Excitation and Removal of Neoclassical Tearing Mode Islands in Tokamaks

A principal pressure limit in tokamaks is set by the onset of neoclassical tearing modes (NTMs), which are destabilized and maintained by helical perturbations to the pressure-gradient driven ``bootstrap'' current. The resulting magnetic islands break up the magnetic surfaces that confine the plasma. The NTM is linearly stable but nonlinearly unstable, and generally requires a ``seed'' to destabilize a meta-stable state. Thus a good analogy is an avalanche. Once excited NTM islands are very robust but can be removed by reducing the plasma pressure (i.e. bootstrap current) sufficiently so that the meta-stable parameter space is gone; self-stabilization then occurs. Examples from the DIII-D tokamak will be presented.