J. Milnes

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 /spl sim/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/m/sup 2/ 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.

Influence of accelerator grid misalignment on multi-aperture particle beam properties

Accurate accelerator grid alignment is a major factor governing the properties of multi-aperture particle beams. It is particularly important for JET positive ion neutral injectors (PINIs), where accelerators are constructed of two grid halves. Grid misalignment can lead to increased power loading of beam intercepting components, to reduced beam transmission, and, finally, to reduced neutral beam power injected into plasma. A new PINI accelerator assembly procedure, which utilises a robotic arm measuring device, was recently developed at JET. PINI alignment data are used as input to a beam simulation code to predict beam properties prior to testing at the Neutral Beam Test Bed. Since the new PINI assembly procedure and the associated computer code have been implemented the alignment of JET PINIs has been considerably improved.

Upgrade of the JET neutral beam heating system

The programme of the JET Octant 8 Neutral Injector Box (NIB) upgrade is the first, and to date the largest, modification of the JET machine approved within the current EFDA-JET framework. When completed in autumn 2002, this programme will result in the total injected deuterium neutral beam power of /spl sim/28 MW, compared to the present maximum level of /spl sim/20.5 MW. This paper gives an overview of various engineering activities and experimental trials carried out in support of this ongoing upgrade programme.

Operating limits of the upgraded JET neutral beam injector from duct re-ionisation and beam shine-through

Physics modelling and engineering analysis have been carried out to determine the operating limits of the upgraded JET neutral beam injector from duct re-ionisation and beam shine-through. The JET neutral beam duct is only 23cm wide and 90cm tall at its throat and yet it presently has to transmit more than 11MW of D/sup 0/ beam particles, resulting in power densities in excess of 200MW/m/sup 2/. Even at this power level, the copper duct liner can be the limiting component with respect to the pulse length of the Octant 4 injector, depending on plasma current and power. The upgrade to the Octant 8 injector in 2002 will increase the power to /spl sim/15MW of D/sup 0/ at 130kV, so it is necessary to determine the new limits. It is shown that at full power, the duct will become the major limiting component with respect to pulse length for this injector. The shine-through power density and integrated energy for various in-vessel components have also been evaluated for the upgraded injector. Thermo-mechanical finite element stress calculations on elements of the ICRH antenna show that the injector can be operated at full power without further restrictions being imposed on the plasma characteristics, e.g. density and shape. For the CFC inner wall guard limiter tiles and their internal reinforcement, however, there is a bulk temperature limit and an enhancement to the existing real-time protection system is proposed.