The Advanced Adiabatic Demagnetization Refrigerator is a multistage Adiabatic Demagnetization Refrigerator (ADR). Each stage passes the absorbed heat to the next stage in line. The last stage (the "hot" stage) passes the heat to the heat sink, which could be a liquid helium bath or mechanical cryocooler.
The Adiabatic Demagnetization Refrigerator (ADR) is a magnetic cooling system that has been used routinely in the laboratory for cooling to temperatures below the temperature of liquid helium. Astronomers are now developing sensors for x-ray and infrared astronomy which will operate in this temperature range. Since these sensors are more sensitive than their higher temperature predecessors, cryogenic engineers are now hard at work on the systems to cool them in orbit.
For more details on the standard ADR, see the ADR Review.
The ADR must warm up periodically to dump stored heat into the "warm" end temperature sink. During the warm part of the cycle, the whole ADR, including whatever sensors it may be cooling, is warm. One reason that the XRS ADR can have such a long cold part of the cycle (over a day) is that the "warm" heat sink is at a low temperature -- only 1.3 Kelvin. If the temperature of the "warm" heat sink were raised, then the cold part of the ADR's cycle would shrink, and the warm part would lengthen.
In other words, the performance of the ADR decreases as the "warm" heat sink is raised. This decrease in performance makes it difficult to use a mechanical cooler as the "warm" heat sink. Mechanical coolers small enough for satellite use, at present, can cool down only as far as 6 to 8 Kelvin. An ADR operating with a cold temperature of 60 milliKelvin and a heat sink temperature of 6 to 8 Kelvin would have to warm up much more frequently than the XRS ADR would.
Despite this drawback, it would be convenient to use a mechanical cooler instead of a liquid helium bath. The liquid helium bath slowly evaporates, unitl it is completely gone. A mechanical cooler, especially a highly reliable one, has no such limit on its cooling life.
One purpose of the advanced ADR is to combine the high performance of the XRS ADR with the convenience of a mechanical cooler. The advanced ADR is not just one ADR, it's a group. The design uses a series of simple, standard ADR's (each with one salt pill) to bridge the temperature gap between the sensors (at, say 60 milliKelvin) and the mechanical cooler (at 6 to 8 Kelvin.) Each standard ADR would have a relatively small temperature drop across it, and thus would be able to remain cold for a long time.
Here is a schematic diagram of one possible advanced ADR. The ADR shown has 3 salt pills, a hot end salt pill, a cold end salt pill, and a middle salt pill. Each salt pill has its own magnet, which controls the temperature in that pill. Between the salt pills are heat switches and Kevlar supports. The upper two magnets in this design are shown surrounded by magnetic shielding, to prevent the magnetic fields from interfering with other equipment.D
You can also see a text discussion of the Advanced ADR Cycle.
Work is now underway at the Cryogenics Branch to choose the proper materials and designs for the advanced ADR. Some of the issues that must be decided are:
The advantages of the advanced ADR design include:
The first of these advantages - no interruption in cooling - is important in large measure because it allows the other 2 advantages. Here's how the system would work.
In a conventional ADR, the salt pill must warm up to dump the accumulated heat to the heat sink. In the case of the XRS ADR, the heat sink is a bath of liquid helium at 1.3 Kelvin. Thus, the salt pill of the XRS ADR must periodically warm up to above 1.3 Kelvin. While the salt pill is that "hot", the XRS x-ray detectors cannot be run. The need to switch off the detectors every so often is a nuisance, especially if it should happen in the middle of an important observation. Therefore, the XRS ADR was designed to run for a whole day before needing to be warmed up. As you might expect, a salt pill big enough to absorb a day's worth of heat is larger and heavier than one designed to absorb for a shorter period. For spacecraft, we prefer small, lightweight systems.
The Advanced ADR will combine the convenience of a long hold time with the low weight of a small salt pill. As mentioned above, the Advanced ADR uses a series of salt pills. The last salt pill in the series cools the detectors (such as the x-ray detectors of XRS.) The next to the last salt pill cools the last salt pill -- and does it by cooling down so that it is colder than the last salt pill. Thus, the last salt pill never needs to warm up. Even if the last salt pill is so small that it can only accumulate a small amount of heat - say, 3 hour's worth - it will never need to warm up. Thus, the astronomers can schedule their observations without worrying about when the ADR will be cycling, even with a lightweight salt pill.
The flexibility in deciding when to cycle also allows the Advanced ADR to work with mechanical coolers more smoothly than a standard ADR would. A standard ADR, such as the XRS ADR, dumps its heat to the heat sink unevenly. Most of the time, it's not dumping anything. When it warms up, it dumps a lot of heat at once. This periodic dumping is no problem for a helium bath (such as that used in XRS), but might create difficulties if used with a mechanical cooler. The Advanced ADR, however, can dump small amounts of heat relatively often, thus smoothing out the load that would be seen by a mechanical cooler.