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Title	98: Develop feedback-controlled MHD spectroscopy
Name:	Jeremy Hanson hansonjm@fusion.gat.com	Affiliation:	Columbia University
Research Area:	Stability & Disruption Avoidance	Presentation time:	Requested
Co-Author(s):	M. Lanctot, G. Navratil, P. Martin, P. Piovesan, D. Shiraki, E. Strait, F. Turco, F. Volpe	ITPA Joint Experiment :	No
Description:	Develop and test a new active MHD spectroscopy technique in which the plasma response is held at a fixed amplitude using feedback. The required feedback currents should depend on the stability properties of the closed-loop system. Thus, this technique may provide a real-time stability assessment while simultaneously maintaining control of the stability. In addition, it provides a means of evaluating new feedback algorithms in piggyback. A similar technique was used to demonstrate sawtooth control in RFX-MOD tokamak experiments.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:
Background:	In more traditional active MHD spectroscopy techniques, the plasma response to a fixed amplitude applied perturbation is measured, and can be linked to plasma stability. Less stable plasma equilibria typically have a larger amplitude plasma response, making the driven plasma response a useful measurement for disruption avoidance. However, the plasma response begins to increase more quickly as marginal stability is approached, and can lead to detrimental effects, such as plasma rotation braking. This new technique would automatically vary the perturbation level needed to maintain a fixed, low-amplitude plasma response, independent of plasma stability. Thus, measurements can be made in close proximity to the closed-loop marginal stability point. There are several other possible advantages conferred by this technique: (a) feedback algorithms can be evaluated without the requirement of unstable plasma modes, (b) the closed-loop marginal stability point likely has higher plasma performance (eg beta) than the open-loop marginal point, (c) plasma modes driven using this technique have lead to tokamak sawtooth suppression in RFX-MOD, and (d) the feedback currents may be sensitive to residual uncorrected error fields. This experiment can be done using the present capabilities of the PCS Error Field category. That is, no PCS algorithm modifications are needed.
Resource Requirements:	H-mode plasma, SPA or AA power supplies on I-coils
Diagnostic Requirements:	Magnetics, MSE, Thomson scattering, CER, ECE radiometer, density interferometer.
Analysis Requirements:
Other Requirements: