Enabling co-simulation of tokamak plant models and plasma control systems

We describe the key use cases that motivate the co-simulation capability and the features that must be provided by the Plasma Control System to support it. These features could be provided by the PCS itself or by a model of the PCS. If the PCS itself is chosen to provide them, there are requirements imposed on its architecture. If a PCS model is chosen, there are requirements imposed on the initial implementation of this simulation as well as long-term consequences for its continued development and maintenance. We describe these issues for each use case and discuss the relative merits of the two choices. Examples are given illustrating uses of co-simulation to address plasma control problems during operation of DIII-D and other devices that use the DIII-D PCS.