Modeling of coupled edge stochastic and core resonant magnetic field effects in diverted tokamaks

Attaining the highest possible performance in poloidally diverted tokamaks requires resonant magnetic perturbation coils to avoid core instabilities such as locked, resistive wall and neoclassical tearing modes. These coils also perturb the pedestal and edge region, causing varying degrees of stochasticity with remnant islands. In this paper, the effects of the DIII-D locked mode control coils are modeled with a field line integration code and compared with experimental measurements. We find that without a detailed profile analysis and field line integration modeling, it is difficult to establish whether a given response is due to a ‘core mode’ or an ‘edge stochastic boundary effect’. We also find in diverted Ohmic plasmas, stochastic boundary layers display many characteristics associated with edge stochasticity in non-diverted tokamaks but can have significant differences such as a ‘focusing’ of the open magnetic field lines into the divertor.