CRYOGENICS AND ITS SPACE
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
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.
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 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 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
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.
The basic ADR
components are as
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
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
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
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
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
X-Ray Spectrometer (XRS)
The XRS is an instrument designed to study x-
rays emitted by black holes and other
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
The signals from the
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
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
Temperature control is accomplished by adjusting the
magnetic field via a feedback loop.
Top view of the Astro-E2 XRS