contribution of local current density to neoclassical tearing mode stabilization

Abstract

Neoclassical tearing modes are driven by the reduction of bootstrap current inside the island due to the flattening of the pressure profile. This current perturbation enhances the magnetic perturbation responsible for the island formation. Therefore it is well known that local current drive (CD) can be used to compensate this perturbation and stabilize the mode. Several forms of the current drive contribution to the modified Rutherford equation have been proposed. Analytical fits of these contributions are provided in order to facilitate their comparison with experiments and another contribution is proposed. Since the bounce and transit frequencies are much larger than the collision frequency and the modulation frequency of the CD source in phase with the island, it is argued that the effective current driven density profile is a flux function. Assuming an exponential profile leads to no difference between modulated and continuous application of current drive. The various forms differ mainly at small island width and the possibility to differentiate amongst them experimentally is discussed.