Cooldown Symulations for TESLA Test Facility (TTF) Cryostats

Abstract

The first TTF cryostat has been tested in June 1997 and monitored to study the deformations produced during the cool-down procedure. A numerical study of the thermal shields of the cryostat has been carried out with finite element analysis simulations. The results allowed to understand the effect of the cooldown on the TTF cryostat. The new cryostat design, with the helium pipe directly welded to the thermal shields, needs a careful analysis of the cooldown procedure. To avoid damages to the cryostats during the cooldown a proper procedure has been devised and tested. According to the DESY cryogenic system capabilities, we have studied the possibility to use a mixture of cold and hot helium. By varying the relative percentage of the components, the shields will feel a smooth and progressive cooldown. Analysis results show that a single cryomodule can be cooled in about two days without damaging the cryostat. This paper reports thermodynamic considerations and the results of the analysis performed for the first and second generation TTF cryostats.