Design of a 2 Slot VLPC Cryostat Cooled by a Cryocooler

Rucinski, Russell A.

Fermi National Accelerator Laboratory

关键词: Heat Transfer;    Spacers;    Titanium Experiment-Hep;    Flanges;    Backfilling;   

DOI  :  10.2172/1033659
RP-ID  :  FERMILAB-D0-EN-573
RP-ID  :  AC02-07CH11359
RP-ID  :  1033659

美国|英语

来源: UNT Digital Library

【 摘 要 】

The conceptual design and preliminary engineering calculations have been completed for a two cassette cryostat. This report summarizes the design. A cryocooler is permanently mounted in the center of a stainless steel, 0.75 inch thick top lid. The cryocooler sits upon a spacer which raises the cooling stage elevations to favorably match the cassette heat intercept elevations. The top lid (32.0-inch outside diameter) mates to a 24-inch pipe size flange with o-ring. The 24-inch pipe size vacuum vessel with end plate has a minimum internal depth of 16-inch to give adequate clearance for the depth of the cryocooler and multilayer insulation blankets. Support stand legs elevate the container to a convenient height and allow for placement of the AFE power supply underneath. Two cassette slots are located on either side of the cryocooler. The slots are positioned parallel to each other, 10.5-inch center to center (6 standard cassette slot widths) so that the standard 8 slot AFE backplane can be used. The slot opening through the lid is approximately 1.422-inch x 16.782-inch. A 0.016-inch thick titanium (Ti-6AI-4V) envelope with sealing lip is inserted through lid and defines the gas helium boundary that the VLPC cassette resides. The internal dimensions of the titanium envelope are 1.390-inch x 16.75-inch x 10.531-inch deep. When the cassette is inserted the clearances will be 0.015-inch on the long side, 0.063-inch on the short side, and 0.032-inch at the bottom. The cassette gasket seals against the top lip of the titanium envelope. A soft gasket or thin vacuum sealant tape seals the underside of the titanium envelope to the top surface of the lid. A clamping hold down bar may be necessary to make this vacuum seal. Gas helium for the cassette space is supplied through a fitting and port that is added to the cassette bulkhead assembly. This is the only modification necessary the standard D-Zero cassette. Evacuation and backfilling and then stagnant positive pressure are achieved through this one port. After the titanium envelope/pocket and cassette are mounted to the top lid, the lid is raised so that assembly work can be accomplished below. Three pneumatic cylinders mounted to the lid are pressurized (with air or the gas helium source) to raise the lid. Safety linkages are installed to prevent accidental lowering of the lid while work is being performed. Honeycomb sandwich panels are attached to the titanium envelope to support the thin envelope against the pressure differential. A thin thermoplastic mesh placed between the honeycomb panel and titanium envelope helps to insulate the panel so it is not a significant parallel heat path down the wall. There are two panel sets for each cassette. One for the area above the upper intercept and one set for the area below. It is reasonable that a 0.40-inch thick panel will suffice. A high conductance thermal link is made between the first stage of the cryocooler and the uppermost intercept location. This is solid OFHC copper part with a short section of flexible copper braid that connects to bars that clamp solidly to the long sides of the cassette envelope. Apiezon 'N' grease is used at assembly to enhance heat transfer at the clamp bar to titanium envelope interface. A second high conductance thermal connection is made between the second stage of the cryocooler and the bottom of the cassette envelope. The link is mostly solid OFHC copper along with a short flexible braid section in between. The two piece clamping bar encases the bottom 1.75-inch of the cassette. Thermal grease is used at the interface.