Another set of porous plugs controlled the escape of helium vapor from the SHOOT dewars. Although the dewars were well insulated, some heat leaked in. This heat (plus the heat from the thermomechanical pumps) slowly evaporates the liquid helium. The helium vapor must be allowed to escape -- if it's held in the dewar, the pressure will build up.
How can we let the vapor escape while making sure the liquid stays in the dewar? On the ground that's easy: the liquid sits in the bottom of the tank, so we put the vent on the top of the tank. In zero gravity, however, there is no top or bottom. We still have to put vents in the tank, but in the vents we install porous plugs. These porous plugs are slightly different from the plugs used in the thermomechanical pumps. The vent plugs have pores that are large enough that both the normal and superfluid components of helium can flow through. When the liquid helium reaches the outside surface of the plug, that is, the surface exposed to the vacuum of space, it evaporates. As it evaporates it cools. To the "cool" helium on the outside surface of the plug, the "warm" helium inside the dewar now looks like a hot spot. Since superfluid helium flows from cool towards warm, the superfluid at the outer surface of the plug flows back into the dewar. As superfluid leaves the outer surface of the plug, normal heium at the outer surface converts to superfluid helium to keep the mixture appropriate to the temperature. Thus, there is a combination fluid flowing from the inside to the outside of the plug, fluid evaporating and escaping, and fluid flowing back into the dewar. If the porous plug is designed correctly, the cooling caused by evaporation at the plug will hold the dewar at the desired temperature.D
This use of the porous plug, to allow helium to evaporate and leave the dewar, is called phase separation. The liquid and vapor forms of helium are called "phases", so a plug which can separate the liquid phase (which stays in the dewar) from the vapor phase (which boils away) is called a "phase separator".
Porous plugs were used as phase separators on the COBE and IRAS satellites, as well as on SHOOT. On these satellites, liquid helium was used to cool sensors which measured infrared radiation. Infrared radiation is sometimes called "heat radiation". Warm objects (including people) give off measurable amounts of infrared radiation. The infrared receivers on COBE and IRAS needed to be cold enough that infrared radiation from the receivers themselves would not overpower the weak infrared signals coming in from faraway astronomical objects.
A small amount of liuqid helium leaks from the porous plug. The amount is so small that in most cases it can be ignored. However, the XRS Instrument has such a small helium supply that even this leak must be fixed. XRS engineers devised a system which evaporates the thin film of leaking liquid helium, using the evaporation to help cool the main helium tank. For an introduction to this heat exchanger, see the film killer section of our XRS Intro section. For technical details, see the film killer page.
The helium which leaks through the porous plug is cooled by the plug, and is thus colder than the helium in the tank.
There is a process, called the Castles Catastrophe, by which all the liquid helium in a satellite may drain off into space. For the catastrophe to occur, there would have to be liquid helium on the outside end of the porous plug, that is on the end away from the tank. This outside liquid helium would also have to be warmer than the helium in the tank. (In normal operation, liquid helium outside the porous plug would be cooled by the plug, and thus would be colder than the helium in the tank.) If both these conditions were met, then superfluid helium would flow from the tank, through the plug, to join the outside helium, just as happens in the fountain effect. (Remember that superfluid helium flows from a colder to a warmer area.) The process might continue till all the helium had been lost in space.
No satellite has yet suffered a Castles Catastrophe, in part because everyone's trying to avoid it. The process is named after its discoverer, Dr. Stephen Castles, who was then at NASA Goddard's Cryogenics & Fluids Branch.