A liquid helium cryostat for Herschel [ Return towards  Space programs  ]

The technical challenge for Air Liquide lay mainly in the design and production of the main liquid helium tank. This tank will store the 2,400 liters of superfluid helium required to supply cold to the satellite and its three observation instruments. It is completely leak tight so that the superfluid helium can be contained at an absolute temperature of 1.6 Kelvin used to cool the telescope throughout its mission.
Other equipment is supplied by Air Liquide:
- a second liquid helium tank for ground operations to test and prepare for launch
- lines for transmitting helium from the optical bench and the thermal links used to distribute the cold from the cooling fluid to the optical instruments
- the thermal shields that insulate the tanks and the optical bench from the external environment.
All material supplied is covered by a contract signed between Air Liquide and EADS Astrium on behalf of the European Space Agency.

The project is very demanding in terms of leak tightness (10-9 mb.l/sec, i.e. 1 million times better than the fuel tanks on the ARIANE rocket), with leak rates at the limit of detection by spectrometers. To achieve this, specialized materials had to be used, new welds qualified etc.
Ultra-cleanness was another challenge: the tank was assembled in a specially-built category 100 clean room.

The cold is brought to the instruments using very specific high-conductivity thermal links. Some links at 1.6 K are protrusions from the main tank enabling the superfluid to be brought as close as possible to the instruments through the use of the "fountain" effect.