Vertical plasma oscillations as a tool to perturb the pedestal in the DIII-D tokamak

Previous experimental approaches used vertical oscillations, also called ‘jogs’ or vertical kicks, for ELM pacing. This contribution presents experiments, where such oscillations were applied to probe inter-ELM pedestal instabilities. Since fast vertical plasma movements induce current mainly at the plasma edge, the applied oscillation can be a strong actuator on pedestal microinstabilities. Such instabilities typically appear as high frequency fluctuations during the last phase of the ELM cycle, when the edge pressure gradient is saturated. In this phase the pedestal is stable but marginal to the stability limit. Therefore, if a perturbation is applied by a vertical plasma oscillation, it becomes highly probable that an ELM crash is triggered. Further, the oscillations modify the detected frequency of the inter-ELM fluctuations, depending on the direction of the induced current. Not all frequency bands are similarly modified, especially the broad band high frequency fluctuations remain unchanged. It is suggested that the frequency changes are related to a shift of the instability location with respect to the rotation profile or a direct modification of the edge rotation.