Cryogenics and its space applications


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Cryogenics and its space applications

  2. 2. Introduction Cryogenics is the study of how to attain low temperatures and how materials behave when these low temperatures are attained. What cryogenics is not: It is not the study of freezing and reviving people. This is known as Cryonics, a confusingly similar term. Cryogenics deals with low temperatures, from about 100 Kelvin to absolute zero. 2
  3. 3. Concept Of Absolute Zero Absolute zero is the lowest temperature that could ever be. The first clue to its existence came from the expansion and contraction of gases. Scientists noticed that for all gases the temperature for which they reached zero volume was about -273 Celsius. This temperature is known as absolute zero and is the zero for Kelvin temperature scale. 3
  4. 4. Cryogenic Liquids Commonly used gases, in their liquid form, are nitrogen and helium. These are the common cryogenic liquids. Liquid Helium and Nitrogen are usually stored in vacuum insulated flasks called Dewars. Nitrogen Nitrogen condenses at -195.8 C (77.36 Kelvin) and freezes at -209.86 C (63.17 Kelvin). Liquid nitrogen is used in many cooling systems. 4
  5. 5.  Helium Helium boils at -268.93 C (4.2 Kelvin) and does not freeze at atmospheric pressure Liquid helium is used in many cryogenic systems when temperatures below the boiling point of nitrogen are needed. The helium we deal with is basically of 2 types: Helium 3 and Helium 4. Both of these can be cooled to below their boiling temperatures. Liquid helium boils at a lower temperature when the pressure is lower. To attain temperatures colder than liquid helium we use an Adiabatic Demagnetization Refrigerator (ADR). 5
  6. 6. Adiabatic Demagnetization Refrigerator (ADR) ADR’s are refrigerators 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. It works in cycles alternating between cooling and recycling. Cooling is done by absorbing heat isothermally in a magneto caloric material in the presence of a decreasing magnetic field. Recycling is done by dumping this absorbed heat. 6
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  8. 8. ADR Components The basic ADR components are as shown : Calorimeter - These are sensors that measure heat input eg. in XRS instrument. Heat switch – The heat switch is used to allow heat to be dumped periodically to the helium bath. Thermal Bus – These are copper rods that connect 8
  9. 9.  Salt Pill – The salt pill is where the cooling action takes place. The pill (actually a cylinder) is made of ferric ammonium sulfate. Suspension – The outer structure of the ADR consists of metal rings and tubes, which allow the ADR to fit securely within a superconducting magnet. Heat Switch Shell – The shell is a cylinder made of Vespel, a polyimide 9 material, which provides
  10. 10. Adiabatic Demagnetization Refrigerator 10
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  12. 12. Limitation of the ADR The performance of the ADR decreases as the “warm” heat sink is raised. Hence a mechanical cooler cannot be used as the “warm” heat sink. Mechanical coolers small enough for satellite use can cool down only as far as 6 to 8 milli Kelvin. However it would be extremely convenient to use a mechanical cooler instead of a liquid helium bath. The liquid helium bath slowly evaporates, until it is completely gone. 12
  13. 13. Advanced Adiabatic Demagnetization Refrigerator (AADR) The AADR is a multistage adiabatic demagnetization refrigerator. Each stage passes the absorbed heat to the next stage. The AADR is not one ADR but a group of ADR’s. Each standard ADR would have a relatively small temperature drop across it and would hence be able to remain cooler for a longer time. Its purpose was to combine the high performance of an XRS ADR with the convenience of a mechanical cooler. 13
  14. 14.  This AADR 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 are surrounded by magnetic shield to prevent magnetic fields from interfering with 14 other equipment.
  15. 15. Working Heat is constantly leaking into the AADR from warmer surroundings. This can be from the physical supports, or as infrared radiations from the sensors studying the radiation . The cold end salt pill – The purpose of this pill is to absorb this heat so that the sensors can stay at their best operating temperature. The middle salt pill – This pill is designed in such a way that it can be cooled to a temperature slightly colder than the cold end salt pill. This pill is cooled by reducing the magnetic field produced by the magnets surrounding it. 15
  16. 16.  The Hot end salt pill – From the middle heat salt pill the heat is transferred to the hot end salt pill. Before the heat can be transferred, the middle pill should be brought to the top and the hot end pill, to the bottom, of its temperature range. From the hot end salt pill to the Heat sink – Once the hot end salt pill is hot enough the heat switch connecting this pill to the heat sink is activated. The sink might be liquid helium bath or a mechanical cryo-cooler. While the middle and hot end salt pills have been transferring out the cold end salt pill absorbs heat, ready to start the cycle again. 16
  17. 17. Advantages of the AADR Greater temperature range Mechanical cooler as heat sink Continuous cooling Lower weight 17
  18. 18. X-Ray Spectrometer (XRS) The XRS is an instrument designed to study x- rays emitted by black holes and other astronomical objects. It shows how liquid helium cooling and an ADR can work together as part of a satellite cooling system. The system included some features that were intended to lengthen the lifetime of liquid helium. To function properly, the x-ray sensors in the XRS needed to be cooled to sixty thousandths of a degree above absolute zero. For this we chose an ADR. 18
  19. 19. Working of the XRS First the x rays must be focused onto the detectors. The detectors need to be kept extremely cold. This requires a complex cryogenic system The signals from the detectors are amplified and shaped 19 by a package of
  20. 20. XRS Cryogenic System The XRS must operate at a low temperature to minimize phonon noise and maximize the sensitivity of the resistive thermometer. The primary source of cooling is a 130 liter solid neon dewar. The life of the neon is extended by use of a mechanical cooler. The solid neon maintains a temperature of approx. 17 K and surrounds a tank filled with helium. The liquid helium is vented to space. The final stage of cooling is accomplished via the use of an ADR. Temperature control is accomplished by adjusting the magnetic field via a feedback loop. 20
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  22. 22. Top view of the Astro-E2 XRS 22
  23. 23. Thank You 23
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