On locating the poloidal field coils for tokamak vertical position control

The TCV tokamak has 16 poloidal field coils for plasma shaping, distributed around the vacuum vessel. Experiments are described that determine the relative merits of different coil positions for controlling the unstable vertical movement of elongated plasmas. The results show over an order of magnitude variation in the amplitude of the plasma vertical position response to voltages applied to the different coils. The currents induced in the vessel, the required reactive feedback power and the ohmic power dissipated in the vessel all depend strongly on the coil location. Coils on the inboard, small major radius, side of the vacuum vessel are particularly effective at producing fast vertical displacements. A comparison of the results of two separate experiments with the predictions of a rigid current displacement model shows that the relative merits of the different coil positions are adequately explained by this model.