Validating multi-n plasma response simulations

Frequency-dependent measurements of the magnetic plasma response to n=2 perturbations in DIII-D exhibit apparent multi-mode behavior. Response measurements from the high-field side midplane show a double resonance in the frequency range of -100 to 100 Hz, in discharges near the n=2 ideal MHD no-wall pressure limit. The poloidal structure of the response varies with the perturbation frequency, becoming more peaked on the midplane at low frequency. Thus far, the observations have not been explained by plasma models that simulate a single-n plasma mode. However, the perturbing coils have a significant n=4 sideband, and ideal MHD analysis indicates that the plasma n=4 mode is also near marginal stability. A possible explanation for the observations is that the n=4 sideband couples to the n=2 response via symmetry-breaking wall eddy currents. We will investigate this hypothesis using a newly developed version of the VALEN code that can incorporate multiple n-numbers in response calculations.