Modelling of the FPS for the internal coils and investigating controller discretization effects in TCV

The tokamak in Lausanne, Switzerland (TCV), has been designed to investigate highly elongated plasmas. The elongation improves performance but also introduces a vertical position instability which requires active feedback control to stabilize. The potential high growth rate of this instability requires a fast response time. For this reason TCV has a set of magnetic coils which are placed inside the vacuum vessel. These coils are able to act on the plasma faster than the coils that are located outside of the vessel. At the moment, control of the internal coils is done using an analog control circuit. However, because of the introduction of a new control system with greater computational power it might be possible to control the internal coils with a digital controller which allows for the application of more advanced control techniques. The goal of this project has been to determine what sample rate of controller for the internal coils is needed such that vertical instability can be stabilized. To determine this, a model of the control loop for the internal coils has been revised by improving the linearized rigid plasma model using experimental data and by introducing an advanced model for the fast power supply (FPS) of the internal coils. Using this improved model it can be concluded that a small to medium elongated plasma can be stabilized using a sample rate of 25 kHz. A highly elongated plasma will need a sample rate of at least 50 kHz which is approaching the limit of the new control system.