Design and commissioning of the MAST neutral beam power supplies for the long pulse beam upgrade

The original beam injection system used in the Mega Amp Spherical Tokamak (MAST) at Culham was unable to deliver the long pulse plasma heating required to fully explore the potential of the spherical tokamak. A decision was taken to upgrade the neutral beam heating system and install two Positive Ion Neutral Injectors (PINIs), delivering 2.5 MW of deuterium neutral beam power each, when operated at 75 kV and 65 A. The existing power supplies required extensive upgrading to achieve the performance needed to drive the PINIs to their full potential. The upgraded power supply requirements were: a significantly increased beam pulse length of 5 seconds, precise regulation of the beam voltage and rapid termination after breakdowns between PINI grids, then beam extraction recommencing after 20 ms. This paper describes the design, development and commissioning of the PINI power supplies from conception, to extensive dummy load testing and finally operational experience with the two neutral beams. Finally a brief description is given of the foreseen capability of this system to modulate and vary the beam power during a MAST pulse. This will provide a unique opportunity to explore the plasma characteristics over a range of beam powers within a single pulse.