System modelling and identification of Mega-Amp Spherical Tokamak

A dynamic system response model for plasma in Mega-Amp Spherical Tokamak (MAST) has been developed using RZIP. This is a linearized tokamak model. It assumes a constant plasma current distribution and axisymmetry. The plasma and conducting structures are approximated by discrete toroidal current filaments. RZIP generates a state-space model of the system behaviour, with PF coil voltages as inputs and various predicted diagnostic measurements as outputs. A separate SIMULINK model has been developed to represent the MAST plant, power supplies and analogue feedback controller. The RZIP and SIMULINK models were combined to produce a complete system model. This combined model was used to simulate test shots under both vacuum and plasma conditions. The two sets of traces cohere reasonably well, with some difference in attenuation levels. System identification experiments were carried out on the PF coils, with particular attention paid to the vertical field coils, under vacuum conditions. The frequency response behaviour was found using an excitation signal comprising a number of sinusoids. The RZIP model was verified by comparing its predicted response with experimental data.