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Extreme cryogenics keeps LHC cool

17 August 2000

When it comes into operation in 2005, CERN’s Large Hadron Collider (LHC),
which will use thousands of superconducting magnets operating at superfluid
helium temperatures, will also be the largest cryogenic system of its kind in the
world. The LHC has to operate below 2K to achieve the strong magnetic fields
required to hold protons in orbit in the confines of CERN’s existing 27 km
tunnel. Supplying all of this liquid helium is a major cryoengineering
challenge.

Pre-series test cells for the LHC cryogenic distribution line
(QRL) went on test at the laboratory at the beginning of June. Made by three
European industrial groups, the test cells have been produced following a 1995
decision to separate the accelerator’s cryogenic distribution system from the
magnet cryostats.

The original design for the LHC included the
machine’s cryogenic distribution system in the same cryostat as the magnets.
This was abandoned in favour of the present solution to avoid unnecessary
complexity for the magnet cryostats, their interconnections and the
commissioning of the cryogenic system.

This choice had already been
successfully adopted for the HERA collider at Germany’s DESY laboratory. In
the present system, eight cryogenic plants distributed over five access points
around the LHC ring will feed the superconducting magnets via eight
approximately 3.2 km long QRL sectors, each of which will operate
independently.

Helium at different temperatures and pressures will be
supplied to the magnets via service modules joining the accelerator’s cryogenic
components to the QRL every 107 m within the accelerator’s bending arcs, a
distance that corresponds to a single LHC cell of six dipole and two quadrupole
magnets. Elsewhere the interconnections will be at varying
distances.

Each of the three test cells currently being put through its
paces at CERN is a section about 112 m long in which two service modules
are joined by a pipe module made up of several straight pipe elements.
Cryogenic supply infrastructure and a number of so-called end-boxes made at
CERN complete the test set-up. Two end-boxes close the interconnections that
will join units of the final cryogenic distribution line together. A further two cap
the service modules where connections to the magnet cryostats will be made.
The whole test set-up is extensively instrumented to allow the thermal and
mechanical measurements necessary for the technical validation of the system
to be made.

Owing to the huge scale of the LHC – some 25.6 km of
cryogenic distribution line involving about 200 km of piping needing thousands
of welds, around 3300 bellows and 1700 control valves at low temperature – a
combination of precision stainless steel piping experience and cryogenics
expertise is required from contractors.

Moreover, the finished system
will not only be very large and technically challenging but will also be required
to operate with unfailing reliability for 6600 h a year over the LHC’s projected
20 year lifespan.

When a market survey was launched in 1996, the
results revealed few companies with the relevant expertise in both areas, so
industrial consortia were sought. Seven groups were invited to tender in 1997,
five replied and three were retained. They are France’s Air Liquide, the
German consortium Linde-Babcock and a larger consortium headed by
Switzerland’s ABB Alstom Power (the other members are Nordon from
France, Kraftanlagen Nukleartechnik, Messer Griesheim and Alcatel Kabel
from Germany).

The LHC cryogenic team is careful to stress that the
modules currently at CERN are pre-series test cells and not prototypes. The
technology is known and the challenge is the large-scale series production of
sophisticated cryogenic transfer line components to produce a reliable
cryogenic distribution system.

The QRL is four times as long as any
existing system and requires the lowest heat inleak ever demanded. Altogether
this makes stringent quality control a key issue. The three test cells at CERN
are therefore being used not to demonstrate a completely new technology but
to qualify the chosen design and test its thermal and mechanical performance
before a final call to tender for full-scale production is launched.

A
healthy spirit of competition is being maintained between the three suppliers,
each of which will be bidding to build the eight sectors of the final system, or a
share of it. Final contract adjudication is expected next year. Installation and
commissioning of the cryogenic distribution line, which precedes installation of
the magnets, is scheduled to run from mid-2002 until summer 2004.

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