Performance of the plasma shaping control system on the PBX-M tokamak

The plasma-shaping control system for the PBX-M (Princeton Beta Experiment-Modified) tokamak generates highly indented plasmas inside a closely fitting conducting shell. The feedback system has been linearized about a predetermined plasma shape. Three modes of a singular value decomposition (determined from the design shape) are used to control the plasma position and shape. Plasma shape is determined using flux-difference measurements around the circumference of the plasma. An analog computer reduces the magnetic measurements to current corrections in seven sets of symmetric external shaping coils. Though limited to a single design shape, this system offers sufficient flexibility to control the plasma shape throughout the discharge