Philippe Lebrun

Cryogenics for the Large Hadron Collider

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.

Superfluid helium cryogenics for the large hadron collider project at CERN

Abstract The Large Hadron Collider (LHC) at CERN will be the next research instrument of high-energy physics. Colliding protons at 14 TeV center-of-mass energy and high luminosity, it will probe the structure of matter down to an unprecedentedly fine scale, thus allowing to reproduce in the laboratory phenomena which occurred in the very early universe. On the technological side, the LHC makes use of high-field superconducting magnets for guidance and focusing of the particle beams around the 26.7 km circumference of the machine, to be installed in the existing LEP tunnel. The nominal bending field of 8.65 T is produced in some 1300 twin-aperture dipoles, wound with small-filament Nb Ti conductor, and operated below 1.9 K in static baths of pressurized helium II, thus taking advantage of its specific properties as cooling fluid. We present the main technical challenges of the LHC cryogenic system, and review the actions of development and the preparatory work in progress.

submitter : Pressure drop and transient heat transport in forced flow single phase helium II at high Reynoldsnumbers

Abstract Pressure drop and transient heat transport measurements had been performed in the superfluid helium test loop at CEA/CEN-Grenoble, France. Single phase helium II was circulated through a 28 mm I.D. tube, over a length of 200 m for mass flow rates between 0.02 and 0.1 kg/s. Steady-state pressure drop measurements allow us to estimate the friction factor for Reynolds numbers in the 10 6 range. Rectangular heat pulses were applied at mid-length, and the fluid temperature was measured at several locations along the flow, upstream and downstream. A code has been developed which uses the two-fluid model to simulate time-dependent heat transport helium 11 flow. Simulation results are in good agreement with experimental measurements.

Design concept and first experimental validation of the superfluid helium system for the Large Hadron Collider (LHC) project at CERN

Abstract The superconducting magnets of the CERN Large Hadron Collider (LHC) project must be kept at their operating temperature of 1.9 K all around the 26.7 km circumference of the machine ring. For this purpose, they are immersed in static baths of pressurized helium II, acting as heat transport medium to a heat exchanger tube running along the magnet string, in which flowing saturated helium II absorbs the deposited heat at constant temperature. Proper operation of this cooling scheme requires stability of the two-phase helium flow and adequate heat transport across the helium II heat exchanger. To investigate these issues, we have conducted tests on heated two-phase flow of saturated helium II in a 24 m long tube simulating the magnet heat exchanger.

Thermohydraulics of quenches and helium recovery in the LHC prototype magnet strings

In preparation for the Large Hadron Collider project, a 42.5 m-long prototype superconducting magnet string, representing a half-cell of the machine lattice, has been built and operated. A series of tests was performed to assess the thermohydraulics of resistive transitions (quenches) of the superconducting magnets. These measurements provide the necessary foundation for describing the observed evolution of the helium in the cold mass and formulating a mathematical model based on energy conservation. The evolution of helium after a quench simulated with the model reproduces the observations. We then extend the simulations to a full LHC cell, and finally analyse the recovery of helium discharged from the cold mass.

Superconducting instrumentation for high Reynolds turbulence experiments with low temperature gaseous helium

Turbulence is of common experience and of high interest for industrial applications, despite its physical grounds is still not understood. Cryogenic gaseous helium gives access to extremely high Reynolds numbers (Re). We describe an instrumentation hosted in CERN, which provides a 6 kW @ 4.5 K helium refrigerator directly connected to the experiment. The flow is a round jet; the flow rates range from 20 g/s up to 260 g/s at 4.8 K and about 1.2 bar, giving access to the highest controlled Re flow ever developed. The experimental challenge lies in the range of scales which have to be investigated: from the smallest viscous scale η, typically 1 μm at Re=107 to the largest L∼10 cm. The corresponding frequencies: f=v/η can be as large as 1 MHz. The development of an original micrometric superconducting anemometer using a hot spot and its characteristics will be discussed together with its operation and the perspectives associated with superconducting anemometry.

Operation of a forced flow superfluid helium test facility and first results

Abstract Cooling techniques based on forced flow He II appear as a possible solution for very large superconducting magnet systems with field strengths in the order of 10 T. To prepare the definition of such cooling systems, a He II test facility of significant size has been built at CEN Grenoble, in collaboration with CERN in Geneva and the EURATOM-CEA fusion research group in Cadarache, France. Its main features are a nearly 230 m long test section, 28 mm in diameter, and mass flow rates of up to 100 g/s of 1.8 K He. First experimental results are presented and compared with solutions predicted by a model calculation.

Precision heat inleak measurements on cryogenic components at 80 K, 4.2 K and 1.8 K

Abstract We have built and are operating a test bench for precision heat inleak measurements and thermal qualification of components for the superfluid helium cryostats of the CERN Large Hadron Collider (LHC) project. The bench features the three temperature levels used for heat interception in the LHC cryostats. The heat loads are measured using both calibrated thermal conductances (“heatmeters”) and standard boil-off methods. After validation of the measurement methods, we compare experimental results with thermal design calculations on a prototype magnet support post.

Investigation of quench pressure transients in the LHC superconducting magnets

Abstract In case of resistive transition of a LHC superconducting magnet (“quench”), the stored energy is dissipated in the winding within a few tenths of a second. A fraction of this energy is transferred to the helium inside the windings and around the beam tubes, causing expansion and axial flow. This results in a fast pressure peak (several MPa) at the midlength of the magnet. We present a one-dimensional thermo-hydraulic model aiming at simulation of the quench pressure peak and study the influence of geometrical parameters, such as magnet length and beam pipe diameter. Results of simulations are compared to measurements performed on a short model and on a quasi-full scale prototype of the LHC dipoles.

Does one need a 4.5 K screen in cryostats of superconducting accelerator devices operating in superfluid helium? lessons from the LHL

Superfluid helium is increasingly used as a coolant for superconducting devices in particle accelerators: the lower temperature enhances the performance of superconductors in high-field magnets and reduces BCS losses in RF acceleration cavities, while the excellent transport properties of superfluid helium can be put to work in efficient distributed cooling systems. The thermodynamic penalty of operating at lower temperature however requires careful management of the heat loads, achieved inter alia through proper design and construction of the cryostats. A recurrent question appears to be that of the need and practical feasibility of an additional screen cooled by normal helium at around 4.5 K surrounding the cold mass at about 2 K, in such cryostats equipped with a standard 80 K screen. We introduce the issue in terms of first principles applied to the configuration of the cryostats, discuss technical constraints and economical limitations, and illustrate the argumentation with examples taken from large projects confronted with this issue, i.e. CEBAF, SPL, ESS, LHC, TESLA, European X-FEL, ILC.