The Large Hadron Collider (LHC) under construction at CERN is based on the use of high-field superconducting magnets operating in superfluid helium. For the validation of the machine dipoles and quadrupoles, a magnet test plant is under construction requiring 12 so-called Cryogenic Feeder Units (CFU). Based on experience done at CERN, two pre-series CFUs were designed and built by industry and are currently in use prior to final series delivery. This presentation describes the features of a CFU, its typical characteristics and the experience acquired with the first units

Benda, V

Schouten, J A

Vullierme, B

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsEXPERIENCE WITH A PRE-SERIES SUPERFLUID HELIUM TEST BENCHFOR LHC MAGNETSV. Benda, B. Vullierme, J. Schouten*The Large Hadron Collider (LHC) under construction at CERN is based on the use of high-fieldsuperconducting magnets operating in superfluid helium. For the validation of the machine dipoles andquadrupoles, a magnet test plant is under construction requiring 12 so-called Cryogenic Feeder Units(CFU). Based on experience done at CERN, two pre-series CFUs were designed and built by industryand are currently in use prior to final series delivery. This presentation describes the features of a CFU,its typical characteristics and the experience acquired with the first units.LHC Division*Oxford Instruments Ltd, Tubney Woods, Abingdon, Oxford OX13 5QX, UKPresented at the Eighteenth International Cryogenic Engineering Conference (ICEC 18)21-25 February 2000, Bombay Mumbai, IndiaGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 388Abstract26 July 2000Experience with a Pre-Series Superfluid Helium Test Bench for LHC MagnetsVladislav Benda, Bruno Vullierme, Joseph Schouten*LHC Division, CERN, 1211 Geneva 23, Switzerland*Oxford Instruments Ltd, Tubney Woods, Abingdon, Oxford OX13 5QX, UKThe Large Hadron Collider (LHC) under construction at CERN is based on theuse of high-field superconducting magnets operating in superfluid helium. For thevalidation of the machine dipoles and quadrupoles, a magnet test plant is underconstruction requiring 12 so-called Cryogenic Feeder Units (CFU). Based onexperience done at CERN, two pre-series CFUs were designed and built byindustry and are currently in use prior to final series delivery. This presentationdescribes the features of a CFU, its typical characteristics and the experienceacquired with the first units.1 INTRODUCTIONThe main magnetic system of LHC will consist of about 1200 twin-aperture, high-field superconductingdipoles and 400 twin-aperture, high-gradient superconducting quadrupoles operating in pressurizedsuperfluid helium below 1.9 K [1]. All these magnets will be tested at CERN before their final installationin the accelerator tunnel.In 1994 the first magnet test bench for LHC magnet prototypes was designed and built by CERN [2].It was used to test 10 magnet prototypes before its dismantling in 1999. On the basis of the experiencedone with this first bench, two pre-series CFUs for testing of LHC pre-series magnets were designed andbuilt by Oxford Instruments Ltd, installed and tested at CERN.The functions of the CFU are to control the cool down and warm up of a magnet, to maintain amagnet in saturated liquid helium at 4.5 K or in pressurized superfluid helium at 1.9 K for magneticmeasurements and quench training, to recover as much liquid helium as possible after a quench and toautomatically cool down the magnet again. The CFU also contains all the current leads for the main coiland for the auxiliary magnets.2 PRINCIPLE OF OPERATIONThe magnet under test is fixed to three support posts, which are anchored to the concrete floor of the testhall. Cryogenic and electric feeding is done through the magnet feed box of the CFU, connected to oneend of the magnet, the opposite end of the CFU being closed by a magnet return box. As the magneticmeasurement equipment only operates at ambient temperature, both magnet apertures are equipped withwarm bore inserts, which bring a heat load of 25 W into the superfluid helium bath.The process flow diagram is presented in Figure1. The main operation modes are as follows:2.1 Magnet pre-cooling (300K to 90 K)When all electric and hydraulic CFU/magnet connections are completed and tested, the vacuum space ofCFU/magnet is pumped down and globally leak tested. As the cold mass is about 35 tonnes, the cool-down from 300 to 90 K is done by helium gas pre-cooled with liquid nitrogen in a dedicated heatexchanger, which is not part of the CFU.The cold helium gas at 10 bar enters the system at interface L1, passes through the phase separatorand control valve FV165 before flowing through the magnet from the far end. The cold helium gas passesthe magnet before flowing through FV149 into the decanter, then the flow is split into two. The main flowgoes through FCV177 while the rest passes through the CFU and the magnet thermal shields. Both flowsjoin again and leave the CFU through interface L2 at about 2 bar. This flow splitting is controlled such asto pass the maximum possible flow through the thermal shield without limiting the required flow for asimultaneous coldmass cool-down. The maximum mass flow during the pre-cooling phase is 80 g/s; themaximum temperature difference between cooling gas and cold mass is 60K.2.2 Magnet cooling (90 K to 1.9 K)When the cold mass reaches 90 K, the helium gas cooling is stopped and the cooling continues with liquidhelium through the same circuit. Liquid helium enters the system at interface L3 and leaves CFU throughinterface L4.Whenever a cold mass is filled with saturated liquid helium at 1.3 bar, the level in the phaseseparator and in all the current lead vessels is kept at the required value. In this phase a magnet can beoperated at 4.5 K or it can be cooled further down to its working temperature of 1.9 K. For this purposethe CFU is equipped with a low-pressure circuit connected to a liquid/liquid heat exchanger (LLHX) builtinside the magnet. Saturated helium at 1.3 bar passes through TCV145 which is the main impedance inthis circuit and which controls the helium level inside the LLHX. The liquid helium is pre-cooled in aliquid/gas heat exchanger with cold low pressure gas which is pumped via PCV144 controlling thepressure in the LLHX. The pre-cooling liquid continues through an internal pipe to the opposite end ofthe LLHX, thermally coupled to the cold mass. The cold low-pressure gas leaves the CFU, passes theVery Low Pressure Heater [3] before entering the Warm Pumping Unit [4]. The maximum flow capacityof this circuit is 6 g/s at 10 mbar. The gas/liquid heat exchanger is of the perforated late type [5].2.3 Magnet warm-upAfter the magnetic measurement campaign, the magnet is quenched and the heaters inside the decanterand in the phase separator will boil off the liquid helium. Any remaining liquid helium is evaporated by asmall flow of gaseous helium.Once the liquid helium is evaporated, the magnet is warmed up to room temperature with alarge (80g/s) flow of warm helium gas using the same circuit (via L1 and L2) as during the pre-coolingphase.2.4 Magnet poweringIn order to power all magnet coils, the CFU is equipped with one pair of main 13 kA current leads, twopairs of 1 kA and two pairs of 0.6 kA auxiliary current leads. In order to minimize the number ofcryogenic valves, flowmeters, control loops etc., the 4 pairs of auxiliary current leads are built as twins.The cold terminals of the main current leads are connected to terminals of the main coil by a doublesuperconducting cable, partially s abilized with copper. This double cable is soldered to the cold currentlead terminal and mechanically clamped to a magnet coil terminal. Cold terminals of the auxiliary currentleads are connected to auxiliary coils via a copper stabilized superconducting cable soldered at both ends.Each current lead and bus-bar is designed to withstand 2 kV with resistance to ground better than 109W inpure gaseous helium at 1 b r and ambient temperature. In addition, the main current leads withstand 5 kVwith resistance to ground better than 109W u der cold conditions, when a cold mass is filled with liquidhelium.The magnet coils are operating in pressurized superfluid helium at 1.9 K and 1 bar and all currentleads are operating at saturated liquid helium at 4.5 K. Hence the two helium circuits are separated by so-called lambda plates. The key component of the lambda plate is the ceramic feedthrough that wasspecially developed for this purpose [6].2.5 Magnetic measurements and magnet trainingIn order to validate the required maximum magnetic field, each magnet must pass magneticmeasurements and “quench training” [7]. “Quench” is a transient mode of a superconducting coil when itlooses its uperconducting state, because of poor cooling or high current/magnetic field. During thequench the liquid helium around the coil vaporises and the pressure increases rapidly. A mixture of gasand liquid helium passes through a “quench” discharge valve (FV149) into a decanter ofabout 400 litres volume. The gas leaves partially through the recovery line and partially through the low-pressure line (interface L4). The liquid helium remaining in the decanter is used for recooling the coldmass, which may have warmed up to 35 K during the “quench”.3 CONTROL AND INSTRUMENTATIONThe CFU is fitted with 12 pressure and vacuum transmitters, 23 mperature sensors and 3 cold flowmeters. Also each current lead incorporate 3 temperature sensors and a dedicated flow sensor with controlvalve.The complete CFU is controlled by an industrial PLC with remote input/output units using aProfibus interface. The PLC monitors all parameters and devices. The software is written such that theCFU can be operated with minimum operator interaction. Once a magnet has been connected and leaktested the control system will perform all warm tasks, like pumping and purging of helium and vacuumcircuits, cooling down of the magnet through its 3 phase (pre-cooling, 4.5 K and 1.9 K cooling),automatic quench recovery and warm-up. The interface with the operator is using a separate SCADAsoftware package called PCVUE ™.4 TEST RESULTSThe first CFU has been in operation for testing of two 15 m dip le magnet prototypes for 7 m nths. Thesystem has handled more than 150 quenches. The second CFU has been commissioned and successfullytested at the end of 1999.The duration of a typical magnet test cycle with the CFU is at present as follows:Magnet installation: 5 days ;Vacuum pump down: 8 hours ; Cool-down: 30 hours at m= 80 g/s anddT = 60 K ; Magnetic measurements: about one month ; Quench recovery: 3 ours ; Warm-up: 50 hoursat m = 80 g/s and T = 60 K ; Dismantling: 5 days. In future the CFU/magnet interfaces will be modifiedand the gas flow and temperature difference between gas and magnet will be increased in order to shortenthe global time of a magnet test.The total heat loads of CFU in a dummy configuration (without magnet) are shown in Table 1.Temperature level(K)Heat load at I = 0A(W)Heat load at I = 13kA(W)1.9 9.1 ~ 104.3 27 ~ 4550 48 48Table 1  CFU Heat Loads™ : PCVUE is a registered Trade Mark of ARC Informatique – France.REFERENCES1. Lebrun, Ph., Superfluid Helium Cryogenics for the Large Hadron Colli er Project at CERN, Cryogenics (1994) 34 1-82. Benda, V. et al, Cryogenic Benches for Superfluid Helium Testing of Full-Scale Prototype Superconducting Magnets forthe CERN LHC Project, Cryogenics (1994) 34 733-7363. Benda, V. et al, Electrical heater for very-low pressure helium gas, Proceedings, 4th Int rnational ConferenceCryogenocs ’96 (1996), Praha, Czech Republic, pp. 40-43.4. Benda, V. et al, Cryogenic infrastructure for s perfluid helium testing of LHC prototype superconducting magnets CEC,ICMC (1993), Albuquerque, USA.5. Viargues, F. et al, Construction and Preliminary Testing of Perforated Plate Heat Exchanger for Use in Helium IIRefrigerators, Cryogenics (1994),.34, ICEC Supplement, 325-328.6. Benda, V. et al, Current f edthroughs for superconducting magnets operating below 2K, ICEC 17 (1998), pp.739-742.7. Billan, J. et al, Manufacturing features and performances of long models and first prototype for the LHC project, EPAC 98(1998), Stockholm, SwedenFigure 1  The process flow diagram

Experience with a Pre-Series Superfluid Helium Test Bench for LHC Magnets

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