This section deals with the Lar cryogenic system consisting of:
The main design requirements for this vessel were to maximize the space available for calorimeter modules as well as to minimize the wall thickness in front of the e.m. calorimeter and between the hadron calorimeter and the iron. Furthermore, the vessel should withstand a maximum pressur of 3 bars and support the total load of the calorimeter modules (about 500 t).
The cryostat is a cylindrical dewar consisting of two shells separated by vacuum insulation and superinsulation. Figure xx1 shows the overall layout and table nn lists the main characteristics of this cryostat. All walls are made of stainless steel except for the inner wall of the vacuum vessel, around the beam-pipe and the central tracker, which is made of aluminium alloy. A series of longitudinal spars and circular ribs on the inner part of the cold vessel contribute to the mechanical stiffness of the vessel and allow the integration of the stack support system. Something about weldings..
The full cold vessel weight (about 600 t including the calorimeters and about 80 t of LAr) is transferred directly down through the cryostat walls to the central iron yoke. These forces are taken through the insulating vacuum on four low thermal conductivity feet made of fiber glass and located between the end flange of the magnet cryostat and the muon chambers. These feet stand on warm teflon-coated sliding surfaces thus allowing for the differential displacements occuring during cool down and warm up.
A 1.5m3 expansion vessel located at the rear of the cryostat and partly buried in the iron yoke, is directly connected to the cold vessel through a chimney with no valves to avoid accidental shutting off of this relief volume. This chimney also houses the argon and nitrogen lines needed for the cryogenic operation as well as the calorimeter HV cables. The liquid nitrogen for the main cooling coils is fed in at the front part of the cryostat, whereas a port located at the bottom of the rear part of the cryostat is used in the circulation scheme for cooling down by injection of cold helium and for filling or emptying of the liquid argon.
The flat cables for up to 65000 channels are brought out of the cryostat through 24 special tubular ports distributed at both ends around the top half of the vessel. Fig xx2 shows the principle of such feed-throughs . The cables within these tubes are packed to form a heat-exchanger. (will be detailed a bit more )
The 1504 HV cables exit from the cryostat through the top of the expansion vessel by a specially developed single feed-through.
Refrigeration for the cool-down,stable operation and argon storage dewar is provided by liquid nitrogen supplied from a 25000 l storage dewar filled by a liquid nitrogen liquifier or an external supply.
The cryogenic system was designed to allow a maximum cool down time 30 days, transfer of LAr into the cold cryostat in less than 24 hours, transfer of LAr out of the cryostat in less than 2 hours and a maximum warm-up time of 30 days. Cooling down to LAr temperature is achieved by circulation of helium gas cooled in an external and moveable liquid nitrogen heat exchanger. Stable operation is ensured by a regulated flow of liquid nitrogen through three sets of coils, the main one located along the top half of the cold vessel, another one located around the bubble shield which is on the top half of the larger radius part of the inner cold wall, and the third one in the expansion vessel.
Complete control of all processes is done by a system of VME automates with an OS9 operating sytem coupled to a GRAFCET control process. This enables all operations including cooldown/warmup and filling/emptying to be done automatically under computer control.
The complete calorimeter was effectively cooled down in 25 days in early 1991 and has remained cold since then. The average liquid nitrogen consumption was of 600 l/h during the cool-down and about 95 l/h during stable operation. 250 m3 of helium gas were needed for the whole cool-down process and 190 m3 of liquid nitrogen. Time needed for filling with LAr was about 5 hours and for emptying about 2h30.
The pressure in the gaseous part of the expansion vessel was regulated to 1.35 +/-0.05 bar thus enabling the temperature of the liquid argon to be kept at a stable value of 90.2 +/- 0.1 K .
When moving into the beam in early 1992, the cold vessel was emptied to comply with the local safety rules and the cooling lines were disconnected, effectively stopping the cooling for ten days. During this period the average cryostat temperature rose by about 15 degrees, with a bottom to top temperature gradient of 30 degrees on the cryostat walls and 25 degrees on the calorimeter modules. Refilling was done within 5 hours after a one day cool down to bring the calorimeter to about 100 K .
Jürgen Naumann Last modified: Tue May 18 20:09:09 METDST 1999