The Large Hadron Collider (LHC), a 26.7 km circumference superconducting accelerator equipped with high-field magnets operating in superfluid helium below 1.9 K, has now fully entered construction at CERN, the European Laboratory for Particle Physics. The heart of the LHC cryogenic system is the quasi-isothermal magnet cooling scheme, in which flowing two-phase saturated superfluid helium removes the heat load from the 36000 ton cold mass, immersed in some 400 m/sup 3/ static pressurised superfluid helium. The LHC also makes use of supercritical helium for nonisothermal cooling of the beam screens which intercept most of the dynamic heat loads at higher temperature. Although not used in normal operation, liquid nitrogen will provide the source of refrigeration for precooling the machine. Refrigeration for the LHC is produced in eight large refrigerators, each with an equivalent capacity of about 18 kW at 4.5 K, completed by 1.8 K refrigeration units making use of several stages of hydrodynamic cold compressors. The cryogenic fluids are distributed to the cryomagnet strings by a compound cryogenic distribution line circling the tunnel. Procurement contracts for the major components of the LHC cryogenic system have been adjudicated to industry, and their progress will be briefly reported. Besides construction proper, the study and development of cryogenics for the LHC has resulted in salient advances in several fields of cryogenic engineering, which we shall also review.

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Cryogenics for the Large Hadron Collider

http://cds.cern.ch/record/266586/files/P00024691.pdf

Superfluid Helium Cryogenics for the Large Hadron Collider Project at CERN.

An initial series of six LHC 10 m long prototype dipole magnets and cryostats have been manufactured by European Industry. The assembled cryo-magnets were tested singly and connected in series in a test string at CERN between March 1994 and December 1996. During the same period, an evolution in the requirements for LHC cryogenics distribution has lead project management to adopt a seChapaute cryo-distribution line running Chapaullel to the LHC machine.1 The former standard LHC half-cell, made up of a short straight section unit and four 10 m dipoles, has been discarded and replaced with one composed of a short straight section unit and three 15 m dipoles. The new 15 m LHC dipole magnet cryostats are described. These units house the dipole magnet cold mass, which stands on three low heat in-leak support columns, and is enclosed within an actively cooled radiation screen operating at 4.5–20 K and an actively cooled thermal shield operating at 50–75 K.

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Design of the Second Series of LHC Prototype Dipole Magnet Cryostats

The capacity demands on the cryogenic installations for the Large Hadron Collider (LHC) at CERN have been recently updated [1]. Unlike the LEP energy upgrade using superconducting acceleration cavities LHC will require high power refrigeration at 1.9 K, as well as non-isothermal cooling at 4.5 K to 20 K and at 50 K to 75 K. This paper presents the assessment of cryogenic capacity that has to be supplied by the eight refrigerators for LHC in relation with the foreseen operating modes of the machine.

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Demands in Refrigeration Capacity for the Large Hadron Collider

A major milestone in the preparation of the Large Hadron Collider (LHC) project is the testing and operation of a 50-m long superconducting magnet string, representing a half-cell of the machine lattice. This also corresponds to the length of the elementary cooling loops providing refrigeration at the 1.9 K, 4.5-to-20 K, and 50-to-75 K levels to the LHC cryomagnets. Based on existing large-capacity cryogenic infrastructure, we have designed, built and are operating a dedicated cryogenic system feeding the LHC Test String, with installed capacities of 120 W @ 1.8 K and 10 g/s supercritical helium at 4.5 K. The system also includes 15 kA, 1.6 kA, 500 A, 250 A and 50 A current lead pairs for powering of main and auxiliary magnet circuits, as well as a 120 kW liquid nitrogen vaporizer for controlled cooldown of the 105 kg cold mass. The system is fully instrumented, controlled by dedicated industrial PLCs connected to an industrial supervision system. We report on performance in operation, including response of the system to transients such as current ramp and discharge, as well as magnet resistive transitions.

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The superfluid helium cryogenic system for the LHC test string : Design, construction and first operation

A full-scale thermal model of a prototype dipole cryomagnet for theCERN LHC project

The heat inleaks into the cryostats represent a significant fraction of the total heat loads to the Large Hadron Collider (LHC). In view of the large size of this machine and the high thermodynamic cost of low-temperature refrigeration, it is essential to maintain the heat inleaks within budgeted values. Following experimental validation of thermal design of critical components (e.g. multilayer reflective insulation, support posts, cold valves), thermal performance assessment of complete cryomagnets has been undertaken. Thermo-mechanical and cryogenic behaviour was measured on full-scale prototype units.

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Measurement and Analysis of Thermal Performance of LHC Prototype Cryostats

The high-field superconducting magnets of the Large Hadron Collider (LHC) project at CERN will operate below 1.9 K in static baths of pressurized helium II. In case of resistive transition (“quench”), the resulting pressure rise in the cryostats must be limited to below their 2 MPa design pressure. This is achieved by discharging helium at high flow-rates into a cold recovery header, normally maintained at 20 K. For this purpose, we have designed, built and tested a cryogenic quench relief valve with a nominal diameter of 50 mm and an opening time of below 0.1 s. The valve, which can be opened on an external trigger, also acts as a relief device actuated by the upstream pressure when it exceeds 0.4 MPa. In normal operation, the closed poppet must be helium-tight, for hydraulic and thermal separation of the magnet baths from the recovery header. Following mechanical qualification tests under vacuum, we have mounted the relief valve in a dedicated cryogenic measuring bench, in order to perform precision thermal measurements with pressurized helium II.

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Cryogenic Performance of a Superfluid Helium Relief Valve for the LHC Superconducting Magnets

The LHC machine, composed of eight sectors with superconducting magnets and accelerating cavities requires a complex cryogenic system providing high cooling capacities (18 kW equivalent at 45 K and 24 kW at 18 K per sector produced in large cold boxes and distributed via 33-km cryogenic transfer lines) After individual reception tests of the cryogenic subsystems (cryogen storages, refrigerators, cryogenic transfer lines and distribution boxes) performed since 2000, the commissioning of the cryogenic system of the first LHC sector has been under way since November 2006 After a brief introduction to the LHC cryogenic system and its specificities, the commissioning is reported detailing the preparation phase (pressure and leak tests, circuit conditioning and flushing), the cool-down sequences including the handling of cryogenic fluids, the magnet powering phase and finally the warm-up Preliminary conclusions on the commissioning of the first LHC sector will be drawn with the review of the critical points already solved or still pending The last part of the paper reports on the first operational experience of the LHC cryogenic system in the perspective of the commissioning of the remaining LHC sectors and the beam injection test

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Commissioning the cryogenic system of the first LHC sector

The superconducting magnets of the Large Hadron Collider (LHC) will be protected by safety relief valves operating at 1.9 K in superfluid helium (Hell). A test facility was developed to precisely determine the heat load and the mass leakage of cryogenic valves with Hell at their inlet. The temperature of the valve inlet can be varied from 1.8 K to 2 K for pressures up to 3.5 bar. The valve outlet pipe temperature can be regulated between 5 K and 20 K. The heat flow is measured with high precision using a Kapitza-resistance heatmeter and is also crosschecked by a vaporization measurement. After calibration, a precision of 10 mW for heat flows up to 1.1 W has been achieved. The helium leak can be measured up to 15 mg/s with an accuracy of 0.2 mg/s. We present a detailed description of the test facility and the measurements showing its performances.

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G. Ferlin

Papers

A full-scale thermal model of a prototype dipole cryomagnet for theCERN LHC project

Measurement and Analysis of Thermal Performance of LHC Prototype Cryostats

Cryogenic Performance of a Superfluid Helium Relief Valve for the LHC Superconducting Magnets

Commissioning the cryogenic system of the first LHC sector

A Facility for Accurate Heat Load and Mass Leak Measurements on Superfluid Helium Valves