Measurements of Multi-Layer Insulation at High Boundary Temperature, using a Simple Non-Calorimetric Method

In spite of abundant literature, the thermal performance of Multi-Layer Insulation (MLI) still deserves dedicated investigation for specific applications, as the achievable insulation strongly depends on installation details. Furthermore, less accurate information is available for warm than for cold boundaries, since errors due to edge effects in small test benches increase strongly with warm boundary temperature. We establish here the thermal performance of MLI between 300 K and 77 K or 4 K, without bringing calorimetric methods into play, through the accurate measurement of a temperature profile. A cylinder in thin copper, wrapped with MLI, is cooled at one extremity while suspended under vacuum inside a sheath at room temperature. For known thermal conductivity and thickness of the tube, the heat flux can be inferred from the temperature profile. In-situ measurement of the thermal conductivity is obtained by applying a know heat flow at the warm extremity of the cylinder. Results, cross-checked with a calibrated heatmeter, compare well with what previously obtained on a large-scale measuring facility

A Microcryostat for Refrigeration at 1.8 K

A microcryostat has been developed in the Central Cryogenic Laboratory at CERN with the purpose of cooling a prototype beam loss monitor for the LHC, based on bolometry at 1.8 K. Its characteristics a re the very compact volume (some cm3 LHe) ensuring short cooldown-warmup times, and its low heat losses (~ 8 mW). The cryostat can be mounted on top of a small dewar through a rigid straight transfer line for continuous feeding

A Large-scale Test Facility for Heat Load Measurements down to 1.9 K

Laboratory-scale tests aimed at minimizing the thermal loads of the LHC magnet cryostat have gone along with the development of the various mechanical components. For final validation of the industrial design with respect to heat inleaks between large surfaces at different temperatures, a full-scale test cryostat has been constructed. The facility reproduces the same pattern of temperature levels as the LHC dipole cryostat, avoiding the heat inleaks from local components like supports and feedthroughs and carefully minimizing fringe effects due to the truncated geometry of the facility with respect to the LHC cryostats serial layout. Thermal loads to the actively cooled radiation screen, operated between 50 K and 65 K, are measured by enthalpy difference along its length. At 1.9 K, the loads are obtained from the temperature difference across a superfluid helium exchanger. On the beam screen, the electrical power needed to stabilize the temperature at 20 K yields a direct reading of the heat losses. Precise in-situ calibration is achieved by subcooling the thermal screen, thereby zeroing radiative heat loads. Minimizing fringe effects has been rewarded by a high precision measurement, yielding one of the more accurate quantifications to date of an industrial application of MLI. The influence of possible openings in the thermal screen is monitored both at the 1.9 K bath and with a radiation sensitive bolometer

Cryogenic R&D at the CERN Central Cryogenic Laboratory

The Central Cryogenic Laboratory operates since many years at CERN in the framework of cryogenic R&D for accelerators and experiments. The laboratory hosts several experimental posts for small cryogen ic tests, all implemented with pumping facility for GHe and vacuum, and is equipped with a He liquefier producing 6.105 l/year, which is distributed in dewars. Tests include thermomechanical qualifica tion of structural materials, cryogenic and vacuum qualification of prototypes, evaluation of thermal losses of components. Some of the most relevant results obtained at the laboratory in the last yea rs are outlined in this paper

Influence of Thermal Cycling on Cryogenic Thermometers

The stringent requirements on temperature control of the superconducting magnets for the Large Hadron Collider (LHC), impose that the cryogenic temperature sensors meet compelling demands such as long-term stability, radiation hardness, readout accuracy better than 5 mK at 1.8 K and compatibility with industrial control equipment. This paper presents the results concerning long-term stability of resistance temperature sensors submitted to cryogenic thermal cycles. For this task a simple test facility has been designed, constructed and put into operation for cycling simultaneously 115 cryogenic thermometers between 300 K and 4.2 K. A thermal cycle is set to last 71/4 hours: 3 hours for either cooling down or warming up the sensors and 1 respectively 1/4 hour at steady temperature conditions at each end of the temperature cycle. A Programmable Logic Controller (PLC) drives automatically this operation by reading 2 thermometers and actuating on 3 valves and 1 heater. The first thermal cycle was accomplished in a temperature calibration facility and all the thermometers were recalibrated again after 10, 25 and 50 cycles. Care is taken in order not to expose the sensing elements to moisture that can reputedly affect the performance of some of the sensors under investigation. The temperature sensors included Allen-Bradley and TVO carbon resistors, Cernox, thin-film germanium, thin-film and wire-wound Rh-Fe sensors

Thermal Characterization of the HeII LHC Heat Exchanger Tube

The LHC magnet cooling scheme is based on a HeII bayonet heat exchanger, which acts as a quasi isothermal heat sink. In order to assess the thermal performance of the oxygen free, annealed/cold worked copper tube, measurements of the total thermal conductance of the tube were performed in a laboratory set-up. This paper describes the experimental technique, which permits to separate the contributio n of the Kapitza interface resistance from the total transverse conductance. The influence of the surface treatment on the Kapitza resistance is also discussed

New cryogenic facilities for testing superconducting equipments for the CERN Large Hadron Collider

CERN's major project, the Large Hadron Collider (LHC), has moved to an implementation phase with machine construction to be completed by 2005. To achieve the design proton-proton centre of mass energy of 14 TeV in the given 27 km circumference LEP tunnel, the LHC will make an extensive use of high-field superconducting magnets using Nb-Ti filament operated at 1.9 K. In order to test, on the one han d, the superconducting cables of the magnets and, on the other hand, the expected performance of several of these magnets assembled in a string representing the lattice period of the machine (107 m lo ng), CERN has installed new cryogenic test facilities. The paper briefly describes these new facilities with all their associated equipments

Heat Flow Measurements on LHC Components

The refrigeration and liquefaction capacity necessary to operate at 1.9 K the 27 km long string of superconducting magnets of the LHC has been determined on the basis of heat load estimates, including static heat inleaks from ambient temperature, resistive heating and dynamic beam-induced heat loads. At all temperature levels, the static heat inleaks determine at least one third of the total heat loads in nominal operating conditions of the machine. Design validation of individual cryocomponents therefore requires a correct estimate of the heat inleaks they induce at all temperature levels, in order not to exceed the allocated heat budget. This paper illustrates the measurements of heat inleaks for several cold components of the future machine, including insulating supports, radiation shields, multi-layer insulation, instrumentation current leads. Distinct methods to determine the heat flow are chosen, depending on the expected heat loads, the temperature range spanned by the heat intercepts, and the working conditions of the component itself

Cryogenics for CERN experiments: past, present and future

Use of cryogenics at CERN was originated (in the 1960s) by bubble chambers and the associated s.c. solenoids. Complex cryoplants were installed to provide cooling at LH2 and LHe temperatures. Continuity (in the 1970s) in He cryogenics for experiments was provided by spectrometer magnets for fixed target physics of the SPS accelerator. More recently (in the 1980s), large "particle-transparent" s.c. solenoids for collider experiments (LEP) have been built demanding new cryoplants. The LHC experiments (in the 2000s) will continue the tradition with s.c. dipoles (ALICE and LHCb), solenoids (CMS, ATLAS) and toroids (ATLAS) of unusual size. Cryogenics for experiments using noble liquids follows the same trend since the development (in the 1970s) of the first shower LAr detectors. A LKr calorimeter (about 10 m3) will be operated in 1996 and the ATLAS experiment foresees a set of three huge LAr calorimeters (almost 90 m3 total volume of liquid) to be installed underground