Vacuum vessel, wall protection, pumping system and poloidal limiters of the Spanish stellarator TJ-II

TJ-II is a medium size stellarator under construction in Madrid, Spain, with an average major radius of R₀=1.5 m and a maximum minor radius a=0.2 m. The average toroidal field is 1 T. The pulse length varies from 0.2 s to 1 s and the duty cycle is 5 minutes. This device is capable to generate magnetic field configurations with rotational transform between 0.9 and 2.5, a shear between 1% and 10% and a magnetic well from 0 to 6%. Start of operation is expected in 1996. An RF heating power of 400 kW/53.2 GHz will be available in stage I of the experiment. Later in stage II, 2 MW NBI power will be applied. In stage III, 4 MW NBI power will be used. This paper reports on the engineering and design of the TJ-II in-vessel components as well as the pumping system. The vacuum vessel (VV) is an all-welded piece. Its groove is protected along the entire toroidal circumference against damage due to the bean-shaped plasma by 3 mm stainless steel sheets for the low and medium power operation (stage I and II) and graphite tiles for the high power operation (stage III). Furthermore, the VV is protected on the areas where the neutral beams deposit a residual shine-through heat flux. Four movable poloidal limiters will be used to limit the heat load on the groove area of the VV, especially during stage III. The limiters will be movable to adjust them to the plasma geometry. The VV will be pumped through four symmetrically spaced bottom ports to a base pressure of 10^-8 mbar. Four identical and independent vacuum pumping subsystems will be used