The Vacuum Chamber of a Tokamak Plasma Experiment (TFR)

Abstract

The magnetic confinement of hot plasmas needs high quality vacuum chambers. The bombardment of the walls by particles (ions, electrons, fast atoms, photons, neutrons) must not involve a large flux of secondary emission products. This report describes the realization of a vacuum chamber needful for a toroidal plasma confinement experiment of Tokamak type (TFR). The geometric characteristics of the torus are: major radius 0.98 m, minor radius 0.20 m, volume of the chamber and observation ports: 1.5 m3, total surface: 35 m2. The vacuum vessel is made of a series of belows of inconel ×750, 0.5 mm thick; it is bakable up to 450°C. The assembling of each toroidal section with the observation ports is obtained by electron-bombardment welding. The pumping circuit consists of a primary pump and two turbo-molecular pumps in series (600 1/s and 70 1/s). Two 0.5 m2 titanium pumps, cooled at liquid nitrogen temperature, are also used in parallel with the turbo-molecular pumps in order to increase the pumping speed between two plasma discharges. The base pressure obtained after baking the chamber itself at 420°C and the observation sections at 200°C is 5·10-10 Torr. The mean outgassing rate of the walls is then 2·10-13 Torr 1/s·cm2.