ADIABATIC DEMAGNETIZATION TEMPERATURE
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12-01-2011, 12:57 PM
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SARASWATI INSTITUTE OF TECHNOLOGY AND MANAGEMENT UNNAO
We report on recent progress in the development of a continuous adiabatic
demagnetization refrigerator (CADR). Continuous operation avoids the constraints of long hold times and short recycle times that lead to the generally large mass of single-shot ADRs, allowing us to achieve an order of magnitude larger cooling power per unit mass. Our current design goal is 10 laW of cooling at 50 mK using a 6-10 K heat sink. The estimated mass is less than 10 kg,including magnetic shielding of each stage. The relatively high heat rejection capability allows it to operate with a mechanical cryocooler as part of a cryogen-free, low temperature cooling system.
This has the advantages of long mission life and reduced complexity and cost. We have
• assembled a three-stage CADR and have demonstrated continuous cooling using a superfluid helium bath as the heat sink. The temperature stability is 8 IxK rms or better over the entire cycle and the cooling power is 2.5 laW at 60 mK rising to 10 laW at 100 mK.
How does an Adiabatic Demagnetization Refrigerator Work?
An Adiabatic Demagnetization Refrigerator (ADR) works by using the properties of heat and the magnetic properties of certain molecules.
Some molecules have large internal magnetic fields, or "moments". Just like a tiny bar magnet, these molecules will align themselves with an external magnetic field. The random thermal motions of the molecules, on the other hand, tend to de-align them. The higher the temperature, the more they de-align. ADRs generally use certain types of salts for the molecules, because they have particularly large magnetic moments. The salt is contained in a cylinder, usually called a "salt pill". This salt pill is thermally connected to the object we want to cool (X-ray detectors, for example).
Suppose the salt pill is first placed in a strong magnetic field. The molecules align with the external magnetic field, and the magnetic energy of each molecule is minimal. If the strength of that field is decreased, then the thermal motion of the molecules starts to twist them out of alignment with the field. This requires energy, which comes from the thermal motion of the molecules. The thermal energy is thus transformed into magnetic energy, cooling the salt pill down. As heat flows into the salt pill from the outside, the magnetic field is slowly reduced. This allows the molecules to twist further out of alignment, absorbing more heat. The rate at which the field is reduced can be regulated so as to keep the salt pill at a constant temperature as it absorbs heat. Conversely, increasing the magnetic field will convert magnetic energy back to thermal energy, raising the temperature of the salt pill.
Eventually the magnetic field is reduced to zero, and no more heat can be extracted by the salt pill. At this point, all that can be done is to increase the field, heating the salt pill. So far we have accomplished the feat of converting thermal energy to magnetic. Now we need to turn it back into heat and dispose of this heat somewhere.
If the magnetic field is increased to a much higher value than it originally was, the temperature of the salt pill will increase. This will also raise the temperature of the detectors, rendering them temporarily inoperable. However, the salt pill temperature can actually be higher than the temperature of the surrounding dewar. (A dewar is a container, like a thermos, which holds a cold material, such as liquid Helium or liquid Nitrogen.) At that point it can be thermally connected to the dewar until it cools to the temperature of the dewar. Thus the heat originally absorbed by the salt pill is dumped to the dewar.
The salt pill is then disconnected from the dewar and the magnetic field is slowly reduced, beginning the cycle again. The temperature of the salt pill quickly reaches a level at which the detectors can operate, and the temperature is again regulated by adjusting the rate at which the magnetic field is reduced.
The Adiabatic Demagnetization Refrigerator:
Temperature and Entropy Cycle
Operating Cycle 1: Brief Description
Here is a quick description of the ADR cycle. We'll go over it in more detail after the temperature-entropy diagram.
The magnetic field is dropped rapidly. The salt pill cools as the molecular magnetic moments drop out of alignment with the field, absorbing entropy.
Once the cooling reaches the desired temperature, the rapid decrease of the magnetic field stops. Heat slowly leaks in from outside. The magnetic field is slowly dropped, to allow the salt pill to absorb the heat without increasing in temperature.
When the salt pill has absorbed all the heat it can, the magnetic field is increased again. The salt pill warms as the magnetic moments align and dump their energy.
When the salt pill is hotter than the thermal sink, the heat switch is turned on. Heat flows from the salt pill into the thermal sink. Then the heat switch is turned off and the cycle begins again.
Operating Cycle 2: Temperature Entropy Diagram
I have a fairly long description of the following graph for those who'd like a guided tour. I wrote it originally for users with text browsers, then realized that its slightly different approach might be useful to anyone with any type browser. Feel free to read my graph description, or to read this page, or to try both.
Calorimeters are sensors which measure heat input. This ADR was designed to cool calorimeters for the X-Ray Spectrometer (XRS) instrument. These calorimeters measure the energy of x-ray photons by measuring the heat energy deposited when the photons are absorbed. The instrument will be used to measure x-rays coming from distant astronomical objects.
The heat switch is used to allow heat to be dumped periodically to the helium bath (not shown.) The main components are: external shell (the brown cutaway part); getter chamber and connecting tube (off the left end); and the interleaved copper endpieces (the yellowish, reddish pieces that almost touch.)
The thermal busses (shown here in yellow) are copper rods that connect the calorimeters (which need to be cooled) with the salt pill (where the cooling action takes place.)
The salt pill is where the cooling action takes place. The pill (actually a cylinder) is made of ferric ammonium alum (FAA), also called ferric ammonium sulfate. FAA was chosen to give good cooling power in the temperature range where this ADR wil operate. (Other ADR's use other materials.) When in use, the salt pill end of the ADR is slid into a superconducting magnet. Changing the applied magnetic field causes the salt pill to cool or heat. See ADR Primer for a description of the cycle. The horizontal lines running through the salt pill represent the wires that provide good thermal contact from the salt pill material to the heat switch and thermal busses.
The outer structure of the ADR consists of metal rings and tubes, which allow the ADR to fit securely within the superconducting magnet. (The magnet is not shown in this drawing.) The salt pill is suspended within this rigid outer structure by means of Kevlar cords. (Kevlar is a DuPont trademark.) Kevlar is strong enough to hold the salt pill in place during the stress of launch, but has low thermal conductivity so that not much heat leaks into the salt pill through the suspension. The ends of the Kevlar lines are attached to bolts (shown in blue.) By turning the bolts, technicians can tighten or loosen the cords.
Heat Switch Shell
The brown part with the cutaway upper edge is the shell of the heat switch. The shell is a cylinder. It is made of Vespel, a polyimide material, which provides high strength with low thermal conductivity. (Vespel is a DuPont trademark.) Not shown in this drawing is a layer of titanium foil on the outside of the Vespel, to block room temperature permeation of helium from the heat switch.