The J.E.T. (Joint European Torus) vacuum vessel
Résumé
The large vacuum vessel of the J.E.T. (Joint European Torus) experiment has been designed as an all metal composite torus of non-circular cross-section. To comply with mechanical stability and ultra high vacuum requirements it has been designed as a completely welded fabrication. The metal structure consists of 32 rigid and wedge-shaped sections (equal in number to the B TOR field coils) which in turn are joined together by parallel bellows to form a torus. The rigid sectors are of box type construction suitably ribbed to withstand the stresses imposed upon them by the external forces. The bellows linking these rigid sectors determine the electrical resistance of the metallic vacuum vessel the long way around the torus, since currents which are induced in parallel with the plasma current should be kept as low as possible. All forces acting on the vacuum vessel are absorbed by the rigid sectors which also incorporate the openings to the interior of the machine, such as ports for pumping, diagnostics, auxiliary plasma heating, etc... This toroidal vacuum vessel has been designed as a double walled structure with the bellows linking the rigid sectors being fitted as pairs, and it is proposed to circulate hot inert gas through out this interspace in order to raise the temperature of the whole vacuum vessel to 500 °C to achieve bakeout conditions which will assist in reaching the required base pressure of 10-10 torr. In order to protect external apparatus from this bakeout temperature the outside of the vacuum vessel will be thermally insulated. Much consideration has been given to the choice of materials from which to construct this vacuum vessel. Stress conditions at elevated temperatures and electrical requirements indicate that one of the most suitable materials would be in the high content nickel alloy range. The final choice was to use Inconel 600 (a trade name of Huntington Alloys, USA) or Nicrofer 7216 (a trade name of Vereinigte Deutsche Metallwerke FRG) for the rigid sectors and Inconel 625 (Nicrofer 6020) for the bellows, the latter material having the required electrical resistivity. The horizontal ports lying on the equitorial plane are connected to pumping boxes which in turn form locations for the turbo molecular and cryopumps. They also provide maximum access to the interior of the vacuum vessel for the beams of energetic neutral particles proposed for additional plasma heating. The vacuum vessel has been so designed that all the stresses are taken by the rigid sectors, whilst the radial inward directed forces are transmitted to and supported by a modular restraint ring which acts as an arch type of structure transmitting pressure forces of 100 tons between adjacent rigid sectors. The forces acting on the vacuum vessel are due to the atmospheric pressure during normal operation and during bakeout operations at 500 °C, the electro magnetic forces due to eddy currents which are induced in the metallic structure when the magnetic fluxes linking the vacuum vessel change with time, and the thermal stresses arising for expansion of the vacuum vessel during the baking and during experimental operation.
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Origine : Accord explicite pour ce dépôt
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