The document describes the Large Hadron Collider (LHC), detailing its status as the world's largest cryogenic system that operates at extremely low temperatures of 1.9 K. It explains the necessity of superfluid helium as a coolant, its properties, and the multi-stage cooling process involved. Lastly, it highlights the role of superconducting electromagnets in maintaining high-energy particle movement within the collider.

1. -By Nishit Desai
Cryogenics
(World’s Largest Cryogenic System)

2. Largest Cryogenic System
 LHC = Large Hadron Collider
 One of the coldest places on the Earth
 Main magnets operate at 1.9 K (-271.3°C), colder
than outer space which is at 2.7 K (-270.5°C)
 Requires 120 tones (120000 kg) of superfluid helium
to keep magnets at 1.9 K

3. Why LHC required low temperatures?
 Uses powerful electromagnets to keep its high-energy
particles on a circular track.
 To provide the strong fields needed to grip its high-
energy particles, the LHC electromagnets should be
superconductors.
 In this way, the LHC magnets can be powered to very
high fields and at minimal cost.

4. Why LHC required low temperatures?
 Most materials which become superconducting only
do so at liquid helium temperatures.
 The critical current at which LHC is operating depends
on temperature.
 To maintain the required high currents, the LHC's
electromagnets are cooled down to just 1.9 K, at
which temperature helium is a superfluid.

5. Largest Cryogenic System
 The Centrifugal
compressors which
operate like spin-driers,
hurling helium outwards
into the compressor
outlets. These operate at
extremely high speeds (up
to 54000 rpm), requiring
special active magnetic
Fig. Helium centrifugal
Compressors at the test
and assembly area for
LHC equipment.

6. Largest Cryogenic System
 With almost no viscosity,
superfluid helium penetrates tiny
cracks, 'soaking' deep inside the
magnet coils to absorb any
deposited or generated heat.
 The heat is quickly transported to
a heat exchanger pipe containing
a mixture of saturated vapour
and superfluid helium arranged in
a series of 107-metre cooling
loops all around the ring.
Fig. The LHC uses eight
18-kW cryoplants, one of
the 18-kW 4.5-K
refrigerator units.

7. Cooling Process
 Entire cooling process consisting of 3 stages
takes weeks to complete
 First Stage = Helium is cooled at 80 K.
 10,000 tones of liquid nitrogen are used in
heat exchangers in the refrigerating equipment to
bring the temperature of the helium down to 80
K.

8. Cooling Process
 Second Stage = Helium is cooled to 4.5 K from
80 K
 Turbines are used for this purpose
 Third Stage = Helium is cooled to 1.9 K from 4.5
K
 The 1.8 KW refrigeration units are used for this
purpose
 The Cryogenics System cools 36,000 tones of

9. Why Superfluid Helium?
 Helium is natural choice of coolant as its
properties allow components to be kept cool over
long distances.
 Superfluid helium (cooled below 2.17 K) has
high thermal conductivity and zero viscosity.
 It is an efficient heat conductor.

10. References
 CERN Document Server
Article: - Cryogenics: Low temperatures, high
performance.
Article: - The LHC cryogenic system