G. Riddone

The Cryogenic Distribution Line for the LHC: Functional Specification and Conceptual Design

The Large Hadron Collider (LHC)1 currently under construction at CERN will make use of superconducting magnets operating in superfluid helium below 2 K. The cryogenic distribution scheme for each of the eight sectors, individually served by a refrigeration plant, is based on a separate Cryogenic Distribution Line (QRL) feeding helium at different temperatures and pressures to the elementary cooling loops. The QRL comprises two supply headers and three return headers including a sub-atmospheric one. Low heat inleak to all temperature levels is essential for the overall LHC cryogenic performance. With an overall length of 25.6 km the QRL has a very critical cost-to-performance ratio. Therefore, following an in-house feasibility study, CERN adjudicated in autumn 1998 three industrial contracts in parallel for the supply of Pre-Series Test Cells (∼ 112 m) of the QRL, which will be tested at CERN in 2000. Installation of the QRL for LHC is scheduled from 2002 to mid 2004. This paper will present the general layout, the functional requirements as well as some aspects of the in-house conceptual design.

A Simplified Cryogenic Distribution Scheme for the Large Hadron Collider

The Large Hadron Collider (LHC), currently under construction at CERN, will make use of superconducting magnets operating in superfluid helium below 2 K. The reference cryogenic distribution scheme was based, in each 3.3 km sector served by a cryogenic plant, on a separate cryogenic distribution line which feeds elementary cooling loops corresponding to the length of a half-cell (53 m). In order to decrease the number of active components, cryogenic modules and jumper connections between distribution line and magnet strings a simplified cryogenic scheme is now implemented, based on cooling loops corresponding to the length of a full-cell (107 m) and compatible with the LHC requirements. Performance and redundancy limitations are discussed with respect to the previous scheme and balanced against potential cost savings.

PRELIMINARY RISK ANALYSIS OF THE LHC CRYOGENIC SYSTEM

The Large Hadron Collider (LHC), presently under construction at CERN, will require a helium cryogenic system unprecedented in size and capacity, with more than 1600 superconducting magnets operating in superfluid helium and a total inventory of almost 100 tonnes of helium. The objective of the Preliminary Risk Analysis (PRA) is to identify all risks to personnel, equipment or environment resulting from failures that may accidentally occur within the cryogenic system of LHC in any phase of the machine operation, and that could not be eliminated by design. Assigning a gravity coefficient and one analyzing physical processes that will follow any of the recognised failure modes allows to single out worst case scenarios. Recommendations concerning lines of preventive and corrective defence, as well as for further detailed studies, are formulated.

Demands in Refrigeration Capacity for the Large Hadron Collider

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.

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

Abstract The design, construction and thermal performance of an LHC Cryostat Thermal Model are described. In place of an LHC prototype dipole cold mass a standard- cryostat houses a cold mass with the same overall dimensions, cold surface area, thermal insulation and support systems. The Cryostat Thermal Model is connected to suitably dimensioned end boxes fixing the required temperature levels, by means of saturated helium baths at 1.9 K and 4.2 K, and controlled flows of gaseous helium at 50 to 75 K. Comprehensive instrumentation and fully automatic process control and data acquisition allow precise calorimetric measurements to be performed at all temperature levels.

Measurement and analysis of thermal performance of LHC prototype cryostats

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.

An experimental study of cold helium dispersion in air

The Large Hadron Collider (LHC) presently under construction at CERN, will contain about 100 tons of helium mostly located in the underground tunnel and in caverns. Potential failure modes of the accelerator, which may be followed by helium discharge to the tunnel, have been identified and the corresponding helium flows calculated. To verify the analytical calculations of helium dispersion in the tunnel, a dedicated test set-up has been built. It represents a section of the LHC tunnel at a scale 1:13 and is equipped with a controllable helium relief system enabling the simulation of different scenarios of the LHC cryogenic system failures. Corresponding patterns of cold helium dispersion in air have been observed and analyzed with respect to oxygen deficiency hazard. We report on the test set-up and the measurement results, which have been scaled to real LHC conditions.

The LHC magnet string programme: status and future plans

String 1, with one twin aperture quadrupole and three twin aperture 10-m dipoles (MB1, MB2 and MB3) powered in series and operating at 1.9 K, has recently been dismantled after four years of operation interrupted by technical stops and shutdowns for upgrading or exchanging equipment. Following the validation of the main LHC systems (cryogenics, magnet protection, vacuum, powering and energy extraction) the experimental programme was oriented towards the optimisation of the design and the observation of artificially induced fatigue effects. The design study for String 2 has been completed. This facility, which will be commissioned in December 2000, is composed of two LHC half-cells each consisting of one twin aperture quadrupole and three 15-m twin aperture dipoles. A cryogenic distribution line housing the supply and recovery headers runs parallel to the string of magnets. An electrical feedbox is used to power, with high temperature superconductor current leads, the circuits as in the regular part of an LHC arc. This paper reviews the experiments carried-out with String 1 and summarises the results obtained after more than 12800 hours of operation below 1.9 K and 172 quenches. It also describes the layout and the components of String 2 and explains the objectives pursued by its designers.

Investigation of Thermal and Vacuum Transients on the LHC Prototype Magnet String

Publisher Summary The prototype magnet string is a full-scale working model of a 50-m length of the future Large Hadron Collider (LHC), CERNs new accelerator project, which may use high-field superconducting magnets operating below 2 K in superfluid helium. As such, it provides an excellent test bed for practicing standard operating modes of LHC insulation vacuum and cryogenics, and assessing accidental behavior and failure modes experimentally, and thus verifying design calculations. This chapter presents experimental investigation of insulation vacuum pumpdown, magnet forced-flow cooldown and warmup, and evolution of residual vacuum pressures and temperatures in natural warm-up, and catastrophic loss of insulation vacuum. In all these transient modes, experimental results are compared with simulated behavior, using a non-linear, one-dimensional thermal model of the magnet string.

Update of a cooldown and warmup study for the large hadron collider

The paper presents the inventory of components and materials for LHC magnets, especially for main dipoles and quadrupoles. A mathematical model for LHC transient modes, such as cooldown and warmup of a magnet, a standard cell and the eight LHC sectors, has been developed on the basis of the up-to-date layout of the LHC machine, and validated by experimental data. The model considers the momentum and continuity equations, as well as the energy equations for helium and materials. Based on the simulation results, the heat transfer in the magnets has been studied and the transient modes optimized.