 Large APEX Bolometer Camera Bolometer Development Group Millimeter & Submillimeter Astronomy Group Max-Planck-Institut für Radioastronomie (MPIfR)
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Cryogenics The bolometer
array of LABOCA is designed to be operated at a temperature lower than
300 mK. This temperature is provided by a cryogenic system made of a
wet cryostat, using liquid nitrogen and liquid helium, in combination
with a two-stage sorption cooler.
 An 8-inch cryostat, built by Infrared
Labs (Tucson, USA), was customized at MPIfR to accommodate the
double stage sorption cooler, the bolometer array and cold optics and
electronics.A high vacuum in the cryostat is provided by an integrated turbomolecular pump backed by a membrane pump. Operational vacuum is reached in one single day of pumping. When the system is cold and operating, the vacuum has been measured to be about 10E-6 mbar for many days. Click here to check the vacuum (updates every minute) The cryostat of LABOCA incorporates a 3-liter reservoir of liquid nitrogen and a 5-liter reservoir of liquid helium. After producing a high-vacuum into the cryostat, the cryostat is filled with the liquid cryogens. The liquid nitrogen will provide a thermal shielding at 77 K (-196 °C) in our labs in Bonn (standard air pressure, 1013 mbar) and at 73.5 K at APEX (5107 m above the sea level) where the air pressure (about 540 mbar) is almost one half of the standard one. The liquid helium provides a thermal shielding at 4.2 K (-268.95 °C) at standard pressure and 3.6 K at APEX. In normal operation, the system must be refilled once per day with about 3 litres of liquid nitrogen and about 5 litres of liquid helium. The refilling operation requires less than 20 minutes.  The
cryostat incorporates a commercial two-stage closed-cycle sorption
cooler, model SoCool
manufactured by Air-Liquide (see
description here). In this device a helium-4 sorption cooler is
used to liquefy helium-3 gas in the adjacent, thermally coupled,
helium-3 cooler. The condensed liquid helium-3 is then sorption pumped
to reach temperatures as low as 250 mK, in the absence of a thermal
load. Therefore, the double stage design allows to cool the bolometer
array down to a temperature lower than 300 mK starting from the
temperature of the liquid helium bath at
atmospheric pressure. This makes the maintenance of the system much
simpler than that of other systems, where pumping on the liquid helium bath is
required. The two sorption coolers are closed-systems, which means they
do not require any refilling of gas and can be operated from the
outside of the cryostat, just by applying electrical power.To keep the bolometers at operation temperature, the sorption cooler needs to be recycled. The recycling is done by application of a sequence of voltages to the electric lines connected to thermal switches and heaters integrated in the sorption cooler. A typical recycling procedure requires about two hours and can be done manually or in a fully automatic way controlled by the frontend computer. At the end of the recycling, both gases, helium-4 and helium-3, have been liquefied and the evaporation of the two liquids provides a stable temperature for many hours. Click here to see the temperature log (updates every minutes, refreshed every day) Click here to see plots of the temperatures (updates every minutes, refreshed every day) After the recycling of the sorption cooler, the bolometer array reaches 285 mK. The hold time of the cooler strictly depends on the recycling procedure. The base temperature is a function of elevation and can be affected by telescope movements, leading to temperature fluctuations of ~3 mK at most. |
| web: gsiringo (at) mpifr-bonn.mpg.de | last edit: G. Siringo, MPIfR - August 2007 |