A bayonet coupling is a demountable joint that allows for quick and easy connection and disconnection of cryogenic components, including transfer lines, cryostats, liquefiers and refrigerators.

Bayonets provide a number of advantages, chief among them the ability to connect and disconnect the bayonets while components they connect are still at cryogenic temperatures. The ability provided by bayonet couplings to easily connect and disconnect components greatly adds to the operational flexibility and maintainability of a cryogenic system. The couplings are very common, for example, in transfer lines used to move cryogenic liquid from a supply dewar to a transport dewar or cryostat.

Figure 1 shows a typical bayonet arrangement where the coupling consists of a set of nested pipes (one “male” and one “female”) connected with a room temperature seal. The use here of long, thin-wall, low conductivity and typically stainless steel tubes decreases heat transfer between the room temperature seal and the cryogenic components, with a very narrow gap between the two parts of the bayonet connection.

There are some disadvantages to bayonets as well. They can be expensive, add additional heat leak to the cryogenic system, are less reliable than an all-welded connection and can be difficult to disconnect once the tube diameters involved become too large.

A particular limitation of the use of bayonets is that they must be oriented so that the female side of the connection is always below the male side of the connection. Any other orientation results in cold gas or liquid being driven by gravity through the annular space in the bayonet connection to the room temperature seal, resulting in high heat leak and possible seal failure. This limitation means that engineers usually design bayonets in vertical or near vertical orientations.

Typically, facilities use bayonets in situations where frequent connection and disconnection of cryogenic components are planned. The couplings are commercially available and generally reliable, but must be designed and machined to high precision to function properly.

Facilities usually purchase bayonets either as catalog items or as a custom design from a number of industrial suppliers. While bayonet systems can be designed and built in-house by end users, there is a great deal of subtlety in the proper design and construction of these couplings. Thus, only very experienced facilities are advised to construct their own bayonets.

A good overview of bayonets is given in J.G. Weisend II (ed), Handbook of Cryogenic Engineering, Taylor & Francis, 1998. Examples of the use of bayonets in cryogenic facilities include L.X. Jia et al, “Cryogenic Facility in BEPCII Superconducting Upgrade,” in Proceedings of the Twentieth International Cryogenic Engineering Conference, 2005; and V. Ganni et al., “FRIB Cryogenic Distribution System and Status,” IOP Conference Series: Materials Science and Engineering Volume 101, Advances in Cryogenic Engineering: Proceedings of the Cryogenic Engineering Conference, 2015.

Some examples of the development of bayonet designs are given in H.H. Tsai et al., “Effect of the Connection Gap on the Heat-load Characteristics of a Liquid Nitrogen Bayonet Coupling,” in Cryogenics 52, 2012; and R.A. Rucinski et al., “Rotary Bayonets for Cryogenic and Vacuum Service,” in Advances in Cryogenic Engineering Vol. 39 A, Proceedings of the 1993 Cryogenic Engineering Conference, 1994. A specific example of bayonets for use in He II is given in G.E. McIntosh et al., “Bayonet for Superfluid Helium Transfer in Space,” in Advances in Cryogenic Engineering Vol. 33, Proceedings of the 1987 Cryogenic Engineering Conference, 1988.