A model for coupled plasma current and position feedback control in the ISX-B tokamak

Abstract

In the ISX-B tokamak, the poloidal fields are supplied by sets of coils which are functionally coupled, in the sense that each set contributes to both the Ohmic-heating (OH) flux and the vertical field (VF), in contrast to the traditional way of grouping coils into ‘decoupled’ OH and VF windings. As a result, control of the plasma current Ip and control of the horizontal position ΔR are also strongly coupled. A transfer matrix model of this system is described, and the model is used to guide both the design and analysis of Ip and ΔR feedback control for ISX-B. The equations are linearized about a plasma equilibrium operating point to facilitate the analysis of stability and noise sensitivity. On slow time scales, the non-linearities are tractable enough to be used in large-signal analysis as well. The model accurately simulates actual ISX-B behaviour and can be generalized to any tokamak, since IP – ΔR coupling in fact always exists. It is especially applicable to future large machines, such as the TFCX and INTOR, which of engineering necessity will have functionally coupled windings.