Simultaneous Control of Multiple n-Number Resistive Wall Modes

DIII-D experiments demonstrate simultaneous stability measurements and control of resistive

wall modes (RWMs) with toroidal mode numbers n=1 and 2 using a novel feedback algorithm

based on the VALEN physics model and implemented using reactor relevant external coils.

Control was maintained for hundreds of wall-times above the n=1 no-wall pressure limit and

approaching the n=2 no-wall limit. Furthermore, a rotating non-zero target was set for the

feedback, allowing stability to be assessed by monitoring the rotating plasma response while

maintaining control. This technique can be viewed as a closed-loop extension of active MHD

spectroscopy, which has been used to validate stability models through comparisons of the

plasma response to applied, open-loop perturbations. The closed-loop response measurements

are consistent with open-loop MHD spectroscopy data over a range of \beta_{N} approaching the n=1

no-wall limit, demonstrating the potential of this technique as a useful tool for measuring

stability while maintaining control even as the marginal stability point is surpassed. This

improved understanding and control of the n=1 and n=2 RWM will allow for more robust

operation above the n=2 no-wall limit.