Control of plasma poloidal shape and position in the DIII-D tokamak

Discusses the techniques used now and in the past to control plasma shape and position of the DIII-D tokamak plasmas. Emphasis is placed on the ongoing transition to more sophisticated control methods which require models of the system to be controlled in order to develop controllers. Much has been learned about the adequacy of plasma models for predicting plasma evolution, but the issue of how sophisticated a model is really necessary for effective multivariable controller design is not yet resolved. We believe that some information about the effect of the plasma is needed. Our conjecture is that a model only slightly more sophisticated than the rigid radial response plasma we use here is necessary, except perhaps for detailed control of field lines around the X-point. We note that, around the X-point, a change in model assumption (flux conserving or flux dissipating) can dramatically alter the assumed motion of the field lines in response to plasma motion and thus to external conductor current changes.