Recent developments in Pellet Fuelling at JET

A number of improvements to the Pellet Fuelling capability at JET have recently been completed and are now operational for the experimental campaigns. The most notable is the addition of the quasi-Vertical, High Field Side (VHFS) pellet delivery system. Deuterium ice pellets in the shape of cubes (4mm characteristic length), can now be delivered with velocities up to 400 m/s at an 8Hz pellet delivery repetition rate, in the direction of the plasma core. This pellet delivery compliments the present High Field Side (HFS) and Low Field Side (LFS) systems positioned at Octant 2 of the JET Machine. The HFS track is limited in velocity to 250m/s and is positioned too low to seed the core directly, the delivery becomes tangential In field lines at p∼0.7, without reliance on ablatant-cloud diffusion mechanisms. The existence of the latter has been established but the scaling of the underlying physics to a reactor is uncertain. In combination with improvements in D₂ ice quality and a straight through design at the selector assembly, it is anticipated that this system will deliver intact pellets (80-85% of the pellet volume) at much improved velocities to the plasma. The detailed engineering design will be presented, together with the most recent operational results. This paper will also present developments in the design of the JET extruders. The extrusion piston has now been changed from a hydraulically operated system to a more direct stepper motor driven electro-thrust cylinder. Optimization of the thermal heat transfer effects should improve dynamic control and provide better ice quality. Advances in cutter performance have also been made. These improvements are being funded under the EFDA JET Operation Contract and EFDA-JET Work Programme, with support from Euratom and the UK Engineering and Physical Sciences Research Council.