Implementation of _N Control in the National Spherical Torus Experiment

We have designed and constructed a system for control of the normalized (_N) in the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion, Vol. 40, p. 557 (2000)]. A proportional-integral-derivative operator is applied to the difference between the present value of _N (from real-time equilibrium reconstruction) and a time-dependent request in order to calculate the required injected power. This injected power request is then turned into modulations of the neutral beams. The details of this algorithm are described, including the techniques used to develop the appropriate control gains. Example uses of the system are shown.