The LHC will be equipped with about 8000 superconducting magnets of all types. The total current to be transported into the cryogenic enclosure amounts to some 3360 kA. In order to reduce the heat load into the liquid helium, CERN intends to use High Temperature Superconducting (HTS) material for leads having current ratings up to 13 kA. The resistive part of the leads is cooled by forced flow of gaseous helium between 20 K and 300 K. The HTS part of the lead is immersed in a 4.5 K liquid helium bath, operates in self cooling conditions and is hydraulically separated from the resistive part. A cryogenic test station has been designed and built in order to assess the thermal and electrical performances of 13 kA prototype current leads. We report on the design, commissioning and operation of the cryogenic test station and illustrate its performance by typical test results of HTS current leads

Ballarino, A

Bézaguet, Alain-Arthur

Gomes, P

Métral, L

Serio, L

Suraci, A

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsA LOW HEAT INLEAK CRYOGENIC STATIONFOR TESTING HTS CURRENT LEADS FOR THE LARGE HADRON COLLIDERA. Ballarino, A. Bézaguet, P. Gomes, L. Metral, L. Serio and A. SuraciThe LHC will be equipped with about 8000 superconducting magnets of all types. The total current to betransported into the cryogenic enclosure amounts to some 3360 kA. In order to reduce the heat load intothe liquid helium, CERN intends to use High Temperature Superconducting (HTS) material for leadshaving current ratings up to 13 kA. The resistive part of the leads is cooled by forced flow of gaseoushelium between 20 K and 300 K. The HTS part of the lead is immersed in a 4.5 K liquid helium bath,operates in self cooling conditions and is hydraulically separated from the resistive part.A cryogenic test station has been designed and built in order to assess the thermal and electricalperformances of 13 kA prototype current leads.We report on the design, commissioning and operation of the cryogenic test station and illustrate itsperformance by typical test results of HTS current leads.LHC DivisionPresented at the 1999 Cryogenic Engineering and International Cryogenic Materials Conference(CEC-ICMC'99), 12-16 July 1999, Montreal, CanadaGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 320Abstract1 December 19991A LOW HEAT INLEAK CRYOGENIC STATIONFOR TESTING HTS CURRENT LEADS FOR THELARGE HADRON COLLIDERA. Ballarino, A. Bézaguet, P. Gomes, L. Metral,L. Serio and A. SuraciLHC Division, CERN1211 Geneva 23, SwitzerlandABSTRACTThe LHC will be equipped with about 8000 superconducting magnets of all types.The total current to be transported into the cryogenic enclosure amounts to some 3360 kA.In order to reduce the heat load into the liquid helium, CERN intends to use HighTemperature Superconducting (HTS) material for leads having current ratings up to 13 kA.The resistive part of the leads is cooled by forced flow of gaseous helium between 20 Kand 300 K. The HTS part of the lead is immersed in a 4.5 K liquid helium bath, operates inself cooling conditions and is hydraulically separated from the resistive part.A cryogenic test station has been designed and built in order to assess the thermal andelectrical performances of 13 kA prototype current leads.We report on the design, commissioning and operation of the cryogenic test stationand illustrate its performance by typical test results of HTS current leads.INTRODUCTIONThe reference design for the Large Hadron Collider1 (LHC) at CERN is based on thegeneralised use of High Temperature Superconductor (HTS) current leads2,3 so as to reducethe total liquefaction rate distributed over the eight cryogenic plants of the 27 kmcircumference future LHC machine.4Cooling of conventional self-cooled leads would require a liquefaction rate of about300 g/s, while the use of HTS leads would reduce the heat load into the liquid helium bathat nominal current by a factor of up to 19,5 making use of the 20 K/0.13 MPa return gas tothe refrigerators from the beam screen and magnet support cooling circuits.The 8000 superconducting magnets of the LHC machine are cooled in a pressurizedbath of superfluid helium at 1.9 K. Two electrical feed boxes at the end of each of the8 sectors of the machine provide the warm-to-cold transition via the HTS leads down to4.5 K, and via a lambda plate to feed the magnets at 1.9 K. The leads are integrated in theelectrical feed boxes and connected to the magnets via Low Temperature Superconductingbus bars. A cryogenic distribution line, running along the magnets, provides the 4.5 K LHe2Figure 1. Prototype current leads prior to their installation in the test cryostat.feeding of the leads bath and the 20 K GHe to ensure cooling of the resistive part. Warmgas from the leads and the LHe bath is recovered in a gas collector connected to the LP lineof the refrigerator.Prototype 13 kA HTS current leads have been specified by CERN and produced byseveral manufacturers in Europe, Japan and the US.The specification defines the required thermo-electric performance and the geometriclimitation imposed by the underground constraints of the accelerator. The proposed leadsare composed of a resistive heat exchanger in which a flow of 20 K GHe is circulated and aHTS part that dips into the 4.5 K saturated helium bath. The temperature of the warm endof the HTS is controlled at or below 50 K by the flow of 20 K helium in the resistive heatexchanger. The vapor generated in the liquid helium bath is guided in a “skirt” surroundingthe HTS element that is therefore operated in a self-cooling condition. The leads areoptimised for operating at 13 kA with the warm end of the HTS below 50 K and the toppart of the resistive heat exchanger at  290 K. Without current, the temperature of the gasoutlet of the leads is maintained at 290 K by a thermostatically controlled heater at thewarm terminal of the heat exchanger.SYSTEM LAYOUT AND PRINCIPLES OF OPERATIONA low leat inleak cryogenic station for testing HTS current leads was designed inorder to assess the thermal and electrical performance of the prototype leads ordered fromindustry. The cryostat provides the required working conditions: 4.5 K helium bath to coolthe bottom part of the HTS leads, up to 2 x 1.2 g/s of 20 K helium gas to cool the resistivepart of the leads, warm helium gas to quench the HTS, recovery of helium gas and thenecessary instrumentation and valve actuators for control and diagnostic measurements.Furthermore, the design allows flexibility in the operation so as to optimize the control lawfor the optimum performance of the HTS current leads. The control and data acquisitionsystem allows to monitor and automatically control transients such as cooldown, warm-upand current ramping/de-ramping, as well as steady-state operation.3Figure 2.  Inner and outer cryostat view.System layoutAs shown in figure 2, the test cryostat is composed of an annular (outer) cryostatproviding radiation shielding at 4.5 K, neck thermal interception of the inner cryostat, andan inner cryostat in which the current leads are housed and cooled to maintain theirnominal operating conditions. The two cryostats share a common insulation vacuum andare wrapped in multilayer thermal insulation.The connection of the 20 K lines to the current leads in the inner cryostat is made viavacuum-insulated lines entering at and exiting from the top flange of the cryostats. Thiswas necessary to allow easy demounting of the top flange of the inner cryostat to exchangecurrent leads. Six radiation baffles in the inner cryostat provide thermal intercept ofradiation from the top flange at ambient temperature. Gas from the inner and outercryostats is warmed up via electrical heaters and recovered in a main manifold connectedto the low pressure side of the refrigerator. The system makes use of the cryogenicinfrastracture available, in particular a 6 kW @ 4.5 K cryogenic plant. Liquid helium isdistributed via a vacuum-insulated and radiation-shielded transfer line, providing coolingand gas recovery for several test station. The outer cryostat is filled with liquid helium andacts as a supply of liquid for the inner cryostat via a cryogenic valve, and for cold gaseoushelium for cooling the resistive part of the leads. An electrical heater can be used toincrease the boil-off from the outer cryostat during powering of the leads when a highermass flow is required. The inlet temperature of the gas supplied to the resistive part of thecurrent leads is adjusted by mixing with 300 K gas.Principles of operationThe test station has been designed for a maximum liquefaction requirement of about4 g/s as shown in detail in Table 1.The total nominal inventory of liquid helium in the two cryostats is about 150 l.Two mass flow controllers adjust the necessary flow-rate to obtain the required gastemperature of 20 K. Additional electrical valves are used to add room temperature gasthereby increasing the temperature (up to 200 K) of the cooling flow in order to quench the4Table 1.  Detailed design liquefaction requirementsType of heat load Equivalent load in g/s1 pair of 13 kA leads in bath – 3 W 0.1520 K cooling of leads 2.0Anular cryostat – 5 W 0.25Inner cryostat – 0.5 W 0.025Transfer line and bayonet – 13 W 0.65TOTAL 3.075Design margin – 1.25 3.8HTS. The gas is then recovered at the top part of each lead via an electro-pneumaticcontrol valve, which regulates the mass flow in order to maintain the HTS warmtemperature at or below 50 K. These valves can also control mass flow-rate depending onthe current in the leads.The pressure in the inner and outer cryostat is controlled via electro-pneumatic valves.The other electro-pneumatic control valve feeding LHe from the outer cryostat controls thelevel of liquid helium to cover the cold end of the HTS and the LTS short-circuit betweenthe leads in the inner cryostat.INSTRUMENTATION AND PROCESS CONTROLInstrumentationAll the circuits’ temperatures are monitored with Platinum 100 sensors (300 to 25 K)and Carbon or CERNOX™ sensors (25 to 4.2 K). The current leads are equipped with ahigh-voltage insulated Platinum 100 sensor for control of the HTS warm-end temperatureand several  uninsulated Platinum 100 sensors distributed over the resistive part of the leadto measure its temperature profile. Two CERNOX™ sensors monitor the HTS middle andcold end temperatures.Liquid levels are measured with superconducting wire gauges that can be pulsed todecrease their heat input to the liquid helium bath. Two gauges of different dimensions(500 mm and 100 mm length) are used in the inner cryostat for control and diagnosticrespectively, in order to achieve higher accuracy. The inner cryostat has been calibrated toassess heat loads by rate of level decrease with time.Helium flow is measured at ambient temperature using thermal mass (0.5 g/s of fullscale reading with 1 % accuracy) and V-cone™ flowmeters (1.2 g/s of full scale readingwith 1 % accuracy). In view of its accuracy over the full range, the first type is used tomeasure the boil-off from the leads bath; the second type is employed at the warm outlet ofthe current leads to measure the flow of 20 K helium gas, thanks to its precision and lowpressure drop. The flowmeters have all been calibrated with GHe in operating conditionsand re-calibrated in-situ at the lower part of their ranges.Differential pressure sensors check the pressure drop in the resistive part of the lead.Process controlThe process control is performed using an industrial Programmable Logic Controller(PLC) in which the number of I/O channels is of the order of 30 and there are about 7closed control loops. The automatic operation modes include cooldown, normal operation,quench and warm-up. This equipment is interfaced with an industrial supervision systembased on PCVUE32™.501234560 1 2 3 4 5 6Applied heat loads (calibration heater) [W]Measured heat loads [W]cryostat heat loadFigure 3.  Test cryostat measured heat loads at various heater power (no leads).The powering system of the leads is interlocked to the liquid level signal, thetemperature of the warm part of the HTS and the cryogenic operator authorization.The process control system is designed to run in fully automated mode 24h/24h. Ashift operator is automatically called if a parameter of importance deviates from itsexpected range. The system can also be remotely accessed via Internet. Most of theexperimental tests not involving powering of the leads are therefore performed overnight.STAND-BY AND TRANSIENT OPERATIONCommissioning of the test cryostatPrior to testing any of the prototype current leads, the cryostat underwent an extensivecampaign of measurements to assess its thermal performance without leads. Furthermore,the cryostat itself, the instrumentation, the control and supervision system were fullycommissioned and the instrumentation, where appropriate, re-calibrated in situ.Cryostat characterisationThe HTS test cryostat needs about 24 h (with a flow of 0.2 g/s in the inner cryostatand 0.5 g/s in the outer cryostat) to reach stable operating conditions; this is mainly due tothe masses involved and to the very good thermal insulation.Once the cryostat is thermalized, the background heat loads (thermal conduction,radiation and superconducting liquid level gauge) in the inner cryostat can vary dependingon the applied heat loads (current leads), which change the thermal characteristics of thecryostat (Figure 3).During transient heat loads (changing the flow of evaporating helium from the bath),the neck and the radiative screen in the neck change their temperatures, thus varying thedensity of the helium in and around the radiative screen and modifying the steady-stateflow measurements.6These two phenomena are taken into consideration to calculate the heat loads of theprototype current leads. A complete mapping of the thermal characteristics of the testcryostat at different applied heat loads prior to the insertion of the current leads allows oneto determine the heat conducted to the 4.5 K liquid helium bath through the current leadswith +/- 50 mW accuracy.CRYOGENIC TESTS RESULTSEach pair of leads requires at least one week to perform the specified performancetests. One week is also necessary to warm-up the cryostat, remove the previous leads,mount the new leads in the cryostat, check the instrumentation, condition the circuits andcool-down. A test program is established to study the behaviour of each lead in transientand stand-by operation.Stand-by operation. The stand-by operation defines the thermal performance(Figure 4) of the leads with no current (heat loads in the liquid helium bath, flow-rate andpressure drop in the resistive part) at various HTS temperatures (30, 40 and 50 K).Transient and steady operation. In transient operation we study the currentramping/de-ramping behaviour of the leads with 13 kA current (Figure 5), the behaviour ofthe lead during a loss of the 20 K helium flow cooling the resistive part with the currentmade to decay exponentially with a 120 seconds time constant, and the resistive transitionof the superconducting part of the lead. In steady operation the thermal performance of theleads with 13 kA was observed.The heat load in the liquid helium bath increases due to contact resistance when theleads are powered to 13 kA.The self-cooling due to the skirt surrounding the HTS decreases the heat loads by pureconduction in the bath, thus reducing the absolute increase in heat loads when the leads arepowered. This behaviour depends on the efficiency of the heat exchange in the HTS part.Figure 4.  Typical steady-state thermal measurements on current leads (zero current).t [hh:mm]010203040506018:08 18:11 18:14 18:17 18:20 18:23 18:25 18:2800.050.10.150.20.250.30.350.40.450.5g/sK%mbargas supply temperature (K)20 K gas mass flow (g/s)warm temperature of HTS (K)liquid helium level (%)pressure drop over the resistive lead (mbar)7Figure 5.  Typical response of cryogenic parameters during current ramping of leads.Table 2 shows the typical cryogenic results obtained from a pair of HTS leads undertest, including precision and stability. During stand-by, transient and steady operation theliquid helium level is maintained at its nominal value within +/- 0.25 cm, the precision ofthe liquid level gauge being +/- 0.1 cm.Figure 6.  Typical current ramping of leads with control of the mass flow through the resistive heat part.t [hh:mm]01020304050607011:13 11:21 11:28 11:35 11:42 11:49 11:57 12:04-0.200.20.40.60.811.21.4current (kA)50 A/swarm temperature of HTS (K)resistive heat exchanger mass flow (g/s)KkAg/s01020304050607080901008:48 9:02 9:17 9:31 9:46 10:00 10:14 10:29-0.200.20.40.60.811.21.4current (kA)50 A/sresistive heat exchanger mass flow (g/s)warm temperature of HTS (K)K, kA g/st [hh:mm]8Table 2.  Typical test results of a HTS lead pairlead Alead BHTSwarmend[K]20 Kmassflow[g/s]0 kAPressuredrop[mbar]0 kAH.L.4.5 Kbath[W/lead]0 kA20 Kmassflow[g/s]13 kAPressuredrop[mbar]13 kAH.L.4.5 K bath[W/lead]13 kACERN Specification < 50 < 0.6 < 50 < 1 < 1 < 50 < 1.5Measurement of atypical lead pair40400.420.452.61.90.730.730.820.863.340.850.85PrecisionStability+/- 0.1+/- 0.5+/- 0.01+/- 0.03+/- 1+/- 1.5+/- 0.05+/- 0.08+/- 0.01+/- 0.1+/- 1+/- 3+/- 0.05+/- 0.08After having completely characterized the current leads in terms of their thermalperformances (20 K mass flow at 0 and 13 kA, pressure drop over the heat exchanger,stability of the heat exchanger at various HTS temperatures and heat loads on the 4.5 Kliquid helium bath), the control system can be set-up to control a constant mass-flow of 20K GHe in the heat exchanger for each current value (Figure 6). This mass flow varieslinearly with the current passing through the leads between its value measured at 0 A and13 kA. The advantage of this control method is the stability and fast response achieved,independent from the temperature of the warm end of the HTS. Therefore the HTS remainsbelow its maximum operating temperature value, otherwise an interlock prevents or abortspowering of the leads. The disadvantages are that the leads needs to be tested to set thecontrol parameters (mass flow at 0 and 13 kA) and additional instrumentation is needed(flowmeter).CONCLUSIONSA low heat inleak test cryostat has been successfully designed, commissioned andoperated at CERN to test prototype 13 kA HTS current leads.Several pairs of HTS current leads have been tested and have shown that thecryogenic requirements set in the technical specification can be met. These leads havedemonstrated that the heat load into the liquid helium bath can be reduce by a factor of upto 19 with respect to conventional self-cooled leads, however at the cost of GHe cooling at20 K.Additional tests to demonstrate the long term reliability (thermal and electrical cycles)will be performed.Another test cryostat for 600 A HTS current leads of a similar design has been builtand will be commissioned shortly for the test of prototype 600 A HTS leads.REFERENCES1. L. R. Evans, The Large Hadron Collider Project, in “Proceedings ICEC16”, Editors Elsevier Science,Oxford UK (1997), p. 45.2. T. Taylor, HTS Current Leads for the LHC, paper presented at ASC’98, Palm Desert, California (1998).3. A. Ballarino, High Temperature Superconducting Current Leads for the Large Hadron Collider, paperpresented at ASC’98, Palm Desert, California (1998).4. A. Ballarino, A. Ijspeert, U. Wagner, Potential of High-Temperature Super Conductor Current Leads forLHC Cryogenics, in “Proceedings ICEC16”, Editors Elsevier Science, Oxford UK (1997), p. 1139.5. A. Ballarino, L. Serio, Test Results on the First 13 kA Prototype HTS Leads for the LHC, paperpresented at PAC’99, New York (1999)

A Low Heat Inleak Cryogenic Station for Testing HTS Current Leads for the Large Hadron Collider

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