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   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.

            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.
                    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.

   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
   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).

    Adiabatic Demagnetization Refrigerator
   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
   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.

                     ADR Components

   The basic ADR
    components are as
    shown :
   Calorimeter - These are
    sensors that measure
    heat input eg. in XRS
   Heat switch – The heat
    switch is used to allow
    heat to be dumped
    periodically to the helium
   Thermal Bus – These are
    copper rods that connect
   Salt Pill – The salt pill is
    where the cooling action
    takes place. The pill
    (actually a cylinder) is
    made of ferric ammonium
   Suspension – The outer
    structure of the ADR
    consists of metal rings
    and tubes, which allow the
    ADR to fit securely within
    a superconducting
   Heat Switch Shell – The
    shell is a cylinder made of
    Vespel, a polyimide            9
    material, which provides
Adiabatic Demagnetization Refrigerator

              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
    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.

     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.
   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.

   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.
   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.

            Advantages of the AADR

   Greater temperature range
   Mechanical cooler as heat sink
   Continuous cooling
   Lower weight

           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
   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.
                 Working of the XRS

   First the x rays must
    be focused onto the
   The detectors need to
    be kept extremely
    cold. This requires a
    complex cryogenic
   The signals from the
    detectors are
    amplified and shaped              19
    by a package of
               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.
Top view of the Astro-E2 XRS

Thank You


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