Design of the vacuum liner for the National Compact Stellarator Experiment (NCSX)

Abstract

The National Compact Stellarator Experiment (NCSX) requires an inner vacuum vessel (liner) which will serve as a structural component, withstanding full atmospheric pressure and magnetic disruption loads. It must also be capable of bake out to 350 /spl deg/C. Without imparting excessive thermal loading on saddle coils which reside on an outer shell surrounding the liner. NCSX will be sited in the Princeton Beta Experiment (PBX) test cell. Many of the existing site assets including the test cell, TF and PF coils, power supplies, neutral beam heating systems, and site utilities will be utilized to minimize the cost of the project. The conceptual design features a stellarator core that is pre-fabricated and dropped into place on the PBX platform. The existing TF and PF coils are then reassembled around the stellarator core. Trade studies have been conducted to explore different configurations for the liner and the saddle coil structure. These included a design which used a low conductance non vacuum liner assembled within an outer vessel which surrounded the NCSX device, and served as the primary vacuum containment. The plasma has three periods and is very convoluted, changing in cross section from nearly circular to bean shaped with a very concave contour on the inner surface. The contour repeats twice every period, that is the shape is identical after 60 degrees toroidally, but with the shape inverted 180 degrees. The liner is required to follow the plasma surface very closely and, as result, is complex in shape. A trade off study has been made to determine the best fabrication technique to produce such a liner. The study included casting, pressing, explosive forming, isostatic pressing, and brake bending from flat developed patterns. The decision on a fabrication method relied heavily on previous experience by other experiments such as HSX, W7AS, and W7-X which dealt with similar, convoluted shapes. The method picked was the flat pattern technique utilized by Wendelstein W7-AS. The liner is designed to be built in three sections corresponding to the plasma field periods, and final assembled around an inner shell core. After welding of the liner sections, the outer shell segments will assembled around the liner. Saddle coils will then installed around the shell insulation must be installed between the liner and shell to protect the shell during liner bake out, and to minimize heat transfer from the cooler shell during operation. Because the radial build up is limited and the shell is designed to operate at cryogenic (LN2) temperatures the choice of insulation materials is limited. A trade off study has been performed and extensive thermal analyses have been done to determine an insulation design which can operate in the extreme temperature range from 77 K to 623 K. The engineering design is being developed by a team from Oak Ridge National Laboratory and Princeton Plasma Physics Laboratory. A Physics Validation Review of NCSX is planned for September, 1999. The Conceptual Design effort will then commence in earnest, culminating in a conceptual design review in March, 7000. First plasma is planned for September, 2004.