The Electrical Distribution Feed Box (DFBS) is a 4.5 K saturated liquid helium cryostat constructed for the Large Hadron Collider (LHC) Prototype Cell (String 2). The thermal design of the DFBS is presented, with emphasis on the modelling of the cooling of the current lead chimneys via the helium bath boil-off gas and on the design of the lambda plate. The expected performance is compared to measurements done during the first operation phase of the LHC prototype cell

Calzas-Rodriguez, C

Capatina, O

Hauviller, Claude

Poncet, Alain

Sacré, P

Serio, L

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsTHERMAL DESIGN AND PERFORMANCE OF THE ELECTRICAL DISTRIBUTIONFEED BOX OF THE LHC PROTOTYPE CELLC. Calzas Rodriguez, O. Capatina, C. Hauviller, A. Poncet, P. Sacré and L. SerioThe Electrical Distribution Feed Box (DFBS) is a 4.5 K saturated liquid helium cryostat constructed forthe Large Hadron Collider (LHC) Prototype Cell (String 2). The thermal design of the DFBS ispresented, with emphasis on the modelling of the cooling of the current lead chimneys via the heliumbath boil-off gas and on the design of the lambda plate. The expected performance is compared tomeasurements done during the first operation phase of the LHC prototype cell.LHC DivisionPresented at the 19th International Cryogenic Engineering Conference (ICEC 19)22-26 July 2002, Grenoble, FranceGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 609Abstract15 November 2002Thermal design and performance of the Electrical Distribution Feed Box of the LHC prototype cellCalzas Rodriguez C., Capatina O., Hauviller C., Poncet A., Sacré P., Serio L.CERN, European Organization for Nuclear Research, 1211 Geneva 23, SwitzerlandThe Electrical Distribution Feed Box (DFBS) is a 4.5 K saturated liquid heliumcryostat constructed for the Large Hadron Collider (LHC) Prototype Cell(String 2). The thermal design of the DFBS is presented, with emphasis on themodelling of the cooling of the current lead chimneys via the helium bath boil-offgas and on the design of the lambda plate. The expected performance is comparedto measurements done during the first operation phase of the LHC prototype cell.INTRODUCTIONIn order to test the magnets, the related cryogenics and the other components of the LHC under realisticoperating conditions, CERN has ordered from industry a number of full-scale prototype superconductingmagnets, which were installed in a 120 m long string, operated and tested. This facility is the LHCPrototype Cell, hereafter called String 2 [1].The Electrical Distribution Feed Box (DFBS)[2] is a 6 meter-long liquid helium (4.5 K /0.135 MPa) cryostat (Figure 1) which supports andcools 32 High-Temperature Superconductor (HTS)current leads [3] used for powering the String 2main dipole and quadrupole superconductingmagnets, together with their correctors. Currentsare rated at 13 kA and 600 A.The DFBS also supports the Lambda-platethat thermally and hydraulically separates itssaturated liquid helium bath from the magnet stringpressurised superfluid helium bath at 1.9 K /0.13 MPa.Figure 1 String 2 Electrical Distribution Feed BoxThe DFBS cryostat is subjected to heat inleaks at 50 K (thermal shield), 4.5 K (saturated heliumbath), and 1.9 K (shuffling module and magnet baths), each one being the sum of several contributions.The paper is structured in 4 parts presenting: the DFBS cryogenic design with emphasis on theLambda plate design, the calculated and then the tested DFBS heat load values and finally, a revisedthermal model used to optimize the heat exchange within the current lead chimneys.DFBS CRYOGENIC DESIGNFigure 2 gives a schematic layout of the DFBS cryogenic structure and a prototype Lambda plate.The temperature of a current lead is controlled by a regulation valve acting on a flow of externallysupplied 20 K gaseous helium. Helium gas from the vessel, produced by heat loads to the 4.5 K bath, isused to cool the HTS part of the lead and exits the latter in the cryostat neck at a temperature of 50 K. Amanual valve then limits this flow to a fixed amount (0.03 g/s per chimney – see later for explanations)necessary to cool the cryostat necks, thus reducing the conduction heat load to the bath. The mass flow inexcess is returned to the helium vessel, then into the 20 K line via a cold valve, prior to injection into thecurrent lead. Liquid helium is provided from the refrigerator through a regulation valve in order to keepthe liquid helium level in the helium vessel constant.D F B c r y o s t a tC u r r e n t l e a d sλ -P l a te4 . 5 K L i q u i dh e li u m b a t hFigure 2 DFBS cryogenic diagram during nominal operation conditions; Lambda plate prototype6 copper stabilized superconducting cable busbars rated 13 kA and 32 superconducting wires rated600 A transport the electrical current through the Lambda plate. A hydraulic separation between theDFBS liquid helium at 4.5 K and the magnet bath at 1.9 K is ensured (leak tightness better than5x10-2 mbar l / s STP) while maintaining the electrical continuity of the superconducting busbars, withno use of resisting splices to avoid Joule heating. It has to withstand hydraulic pressure difference up to2 MPa in case of magnet quenches and remain leak tight at a level better than 0.16 g/s in superfluidhelium with pressure difference of 0.035 MPa [4].DFBS THERMAL CALCULATIONS (DESIGN VALUES)Heat loads at 4.5 KThe global thermal loads to the 4.5 K DFB liquid helium bath can be divided into three main parts:• thermal loads due to the cryostat itself (including radiation from the thermal shield to the heliumvessel, conduction from the cold supports, the pressure control valve, conduction through the 20 K line,instrumentation wires and cryostat chimneys). In static operation (no current in the leads), for the String 2phase 1 configuration, this thermal load was expected to be ideally 7 W. Additional loads of 1 W (due toelectrical splices) and 3.5 W (due to an increase of the 20 K line helium gas temperature) should appearwhen powering the electrical circuits,• thermal loads due to the Current Leads. These values can be estimated from previous measurementsperformed on the Current Leads on the “test cryostat”, giving an estimated global value of 9.5 W [5]. Thisrepresents the static thermal loads to the 4.5 K helium bath due to all Current Leads installed on String 2under the operating conditions,• heat sink crossing the Lambda-plate. The expected value is 10 W (thermal conduction through OFHCCopper – RRR = 100).Heat loads at 1.9 KThe heat loads on the 1.9 K magnet bath are the addition of the radiation through the multi layerinsulation (MLI) at the shuffling module (cf. Figure 4) level and of the heat flow crossing the Lambda-plate. The expected value is 0.2 W + 10 W = 10.2 W.Heat loads at 50 K on the thermal shieldThe heat load on the 50 K of the thermal shield was estimated at 117 W. This important value is due to anincident, which forced the replacement of the composite part of the 3 support posts with stainless steel.The value should be reduced to 77W when using composite support posts.DFBS THERMAL TESTSSeveral tests were performed without electrical power, in order to determine the heat loads to the 4.5 Khelium bath, due to the cryostat itself as well as the heat load crossing the Lambda-plate. The tests werebased on gaseous helium mass-flow measurements for different set-ups.Global heat loads at 4.5 KDuring a first measurement campaign on String 2, the global thermal loads on the 4.5 K helium bath weremeasured statically (i.e. no current in the leads). The liquid helium supply valve was closed as well as thecold valve separating the gas produced in the helium vessel from the gas in the 20 K external line (seeFigure 2). The chimney manual valves were completely opened in order to allow all the helium gasproduced by liquid evaporation to be evacuated through the chimneys. Only one chimney valve wasclosed (housing fragile instrumentation wires).After system stabilisation, the helium boil-offrate of the pressurised bath results from a balancebetween the three main heat sources to the 4.5 Kbath: cryostat itself, current leads, Lambda plate.The amount of gas produced by liquid vaporisationwas determined by two methods (Figure 3): directmeasurements (using a flowmeter) of the gaspassing through the chimneys and monitoring ofthe liquid level evolution into the helium vessel.The global delivered power to the DFBS 4.5 Kliquid helium bath was found to be of 12.6 ± 0.5 W.Figure 3 DFBS thermal testsHeat loads through the Lambda-plate from 4.5 K to 1.9 KIn order to quantify the heat flow through the Lambda-plate by difference between DFBS thermal loadswith and without the contribution of the Lambda-plate, the cryomagnet helium bath was warmed up to4.5 K (see Figure 4) and the measurement presented above was repeated.heliumvesselshufflingmoduleλ-plate busbar4.5 K1.9 KQnominalheliumvesselshufflingmoduleλ-plate busbar4.5 K4.5 KQ = 0Figure 4 Lambda plate test approachThe measured value of the heat through the Lambda-plate during nominal operation was 7.2 ±1.1 W.Substituting the Lambda plate and current leads contributions from the global measurement, thethermal load due to the DFBS cryostat itself was determined to be 10.3 ±1.1 W.OPTIMISATION OF THE CHIMNEY NECK HEAT FLOW: THERMAL MODEL AND TESTSAs said above, during the measurements, one chimney was kept closed because it houses fragileinstrumentation wires. That chimney was not cooled by helium gas, thus the heat conduction through itsneck represents a non-negligible contribution.Since solid conduction through the chimney neck from 300 K (chimney upper flange) to the heliumbath is a very important factor for the final value of the heat load to the 4.5 K bath, a detailed thermalmodel of 13000 A chimney was established (Figure 5) and validated by a measurement campaign.Presently on the DFBS, the enthalpy of the vapour resulting from liquid helium evaporation is usedto absorb some of the heat leaking into the vessel, thus cooling the neck.16TvvTflange54321110987151413122019181716TliqQcondHeaterCurrentLeadTflangeCLTbafl1Tbafl5Tbafl4Tbafl3Tbafl2AirHeliumgasRadiationConvectionConductionFigure 5 Chimney neck analytical modelHowever, an undesired effect may appear ifthe gaseous helium flow passing through thechimney is too important: the neck becomes toocold at the upper flange level and ice appears. Inorder to avoid it, heaters were installed at thatlocation maintaining the chimney upper flange at atemperature of 26 ºC. Heater power as well asflange temperature variation with gas massflowwere determined by calculations and validated bytests.Heat loads to the helium bath were calculatedfor different values of gas flow cooling thechimney neck. Refer to Figure 6 for somecalculation results. For an optimum operation,compromise had to be found between the heat loadto the helium bath (minimised), chimney flangetemperature (maximised) and the top gastemperature (maximised to avoid ice on the pipecollecting the gas at the chimney upper flange).To completely suppress heat load to the bath,0.030 g/s through each chimney was found to bethe optimum value.Heat load to the bath, chimney heater power and top gas tem perature fordifferent gas massflow values (Flange temperature of 300K; no CL heater)0.101.28293271223195204011871261511652971671520.00.20.40.60.81.01.21.40 0.05 0.1 0.15 0.2Gas massflow (g/s)Heatloadtotheheliumbath(W)050100150200250300350Heater power (W)Heat load to thehelium bath (W)Heater power (W )Top gastemperature (K)Figure 6 Chimney neck thermal calculation resultsCONCLUSIONSGood correlation between the measured and calculated thermal loads values of the DFBS wasshown. The DFBS cryostat static heat load measured value is 10.3±0.6 W and the expected value was7 W. The heat load crossing the Lambda-plate was also measured and the value is 7.2 ±1.1 W while theexpected value was 10 W.The heat loads due to conduction through the chimney neck can be reduced to zero. The analyticalmodel built can be used to optimise the chimney design of the DFBs to be installed in the LHC.REFERENCES1. Saban, R. et al., First results and status of the LHC test String 2, EPAC'02, Paris (France) June 20022. Sacré, Ph. et al., The Electrical Distribution Feedbox for the LHC Prototype Cell, EPAC'00, Vienna (Austria) June 20003. Ballarino, A., HTS current leads for the LHC magnet system, Physica C Proceedings, 2002.4. Poncet, A. and Sacre, Ph., Lambda plate leak tightness, LHC CRI Technical Note 2001-05, EDMS: 3173765. Ballarino, A., Bezaguet, A., Gomes, P ., Metral, L., Serio, L., Suraci, A. , A low heat inleak cryogenic station for testingHTS Current Leads for the Large Hadron Collider, LHC-Project-Report-320, December 1999.

Thermal Design and Performance of the Electrical Distribution Feed Box of the LHC prototype cell

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