Hypervapotron design for the long pulse upgrades on MAST NBI

The MAST (mega amp spherical tokamak) fusion research facility at Culham is undergoing an upgrade of its neutral beam heating systems. The two neutral beam injection systems (NBI), presently installed, can each deliver 2 MW of power for a period of ∼300 ms. These injectors will be upgraded to a longer pulse capability by the installation of new ion sources. These will be a variation of the Joint European Torus (JET) 80 kV tetrode positive ion neutral injector (PINI) design with optics customized to suit the MAST duct geometry. They will eventually be capable of pulse lengths of up to 20 seconds with an injectable total power of 2.5 MW / beamline. In anticipation of these enhancements, several beamline components have been upgraded to cope with the increased beam pulse lengths delivered by the new PINI sources. In particular, the residual ion dumps (RIDs) and calorimeters have been redesigned to use actively cooled hypervapotrons as beam stopping elements. These design solutions are discussed in the paper. Hypervapotrons have been used reliably at JET for many years, their thermal performance being optimized to suit specific needs. Their application to this function has required a further enhancement in performance. The changes made and the technical justification for them is discussed along with the results from the subsequent full-scale power handling tests. A new high power handling record for hypervapotrons of 20 MW/m² was established in these tests. A large-scale mock-up of the internal geometry of the hypervapotron was constructed so that flow visualization could be used to confirm the assumed flow patterns within the hypervapotron. High-speed digital video was used to enable the examination of the flow pattern within the cooling channels. The high-resolution film of the seeded flow was then analyzed at low speed with a view to gaining a better understanding of the heat transfer mechanism. The results of this examination are presented.