1. CRYOGENICS AND ITS SPACE APPLICATIONS By:SHRIKANT BASTEPRATHAMESH BANGE(ME THERMAL) 1

2. Introduction Cryogenics is the study of how to attain lowtemperatures and how materials behave when these lowtemperatures are attained. What cryogenics is not: It is not the study of freezing andreviving people. This is known as Cryonics, a confusinglysimilar term. Cryogenics deals with low temperatures, from about 100Kelvin to absolute zero.2 3. Concept Of Absolute Zero Absolute zero is the lowest temperature thatcould ever be. The first clue to its existence came from theexpansion and contraction of gases. Scientists noticed that for all gases thetemperature for which they reached zero volumewas about -273 Celsius. This temperature isknown as absolute zero and is the zero forKelvin temperature scale. 3 4. Cryogenic Liquids Commonly used gases, in their liquid form, are nitrogenand helium. These are the common cryogenic liquids. Liquid Helium and Nitrogen are usually stored in vacuuminsulated flasks called Dewars. Nitrogen Nitrogen condenses at -195.8 C (77.36 Kelvin) andfreezes at-209.86 C (63.17 Kelvin). Liquid nitrogen isused in many cooling systems. 4 5. Helium Helium boils at -268.93 C (4.2 Kelvin) and does notfreeze at atmospheric pressure Liquid helium is used in many cryogenic systems whentemperatures below the boiling point of nitrogen areneeded. The helium we deal with is basically of 2 types: Helium 3and Helium 4. Both of these can be cooled to below theirboiling temperatures. Liquid helium boils at a lower temperature when thepressure is lower. To attain temperatures colder than liquid helium we usean Adiabatic Demagnetization Refrigerator (ADR).5 6. Adiabatic Demagnetization Refrigerator (ADR) ADRs are refrigerators used to cool space baseddetectors to low temperatures to minimize the noise inthe data obtained. An ADR contains a magneto caloric material, which canbe made to absorb or release heat with applied magneticfields. It works in cycles alternating between cooling andrecycling. Cooling is done by absorbing heat isothermallyin a magneto caloric material in the presence of adecreasing magnetic field. Recycling is done by dumpingthis absorbed heat.6 7. 7 8. ADR Components The basic ADRcomponents are asshown : Calorimeter - These aresensors that measureheat input eg. in XRSinstrument. Heat switch The heatswitch is used to allowheat to be dumpedperiodically to the heliumbath. Thermal Bus These arecopper rods that connect8 9. Salt Pill The salt pill iswhere the cooling actiontakes place. The pill(actually a cylinder) ismade of ferric ammoniumsulfate. Suspension The outerstructure of the ADRconsists of metal ringsand tubes, which allow theADR to fit securely withina superconductingmagnet. Heat Switch Shell Theshell is a cylinder made ofVespel, a polyimide9material, which provides 10. Adiabatic Demagnetization Refrigerator 10 11. 11 12. Limitation of the ADR The performance of the ADR decreases as thewarm heat sink is raised. Hence a mechanicalcooler cannot be used as the warm heat sink. Mechanical coolers small enough for satelliteuse can cool down only as far as 6 to 8 milliKelvin.However it would be extremely convenient touse a mechanical cooler instead of a liquidhelium bath. The liquid helium bath slowlyevaporates, until it is completely gone. 12 13. Advanced Adiabatic DemagnetizationRefrigerator (AADR) The AADR is a multistage adiabaticdemagnetization refrigerator. Each stage passesthe absorbed heat to the next stage. The AADR is not one ADR but a group of ADRs. Each standard ADR would have a relativelysmall temperature drop across it and wouldhence be able to remain cooler for a longer time. Its purpose was to combine the highperformance of an XRS ADR with theconvenience of a mechanical cooler.13 14. This AADR has 3 salt pills , a hot end salt pill, a cold endsalt pill and a middle salt pill. Each salt pill has its own magnet, which controls thetemperature in that pill. Between the salt pills are heatswitches and Kevlar supports. The upper two magnets are surrounded by magneticshield to prevent magnetic fields from interfering with 14www.digitalelectronicz.comother equipment. 15. Working Heat is constantly leaking into the AADR fromwarmer surroundings. This can be from thephysical supports, or as infrared radiations fromthe sensors studying the radiation . The cold end salt pill The purpose of this pill isto absorb this heat so that the sensors can stayat their best operating temperature. The middle salt pill This pill is designed in sucha way that it can be cooled to a temperatureslightly colder than the cold end salt pill. This pillis cooled by reducing the magnetic fieldproduced by the magnets surrounding it. 15 16. The Hot end salt pill From the middle heat salt pill theheat is transferred to the hot end salt pill. Before the heatcan be transferred, the middle pill should be brought tothe top and the hot end pill, to the bottom, of itstemperature range. From the hot end salt pill to the Heat sink Once the hotend salt pill is hot enough the heat switch connecting thispill to the heat sink is activated. The sink might be liquidhelium bath or a mechanical cryo-cooler. While the middle and hot end salt pills have beentransferring out the cold end salt pill absorbs heat, readyto start the cycle again. 16 17. Advantages of the AADR Greater temperature range Mechanical cooler as heat sink Continuous cooling Lower weight 17 18. X-Ray Spectrometer (XRS) The XRS is an instrument designed to study x-rays emitted by black holes and otherastronomical objects. It shows how liquid helium cooling and an ADRcan work together as part of a satellite coolingsystem. The system included some features that were intendedto lengthen the lifetime of liquid helium. To function properly, the x-ray sensors in the XRSneeded to be cooled to sixty thousandths of a degreeabove absolute zero. For this we chose an ADR. 18 19. Working of the XRS First the x rays mustbe focused onto thedetectors. The detectors need tobe kept extremelycold. This requires acomplex cryogenicsystem The signals from thedetectors areamplified and shaped19by a package of 20. XRS Cryogenic System The XRS must operate at a low temperature to minimizephonon noise and maximize the sensitivity of theresistive thermometer. The primary source of cooling is a 130 liter solid neondewar. The life of the neon is extended by use of amechanical cooler. The solid neon maintains atemperature of approx. 17 K and surrounds a tank filledwith helium. The liquid helium is vented to space. The final stage of cooling is accomplished via the use ofan ADR. Temperature control is accomplished by adjusting themagnetic field via a feedback loop.20 21. www.digitalelectronicz.com 21 22. Top view of the Astro-E2 XRS 22 23. Thank Youwww.digitalelectronicz.com 23 24. www.digitalelectronicz.com 24