D. Delikaris

A helium freeze-out cleaner operating at atmospheric pressure

Abstract A low pressure helium purification system has been designed at CERN. The helium gas recovered by means of a set of vacuum pumps from subatmospheric cryogenic circuits is cleaned at purity levels permitting direct re-liquefaction into the main cryo-plant cycle. The gas to be cleaned is close to ambient temperature and atmospheric pressure. It is cooled down to 33K by counterflow heat exchanger with the processed gas plus a small amount of cold helium gas derived from the main cryoplant. Impurities in the gas to be processed are condensed on the cold surfaces, and purification is secured by a filtering. The processed gas returns directly to the low pressure suction flow of the cryo-plant compressor. So far two freeze-out cleaners have been designed and built and are currently in operation at two independent cryo-plants with liquifaction capacities of approximately 3.5 g/s. The results obtained on purification performance and “lifetime” before subsequent regeneration of the device, pressure drop depending on impurity contents, cold gas requirements and heat exchanger performance compare well with theoretical predictions. Helium gas with impurity levels of up to close to 13000 ppm by weight have been treated. At 2300 ppm and a processed helium gas flow of 0.7 g/s life times of close to 24 hours could be obtained permitting the deposition of 135 g of solid air. Regeneration cycles with respect to life time are short (15 minutes).

Technical Analysis and Statistics from Long Term Helium Cryoplant Operation with Experimental Superconducting Magnets at CERN

Publisher Summary nCERN regularly uses a large number of liquid-helium cryoplants for cooling the superconducting magnets of large particle detectors. They are installed in the experimental areas of the electron-positron collider LEP and the proton accelerator SPS for the observation of high-energy interactions of elementary particles. The typical cold mass of a detector magnet ranges from 1 to 40 tons, and typical cryoplant cooling capacities are between 400 and 800 W/4.5 K entropy equivalent. Operation must be very flexible to meet the varying experimental requirements. This chapter presents the technical data of the system and statistics from over 180000 running hours during the four years from 1992 to 1995. Operation includes phases of cool-down, steady-state cooling, and recovery after magnet quench or other incidents and warm-up of the superconducting magnets. It emphasizes on the analysis of fault conditions, multiple interaction between perturbations and consequences for the users of liquid-helium supply interruption. It is particularly interesting to analyze the data from stoppages directly linked to faults in the cryogenic equipment in order to gain a better understanding of the necessary improvements to the various systems when the new cryogenic installations are designed for future experiments using superconducting magnets.

Cryogenics for CERN experiments : past, present and future

Publisher Summary Cryogenics for CERN accelerators, limited to some application in the past, has expanded considerably following the recent development of superconducting (s.c.) accelerating cavities and high field bending magnets. Use of cryogenics at CERN has been originated by bubble chambers and the associated s.c. solenoids. Complex cryoplants are installed to provide cooling at LH2 and LHe temperatures. Continuity in He cryogenics for experiments is provided by spectrometer magnets for fixed target physics of the SPS accelerator. More recently, large particle-transparent s.c. solenoids for collider experiments (LEP) have been built demanding new cryoplants. The LHC experiments continue the tradition with s.c. dipoles, solenoids and toroids of unusual size. Cryogenics for experiments using noble liquids follow the same trend since the development of the first shower LAr detectors. A LKr calorimeter may be operated in 1996 and the ATLAS experiment foresees a set of three huge LAr calorimeters to be installed underground.

First assessment of reliability data for the LHC accelerator and detector cryogenic system components

The Large Hadron Collider (LHC) cryogenic system comprises eight independent refrigeration and distribution systems that supply the eight 3.3 km long accelerator sectors with cryogenic refrigeration power as well as four refrigeration systems for the needs of the detectors ATLAS and CMS. In order to ensure the highest possible reliability of the installations, it is important to apply a reliability centred approach for the maintenance. Even though large scale cryogenic refrigeration exists since the mid 20th century, very little third party reliability data is available today. CERN has started to collect data with its computer aided maintenance management system (CAMMS) in 2009, when the accelerator has gone into normal operation. This paper presents the reliability observations from the operation and the maintenance side, as well as statistical data collected by the means of the CAMMS system.

Long term experience with cryoplant operation for superconducting magnets and RF cavities at CERN

Eighteen liquid-helium cryoplants are presently in use at CERN, four of them commissioned in 1992. Unit capacities (entropy equivalent) range from 0.1 to 6 kW/4.5 K. Four even larger cryoplants (12 kW/4.5 K, upgradable to 18 kW/4.5 K) are in the process of installation and commissioning. Apart from feeding laboratories for development and tests of cryogenic equipment, the cryoplants provide cooling for superconducting detector and accelerator magnets and superconducting RF cavities, where their uninterrupted availability is crucial for efficient accelerator operation. Integrated running time in 1992 was of the order of 100000 hours. This paper summarises experience from all phases of operation, normal running, emergencies, cool-down and warm-up. Some information is given on software controls, data acquisition, and fault analysis, and on conclusions concerning corrective or preventive maintenance and advisability of investments for increased availability of cryogenics.<<ETX>>

Commissioning and First Operation of the Cryogenics for the CERN Axion Solar Telescope (CAST)

A new experiment, the CERN Axion Solar Telescope (CAST) was installed and commissioned in 2002. Its aim is to experimentally prove the existence of an as yet hypothetical particle predicted by theory as a solution of the strong CP problem and possible candidate for galactic dark matter. The heart of the detector consists of a decommissioned 10‐m long LHC superconducting dipole prototype magnet, providing a magnetic field of up to 9.5 T. The whole telescope assembly is aligned with high precision to the core of the sun. If they exist, axions could be copiously produced in the core of the sun and converted into photons within the transverse magnetic field of the telescope. The converted low‐energy solar axion spectrum, peaked around a mean energy of 4.4 keV, can then be focused by a special x‐ray mirror system and detected by low‐background photon detectors, installed on each end of the telescopes twin beam pipes. This paper describes the external and proximity cryogenic system and magnet commissioning as w...

THE MANAGEMENT OF CRYOGENS AT CERN

CERN is a large user of industrially procured cryogens essentially liquid helium and nitrogen. Recent contracts have been placed by the Organization for the delivery of quantities up to 280 tons of liquid helium over four years and up to 50000 tons of liquid nitrogen over three years. Main users are the very large cryogenic system of the LHC accelerator complex, the physics experiments using superconducting magnets and liquefied gases and all the related test facilities whether industrial or laboratory scale. With the commissioning of LHC, the need of cryogens at CERN will considerably increase and the procurement policy must be adapted accordingly. In this paper, we discuss procurement strategy for liquid helium and nitrogen, including delivery rates, distribution methods and adopted safety standards. Global turnover, on site re-liquefaction capacity, operational consumption, accidental losses, purification means and storage capacity will be described. Finally, the short to medium term evolution of the Organizations requirements will be reviewed.

Conclusions from 12 years operational experience of the cryoplants for the superconducting magnets of the LEP experiments

The Large Electron Positron Collider (LEP) has ended its last physics run in November 2000, and it is at present being dismantled to liberate the tunnel for the Large Hadron Collider (LHC) project to be completed by end of 2005. The cryogenic systems for the superconducting solenoid and focusing quadrupoles for the two LEP experiments, ALEPH and DELPHI, each supplying a cooling power of 800 W/4.5 K entropy equivalent, have accumulated more then 100’000 hours of running time. The paper summarises the 12 years cryogenic experience in the various operating modes: cool-down, steady state, recovery after energy fast dump, utilities failures and warm-up of the superconducting magnets. The detailed operation statistics is presented and compared to the other CERN cryogenic systems. Emphasis is given to the technical analysis of the fault conditions and of their consequences on the helium refrigeration production time in view of the future operation of the LHC cryogenics.