N. Delruelle

The helium cryogenic system for the ATLAS experiment

The magnetic configuration of the ATLAS detector is generated by an inner superconducting solenoid and three air-core toroids (the barrel and two end-caps), each of them made of eight superconducting coils. Two separated helium refrigerators will be used to allow cool-down from ambient temperature and steady-state operation at 4.5 K of all the magnets having a total cold mass of about 600 tons. In comparison with the preliminary design, the helium distribution scheme and interface with the magnet sub-systems are simplified, resulting in a considerable improvement of the operational easiness and the overall reliability of the system at some expense of the operational flexibility. The paper presents the cryogenic layout and the basic principles for magnets cooldown, steady state operation and thermal recovery after a fast energy dump.

The CERN cryogenic test facility for the ATLAS barrel toroid magnets

The superconducting magnet system of the ATLAS detector will consist of a central solenoid, two end-cap toroidal magnets (ECT) and the barrel toroid magnet (BT) made of eight coils symmetrically placed around the central axis of the detector. The magnets will be tested individually in a 5000 m/sup 2/ experimental area prior to their final installation at an underground cavern of the LHC Collider. For the BT magnets, a dedicated cryogenic test facility has been designed which is currently under the construction and commissioning phase. A liquid nitrogen pre-cooling unit and a 1200 W@4.5K refrigerator will allow flexible operating conditions via a rather complex distribution and transfer line system. Flow of two-phase helium for cooling the coils is provided by centrifugal pumps immersed in a saturated liquid helium bath. The integration of the pumps in an existing cryostat required the adoption of novel mechanical solutions. Tests conducted permitted the validation of the technical design of the cryostat and its instrumentation. The characteristics of one pump were measured and pressure rise of 300 mbar at nominal flow of 80 g/s confirmed the specifications.

ATLAS magnet common cryogenic, vacuum, electrical and control systems

The superconducting magnet system for the ATLAS detector at the LHC at CERN comprises a barrel toroid, two end cap toroids and a central solenoid with overall dimensions of 20 m diameter by 26 m length and a stored energy of 1.6 GJ. Common proximity cryogenic and electrical systems for the toroids are implemented. The cryogenic system provides the cooling power for the 3 toroid magnets considered as a single cold mass (600 tons) and for the CS. The 21 kA toroid and the 8 kA solenoid electrical circuits comprise both a switch-mode power supply, two circuit breakers, water cooled bus bars, He cooled current leads and the diode-resistor ramp-down unit. The vacuum system consists of a group of primary rotary pumps and sets of high vacuum diffusion pumps connected to each individual cryostat. The magnet safety system guarantees the magnet protection and human safety through slow and fast dump treatment. The magnet control system ensures control, regulation and monitoring of the operation of the magnets. The updated design, layout, development and construction of the systems, as well as the first results of prototyping and commissioning are presented.

Results of the Cryogenic Tests of the Superconducting Magnets forming the Barrel Toroid of the ATLAS Experiment

The Barrel Toroid magnet of the ATLAS experiment will be built from eight 25 m × 5 m racetrack shaped superconducting coils that are symmetrically placed around the central axis of the detector. Prior to their final assembly in the underground cavern of the LHC, these magnets are individually tested at ground level in order to verify the expected overall performances. A dedicated facility has been commissioned and the testing of the coils, at their nominal electrical and thermal operating conditions, has been carried out. The paper presents the results obtained during the cool‐down phase from ambient temperature, the steady‐state operation at 4.5 K, the 20 kA current ramping up/down and the thermal recovery after a fast energy dump of up to 138 MJ stored energy. Included are the measurements of the various thermal loads in both static and dynamic conditions.

Performance of a proximity cryogenic system for the ATLAS central solenoid magnet

The ATLAS central solenoid magnet has been designed and constructed as a collaborative work between KEK and CERN for the ATLAS experiment in the LHC project. The solenoid provides an axial magnetic field of 2 Tesla at the center of the tracking volume of the ATLAS detector. The solenoid is installed in a common cryostat of a liquid-argon calorimeter in order to minimize the mass of the cryostat wall. The coil is cooled indirectly by using two-phase helium flow in a pair of serpentine cooling line. The cryogen is supplied by the ATLAS cryogenic plant, which also supplies helium to the Toroid magnet systems. The proximity cryogenic system for the solenoid has two major components: a control dewar and a valve unit. In addition, a programmable logic controller, PLC, was prepared for the automatic operation and solenoid test in Japan. This paper describes the design of the proximity cryogenic system and results of the performance test.

First Cool-Down and Test at 4.5 K of the ATLAS Superconducting Barrel Toroid Assembled in the LHC Experimental Cavern

The large ATLAS superconducting magnets system consists of the Barrel, two End-Caps Toroids and the Central Solenoid. The eight separate coils making the Barrel Toroid (BT) have been individually tested with success in a dedicated surface test facility in 2004 and 2005 and afterwards assembled in the underground cavern of the ATLAS experiment. In order to fulfill all the cryogenic scenarios foreseen for these magnets with a cold mass of 370 tons, two separate helium refrigerators and a complex helium distribution system have been used. This paper describes the results of the first cool-down, steady-state operation at 4.5 K and quench recovery of the BT in its final configuration.

Commissioning of the Cryogenic System for the ATLAS Superconducting Magnets

The paper describes the test results of the helium cryoplant for the superconducting magnets of the ATLAS particle detector at CERN. It consists of two refrigerators used in common by all the magnets and of two proximity cryogenic systems (PCS) interfacing respectively the toroids and the central solenoid. Emphasis is given to the commissioning of the refrigerators: the main unit of 6 kW equivalent capacity at 4.5 K and the thermal shield refrigerator providing 20 kW between 40 K and 80 K. The first unit is used for refrigeration at 4.5 K and for the cooling of three sets of 20 kA current leads, while the second one provides, in addition to the 20 kW refrigeration of the thermal shields, 60 kW for the cool‐down to 100 K of the 660 ton cold mass of the magnets. The tests, carried out with the equipment in the final underground configuration, are extended to the PCS that includes the large liquid helium centrifugal pumps (each providing 1.2 kg/s) for forced‐flow cooling of the magnets and the complex distri...

The chimney and superconducting bus lines for the ATLAS central solenoid

A thin superconducting solenoid magnet for the ATLAS detector in the CERN-LHC project is under construction as a cooperative work between KEK and CERN. A control dewar at the top of the detector is connected with a coil through a long chimney placed in the gap of the outer detectors and toroidal magnets. A set of superconducting bus lines and cooling tubes is arranged in the chimney. The fabrication of the chimney and the control dewar has been completed and the performance test was carried out. The current leads in the control dewar and the superconducting bus lines in the chimney were successfully tested with a current of 10 kA including 2 kA contingency. Quench characteristics of the bus was measured and also analyzed. A superconducting quench detector worked well to pick up quenches.

Cryogenic Characteristics of the ATLAS Barrel Toroid Superconducting Magnet

ATLAS, one of the experiments of the LHC accelerator under commissioning at CERN, is equipped with a large superconducting magnet the Barrel Toroid (BT) that has been tested at nominal current (20500 A). The BT is composed of eight race-track superconducting coils (each one weights about 45 tons) forming the biggest air core toroidal magnet ever built. By means of a large throughput centrifugal pump, a forced flow (about 10 liter/second at 4.5 K) provides the indirect cooling of the coils in parallel. The paper describes the results of the measurements carried out on the complete cryogenic system assembled in the ATLAS cavern situated 100 m below the ground level. The measurements include, among other ones, the static heat loads, i.e., with no or constant current in the magnet, and the dynamic ones, since additional heat losses are produced, during the current ramp-up or slow dump, by eddy currents induced on the coil casing.

Review of the ATLAS B0 model coil test program

The ATLAS B0 model coil has been extensively tested, reproducing the operational conditions of the final ATLAS barrel toroid coils (ten Kate, 1999). Two test campaigns have taken place on B0, at the CERN facility where the individual BT coils are about to be tested. The first campaign aimed to test the cool-down, warm-up phases and to commission the coil up to its nominal current of 20.5 kA, reproducing Lorentz forces similar to the ones on the BT coil. The second campaign aimed to evaluate the margins above the nominal conditions. The B0 was tested up to 24 kA and specific tests were performed to assess: the coil temperature margin with respect to the design value, the performance of the double pancake internal joints, static and dynamic heat loads, behavior of the coil under quench conditions. The paper reviews the overall test program with emphasis on second campaign results not covered before.