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Cryogenics is the study of how to get to low temperatures and of how materials behave when they get there. Besides the familiar temperature scales of Fahrenheit and Celsius (Centigrade), cryogenicists use other temperature scales, the Kelvin and Rankine temperature scale. Although the apparatus used for spacecraft is specialized, some of the general approaches are the same as used in everyday life. Cryogenics involves the study of low temperatures from about 100 Kelvin to absolute zero.
One interesting feature of materials at low temperatures is that the air condenses into a liquid. The two main gases in air are oxygen and nitrogen. Liquid oxygen, "lox" for short, is used in rocket propulsion. Liquid nitrogen is used as a coolant. Helium, which is much rarer than oxygen or nitrogen, is also used as a coolant. In more detail, cryogenics is the study of how to produce low temperatures or also the study of what happens to materials when you have cooled them down.
Cryogenics is the study of low temperatures, from about 100 Kelvin (-280 Fahrenheit) down to absolute zero. In more detail, cryogenics is:
¢ the study of how to produce low temperatures;
¢ the study of what happens to materials when you've cooled them down.
¢ If you're new to cryogenics, check out our Introduction to Cryogenics page.
¢ Cryogenics is not:
¢ the study of freezing and reviving people, called "cryonics", a confusingly similar term.
Some Uses of Cryogenics
Astronomers at the Goddard Space Flight Center are always working to develop ever more sensitive sensors to catch even the weakest signals reaching us from the stars. Many of these sensors must be cooled well below room temperature to have the necessary sensitivity. Here are some examples of how cooling helps:
¢ Infrared Sensors: infrared rays, also called "heat rays" are given off by all warm objects. Infrared telescopes must be cold so that their own radiation doesn't swamp the weak infrared signals from faraway astronomical objects. There will be infrared telescopes on the airborne infrared observatory SOFIA, the Stratospheric Observatory for Infrared Astronomy.
¢ Electronics: all sensors require electronics. Cooling electronics reduces the noise in the circuits and thus allows them to study weaker signals.
¢ X-rays: the sensors for XRS, the X-Ray Spectrometer measure temperature changes induced by incoming x-rays. When the sensors are colder, the induced temperature changes are larger and easier to measure.
Absolute zero, according to current scientific thought, is the lowest temperature that could ever be. In fact, itâ„¢s so low that we can never quite reach it, although research teams have come within a fraction of a degree. So if we can never get there, how do we know itâ„¢s really there
The first clue to the existence of absolute zero came from the expansion and contraction of gasses. We know that hot air rises and cold air falls. Air rises when itâ„¢s heated because it expands, so itâ„¢s less dense than the cooler air around it. It has buoyancy, just like a piece of wood in a pond, which floats because itâ„¢s less dense than the water. Air sinks when it cools because it contracts, so itâ„¢s denser than the warmer air around it.
Suppose we took a certain amount of air and cooled it as much as we could. How much would it shrink When scientists first began studying the behavior of of heated and cooled gasses, they didnâ„¢t have our modern cooling methods. They measured as best they could over the temperature range that they could reach. Then they plotted their data on graphs.
The graph of volume vs. temperature for a sample of gas forms a straight line. (This assumes that you keep the pressure constant.) The lower the temperature, the lower the volume. If you extend this line to low enough temperatures, it will eventually hit zero volume. Scientists noticed that, for all gasses, the temperature at which the graph said they would reach zero volume was about -273 Celsius (about -460 Fahrenheit).
This temperature became known as absolute zero, and is today the zero for the Kelvin and Rankine temperature scales. Nowadays, we know that gasses do not shrink to zero volume when cooled to absolute zero, because they condense into liquids at higher temperatures. However, absolute zero remains one of the basic concepts in cryogenics to this day.
Although nothing can be colder than absolute zero, there are a few physical systems that can have what are called negative absolute temperatures. Oddly enough, such systems are hotter than some with positive temperatures!
Negative Absolute Temperatures
Are there negative absolute temperatures This is an answer yet to be reckoned with. Physicists have defined a negative absolute temperature, but itâ„¢s a bit flakey, for these reasons:
¢ it only applies to certain physical systems (those with a small number of energy levels),
¢ negative temperatures are hotter than (some) positive temperatures,
¢ A system with a negative temperature will eventually cool down (or warm up, depending on how you look at it) to a positive temperature, even if it is insulated perfectly from its surroundings.
Hereâ„¢s a bit more detail. Certain physical systems have small set of energy levels that they can be in. For example, a laser uses this principle. The atoms (or molecules) that produce the lasing effect can be in one of a number of energy states. Normally, only a small percentage of the atoms are in the highest energy states; many more are in the low energy states. Scientists have found equations that describe how many of the atoms are in which energy state. As you might imagine, these equations depend on temperature. The hotter the system, the more atoms are in the higher energy state. In fact, if you
know what fraction of the atoms are in each energy state, you can plug that into the equation and solve for the temperature. A laser operates by pumping energy into the atoms, pushing many of them into the high energy states. When the atoms drop back into the lower energy states, they give off the energy as a beam of laser light. But between the time they get pumped up and the time they drop back, theyâ„¢re in an abnormal state, with lots more atoms than normal in the high energy state. If you plug this abnormal distribution into the equation and solve for temperature, you may get a negative number.
Liquid air sounds like a contradiction in terms. In fact, it's not: air, when cooled enough, condenses into a liquid and even freezes solid. We're familiar with this phenomenon in the case of water: steam condenses to liquid water which freezes to ice. Or, to put it the other way, ice melts to form water at 0 Centigrade and boils to produce steam at 100 Centigrade. (These temperatures change as the pressure changes. At high altitudes, for example, water boils at a lower temperature because of the lower air pressure.) Carbon dioxide is another familiar example of a gas that freezes: it can be cooled and frozen as "dry ice".
All gases, when cooled, condense. Two gases often used in their liquid forms are nitrogen and helium. These are the commonly used cryogenic liquids. Nitrogen gas, when cooled, condenses at -195.8 Celsius (77.36 Kelvin) and freezes at -209.86 Celsius (63.17 Kelvin.) Or, to reverse the order, solid nitrogen melts to form liquid nitrogen at 63.17 Kelvin, which boils at 77.36 Kelvin. Liquid nitrogen is used in many cryogenic cooling systems.
Liquid helium boils at -268.93 Centigrade (4.2 Kelvin). Helium does not freeze at atmospheric pressure. Only at pressures above 20 times atmospheric will solid helium form. Liquid helium, because of its low boiling point, is used in many cryogenic systems when temperatures below the boiling point of nitrogen are needed.
Cryogenicists talk about various kinds of helium. They distinguish between the two naturally occurring isotopes, helium 3 and helium 4. Helium 4 makes up over 99% of naturally occurring helium. Hence, when we speak of "helium", without specifying which isotope, we're usually speaking of helium 4. Helium 4's nucleus consists of two protons and two neutrons, for an atomic
weight of 4.Helium 3, the rarer isotope, has a nucleus of two protons and one neutron. Helium 3 boils at 3.2 Kelvin. This boiling point is one degree colder than that of helium 4. Both helium 4 and helium 3 can be cooled to below their boiling temperatures by reducing the pressure to below atmospheric pressure. Liquid helium, like water, boils at a lower temperature when the pressure is lower. In fact, when liquid helium is kept in containers that are at atmospheric pressure, the helium temperature changes as atmospheric high and low pressure areas pass. These temperature changes are small, but measurable. With vacuum pumps, we can reduce the pressure in a helium container much more than happens with normal atmospheric pressure changes. As a practical matter, a pumped bath of liquid helium 4 can be used to cool down to about 1 Kelvin. A pumped bath of liquid helium 3 can be used to cool down to about 0.3 Kelvin
A convenient way to cool many kinds of apparatus is to submerge them in liquid helium or liquid nitrogen. Liquid helium and nitrogen are usually stored in vacuum insulated flasks, called Dewars, after their inventor, Sir James Dewar. (Dewars are familiar to most of us under the brand name "Thermos".)
Colder than Liquid Helium
To reach temperatures even colder than liquid helium, we use the adiabatic demagnetization refrigerator (ADR).
Adiabatic Demagnetization Refrigerators (ADRs) are used to cool space-based detectors to low temperatures to minimize the noise in the data obtained. An ADR contains a magneto caloric material, which can be made to absorb or release heat with applied magnetic fields. Selection of a magneto caloric material for use in an ADR is based largely on the refrigerant's cooling power, which is a function of its heat capacity. The heat capacity of gadolinium fluoride is being measured at several constant magnetic fields by applying known amounts of heat and recording the resultant temperature changes. Results are not yet available.
An ADR is a refrigerator that operates in cycles, alternating between two states: cooling and recycling. An ADR cools by absorbing heat energy isothermally in a magneto-caloric material (a paramagnetic "salt pill") in the presence of a decreasing magnetic field. When the refrigerant has absorbed the maximum amount of energy it can hold, that energy must be dumped to a heat sink; this is the recycling state. When recycling, the magnetic field is increased, the material warms up, and the heat is drained away by a heat sink. It is necessary to ensure that heat flows in the proper direction, from the detector toward the heat sink; heat switches that can be turned "off" and "on" are used for this purpose. A heat switch provides thermal contact between the detector and the ADR while the ADR is cooling, and another heat switch is used for conducting heat to the heat sink while the ADR is recycling. Both of these switches can be turned off when operation is not required.
Functions of the basic ADR components
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.
Heat Switch
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 end pieces (the yellowish, reddish pieces that almost touch.)
Thermal Bus
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.)
Salt Pill
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. 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.
Limitations of the ADR
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 milli Kelvin 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, until it is completely gone. A mechanical cooler, especially a highly reliable one, has no such limit on its cooling life. This is why the Advanced ADR (AADR) was developed.
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 cryo-cooler. 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.
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 milli-Kelvin) 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.
Heat is constantly leaking into the advanced ADR from warmer surroundings. It can come in through the physical supports. It can also come in as infrared radiation -- perhaps the radiation that is being studied by the astronomical sensor that the advanced ADR is cooling. Also, the electronics in the astronomical sensors might create a small amount of heat
Into the Cold End Salt Pill
The purpose of the cold end salt pill is to absorb this heat, so that the astronomers' sensors can stay at their best operating temperature. To hold the cold end temperature steady while the heat is flowing in, the ADR operators must slowly reduce the magnetic field produced by the magnet at the cold end pill. The operators must remove the stored energy from the cold end salt pill before they ramp the magnetic field all the way down to zero. For that step, they send the heat:
From the Cold End Salt Pill to the Middle Salt Pill
The middle salt pill is designed so that it can be cooled to a temperature slightly colder than that of the cold end salt pill. That's just what the operators do when they're ready to dump the heat from the cold end to middle pill. (They cool the middle salt pill down by reducing the magnetic field produced by the magnet that surrounds it.) Then they activate the heat switch that connects the middle and cold end salt pills. This allows heat to flow from the cold end pill to the (now slightly colder) middle pill.
As heat flows from the cold end salt pill, operators must increase the magnetic field produced by the magnet that surrounds the cold end pill. If they left the field constant, the temperature of the salt pill would drop as the heat flowed out. When they have transferred as much heat, and ramped up the cold end magnetic field, as much as they want, the operators stop ramping up the magnetic field. They then turn the heat switch off, to block any flow of heat back from the middle pill to the cold end pill.
The operators must also start slowly decreasing the field of the cold end salt pill's magnet, to keep the temperature in the cold end pill constant.
From the Middle Salt Pill to the Hot End Salt Pill
Before they can transfer heat from the middle pill to the hot end pill, operators must bring the middle pill up to the top of its temperature range and bring the hot end pill to the bottom of its temperature range. They do this by ramping up the magnetic field of the middle salt pill magnet and ramping down the field of the hot end salt pill magnet. When the middle salt pill's temperature is higher than the hot end pill's, operators turn on the heat switch that connects the 2 pills. Heat now flows from the middle pill to the slightly colder hot end pill.
When the heat has been transferred, the operators turn off the mid to hot end heat switch.
From the Hot End Salt Pill to the Heat Sink
Operators now heat up the hot end salt pill (by ramping up the magnetic field of the hot end magnet.) They then activate the heat switch that connects the hot end salt pill to the heat sink. The sink might be a liquid helium bath (in which case operators can dump the heat quickly) or a mechanical cryo-cooler (in which case operators must dump the heat more slowly.)
All the time that the middle and hot end salt pills have been transferring the heat out, the cold end salt pill has been absorbing heat, preparing to start the cycle over again
Greater Temperature Range
There's a limit to the temperature range of a single stage ADR, that is, the range between the coldest temperature it can reach and the temperature of its "hot" end heat sink. That limit is set by the properties of the salt pill material. A multi-stage ADR can have a greater temperature range because it can use a series of salt pills of different materials with overlapping temperature ranges.
Mechanical Cooler as Heat Sink
A multi-stage Advanced ADR could have a "hot" end temperature as high as 10 Kelvin -- high enough to use a mechanical cooler as heat sink -- while still cooling down to milli-Kelvin temperatures. By contrast, the single-
stage XRS ADR had such a low high-end temperature, 1.3 Kelvin, that the only heat sink it could use was a bath of liquid helium -- a bath that evaporates away as it cools.
Continuous Cooling
A single-stage ADR must shut down periodically to warm up and dump its load of stored heat into the heat sink. In an Advanced ADR, the end stage could be cooled periodically by a slightly colder stage. Thus, the end stage of an Advanced ADR could provide continuous cooling.
Lower Weight
The Advanced ADR could be lower in weight than a long hold time one-stage ADR. The salt pill of a long hold time one-stage ADR needs to be large enough to absorb a large amount of heat energy. The salt pills of
a continuous ADR can be much smaller, since they can be cycled frequently without interrupting the cooling.
The X-Ray Spectrometer (XRS) was a satellite payload with a cooling system that operated down to sixty thousandths of a degree above absolute zero.
The XRS is an instrument designed to study x-rays emitted by black holes and other exotic astronomical objects. The first one was destroyed in a launch attempt from the Kagoshima Space Center in Japan in February 2000. A replacement was then built and launched in July 2005. Unfortunately, a problem developed with the liquid helium coolant supply, which suddenly evaporated only 19 days after the launch. A mishap investigation board is now being organized to find the cause of the unexpected loss of helium coolant. We hope that they will pinpoint a problem which can be avoided on future satellites. In the mean time, however, we know that many of the technologies used in XRS worked well, and we expect that they will be used in future space missions.
XRS shows how liquid helium cooling and an ADR can work together as part of a satellite cooling system. XRS is also interesting for another reason. Because the volume of liquid helium was so small, the system included some unusual design features. These features were intended to lengthen the lifetime of the liquid helium coolant supply by reducing the need for cooling.
To work properly, the x-ray astronomy sensors in XRS needed to be cooled to sixty thousandths of a degree above absolute zero. For this temperature range, we chose an Adiabatic Demagnetization Refrigerator (ADR). The ADR has been used in laboratories on the ground for years, and is
thus a well-established technology. Another commonly used laboratory cooler for this temperature range is the liquid helium dilution refrigerator. For satellite use, the ADR has 2 important advantages over the dilution refrigerator. First, the ADR is more efficient. Efficiency is important in a satellite, where electric power and all other resources are strictly limited. Second, the dilution refrigerator requires a complicated internal plumbing system. This plumbing would be difficult to adapt for a satellite. In one part of the plumbing, a lighter liquid floats on top of a heavier liquid. It would be difficult to design a replacement for this part of the system which would work in zero gravity.
All the really low temperature cooling systems have one thing in common. Unlike the refrigerator in your kitchen, none of these systems will work at room temperature. They all must be cooled to low temperatures in order to produce the even lower temperatures that we are aiming for. The XRS ADR was cooled by a tank of liquid helium at 1.3 Kelvin (1.3 degrees above absolute zero. Surrounding the liquid helium tank was a tank of solid neon at 17 Kelvin (17 degrees above absolute zero.). At Goddard the helium tank, the ADR, the x-ray sensors, were all built and all the equipment attached directly to them.
Liquid helium cools by evaporating as it absorbs heat, just as, on a warm day, we are cooled by the perspiration that evaporates from us. For XRS, we had to design the system to have a tiny evaporation rate, much smaller than had been done before in a satellite. Because of the small space available in the satellite, the XRS helium tank could only carry 18 to 20 liters of liquid helium. This supply of liquid helium had to last for the 2 years of the mission. By comparison, in one laboratory cryogenic system I used recently, that much helium evaporated in a single day.
On the ground, some laboratories have machines that capture the helium vapor and re-condense it to form liquid helium. Unfortunately, such helium liquefiers are much bigger than the space available for the entire XRS instrument. So we had to concentrate on making the helium evaporate as slowly as possible.
Working of the XRS
The detectors in XRS are X-ray micro calorimeters. They work by monitoring the temperature of a tiny piece of silicon, and measuring the temperature rise that result when it absorbs an X-ray photon.
You might imagine that measuring the temperature rise from a single photon is fairly difficult, and you'd be right! Briefly, here is how it is done:
¢ First, the X-rays must be focused onto the detectors. This is done with a set of conical mirrors made of hundreds of layers of very thin foil.
¢ The detectors need to be kept extremely cold (60 milli Kelvin). This requires a complex cryogenic system, including liquid helium and solid neon. It also requires the use of several filters to keep out stray light, radio waves, and any other radiation other than X-rays.
¢ The signals from the detectors are amplified and shaped by a package of analog electronics and then processed digitally to determine the energy of each photon.
Here is a block diagram of the instrument (minus the mirrors), and a brief description of each subsystem.
XRS Cryogenic System
In addition to the need to keep the heat capacity of the absorber to the minimum, the XRS must operate at a low temperature to minimize the phonon noise and maximize the sensitivity of the resistive thermometer. To achieve the required energy resolution, with the required detector size implies that the operating temperature must be below 0.1 K. For the XRS, there are four stages of cooling.
The primary source of cooling is a 130 liter solid neon dewar. The life of the neon is extended by the use of a mechanical cooler which cools the outer radiation shield of the dewar. The solid neon maintains a temperature of 17 K, and surrounds a 32 liter tank filled with liquid helium. The liquid helium is vented to space, and maintains a temperature of K. The final stage of cooling is accomplished via the use of an adiabatic demagnetization refrigerator (ADR). This allows operation down to 50 mK; for the XRS, the nominal operating temperature will be 60 mK. Accurate temperature regulation is crucial, as the detector response depends directly on its
temperature. A change in temperature results in a corresponding change in the energy scale calibration. Therefore the ADR is specified to maintain the temperature to better than 10 K rms over a 10s to 10min timescale. Longer term temperature drifts are accounted for by a dedicated calibration pixel. Temperature control is accomplished by adjusting the magnetic field via a feedback loop. The expected lifetime of the on-board cryogens is years. This corresponds to operating the cooler 50% of the time; a slightly longer lifetime is expected if the cooler can operate at all times.
The ADR operates by aligning the magnetic moments (electron spins) of the molecules in the salt pill with a superconducting magnet, running at A and providing a magnetic field of Tesla. At the start of a cycle, the magnet is ramped up to a full field and the salt pill is connected to the liquid helium bath via a gas-gap heat switch, transferring the heat to the liquid helium bath. Once the salt pill has reached equilibrium, the heat switch is opened, and at this point the magnetic field is reduced to nearly zero. This allows the spins of the electrons in the salt molecules to randomize adiabatically, causing the salt to cool as they do. It is expected that the Astro-E2 ADR can maintain the 60 mK temperature while in orbit for 1 day, at which point the magnetic spins are completely randomized, and no more heat can be absorbed. At this point, a ``recharge'' of the refrigerator is necessary, and the cycle is started again. The ``recharge'' of the refrigerator, typically lasting hour, can be done partially while the observed astrophysical target is behind the Earth.
2. Cryogenics and Refrigeration by Barrom.
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