Thermal and Mechanical Analysis of the Wendelstein7-X Cryo-Vacuum Pump Plug-In

Abstract

The cryo-vacuum pump (CVP) system controls the plasma density by condensing undesirable gases together with a set of turbo molecular pumps. One CVP will be installed under each of the ten units of the actively cooled divertor in Wendelstein7-X for the long-pulse operation up to 30-min duration. Each CVP is operated with supercritical Helium (ScHe) at 4 K and liquid nitrogen (LN2) at 80 K fed by a plug-in, which is installed inside a W7-X port of the plasma vessel (PV). The plug-in made of stainless steel provides for the vacuum boundary between the plasma chamber and the torus hall atmosphere. Four transfer lines ( $12 \times 1$ mm) are positioned inside the plug-in: two for the inlet/outlet of ScHe and two for the inlet/outlet of LN2, respectively. The design has to guarantee the feeding at the specified temperature of ScHe and LN2 while minimizing thermal losses and thermal interactions between pipes. Inside the plug-in the vacuum level is $10^{\mathrm {-3}}$ Pa at room temperature (RT) and $10^{\mathrm {-5}}$ Pa during operation. The pipes of the ScHe are shielded with a multilayer superinsulation. The cryogenic feed lines are protected with a cryo shield against thermal loads in the port as well as in the PV. The thermal analysis confirmed the efficiency of the thermal shielding design, which keeps temperatures of ScHe and LN2 pipes within operation limits. During baking, the PV is heated to 150 °C while the cryostat remains close to RT. The port and transfer lines are equipped with bellows to accommodate this situation. But the plug-in is rigid and the resulting bend induces mechanical loads on the flanges between port and plug-in. The mechanical analysis confirmed that the selected bolts and pretension conditions guarantee the tightness of the flange during baking.