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The technical implementation of the potentially very efficient inboard pellet-refuelling scheme in tokamaks remains so far restricted to low velocities (u = 200- 300 m/s) due to the fragile nature of cryogenic D2. One specific problem is practically unavoidable pellet impacts in components of the pellet guiding system: first, in a funnelling adaptor installed to cope with the angular scatter of pellets from acceleration devices such as centrifuges and, second, inside variably bent guiding tubes required to access the high field side of the torus. In order to inject pellets at speeds close to those available from the ASDEX Upgrade centrifuge (u ?1200 m/s) , knowledge of critical guiding system parameters such as impact angle and speed is needed. In this study design requirements for an optimised guiding scheme are derived by examining grazing incidence pellet impacts on one single and two subsequent flat, rigid plates. The survival of the pellets was found to be determined by the normal impact velocity component. After undergoing two successive collisions under the same angle a critical impact speed of u(perpendicular) = 40 m/s was detected, permitting incidence angles of up to 2degrees at v>1000m/s. Studying impacts under 10-6 degrees reflection angles from 0.10 to 0.8degrees were measured, suggesting a considerable ductility of the D2 ice. This was confirmed in measurements of the plane pellet compression where irreversible deformation was found to set in well below the critical impact speed. Therefore the optimised guiding track geometry is proposed on the basis of an elliptic loop to keep the pellets as close as possible to the track and to guide the pellets through a large number of elastic small angle collisions. This makes also use of the lubricating effect of the vapour film form upon collision between the sliding pellet and the chute.

Additional information

Authors: LORENZ A, Max-Plank-Institut fur Plasmaphysik,EURATOM-Association, Boltzmannstrasse 2, D-85748 Garching (DE);LANG P, Max-Plank-Institut fur Plasmaphysik,EURATOM-Association, Boltzmannstrasse 2, D-85748 Garching (DE);LANG R, Max-Plank-Institut fur Plasmaphysik,EURATOM-Association, Boltzmannstrasse 2, D-85748 Garching (DE)
Bibliographic Reference: Article:Review of Scientific Instruments, Vol. 71 (2000), No. 10, pp. 3736-3743
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