After the assembly, commissioning and successful first operation of a full-scale superconducting magnet string, and as a new prototype dipole magnet was added to approach final configuration, the cryogenic system has been slightly modified to allow the verification of the performance of the superfluid helium cooling loop in counter-current two-phase flow. At the same time the control system strategies have been updated and only two quench relief valves have been installed, one at each end of the string. We report on the cryogenic operation of the extended version of the string and the response of the system to transients

Bézaguet, Alain-Arthur

Casas-Cubillos, J

Guinaudeau, H

Hilbert, B

Lebrun, P

Serio, L

Suraci, A

Van Weelderen, R

CERN Document Server

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Suraci and R. van WeelderenLHC Division, CERN, CH-1211 Geneva 23, SwitzerlandAfter the assembly, commissioning and successful first operation of a full-scalesuperconducting magnet string, and as a new prototype dipole magnet was addedto approach final configuration, the cryogenic system has been slightly modifiedto allow the verification of the performance of the superfluid helium cooling loopin counter-current two-phase flow. At the same time the control system strategieshave been updated and only two quench relief valves have been installed, one ateach end of the string. We report on the cryogenic operation of the extendedversion of the string and the response of the system to transients.INTRODUCTIONA major milestone in the validation of the basic technical choices for the Large Hadron Collider (LHC)project 1, as far as main accelerator systems - magnets, cryogenics and vacuum - are concerned, is thetesting and operation of a full-scale superconducting magnet string, representing a half-cell of themachine lattice.The first version of the LHC test string was assembled, commissioned and successfully operated forthe first time at the end of 1994 2, confirming the basic design choices of the LHC cryogenic system 3.We report on the cryogenic operation of an extended and modified version of the string and inparticular the counter-current flow cooling, the response to transients such as current ramp and discharge,as well as magnet resistive transitions.EXTENSION AND MODIFICATION OF THE STRINGAs a new prototype dipole magnet was added to lengthen the string towards its final configuration, thecryogenic system has been slightly modified to allow the verification of the performance of the superfluidhelium cooling loop in counter-current two-phase flow, an option which would greatly simplify thecryogenic layout of the LHC machine.The main  modification was the introduction of a Joule-Thomson valve, a phase separator fromwhich liquid overflows down the 1.4 % slope, and a connection to the pumping line at the higher end ofthe string as shown in figure 1. In this configuration, pumping is shared between both ends of the heliumII heat exchanger tube. Pumping from the higher end dominates because of pressure drop in the heatexchanger tube. The liquid and gas velocities are opposite one another giving rise to flow instabilitiesabove certain flow rates.At the same time the control system strategies have been updated profiting from the operationalexperience gained during the first run, in order to prepare the cryogenic system for the forthcomingexperiment, that will validate the long term performance of the system during uninterrupted currentcycling at nominal LHC rates.In line with previous results, only two quench relief valves have been installed, one at each end ofthe string. In this fashion, the quench relief valve scheme proposed for the LHC can be betterapproximated, and the requirements for the relief valves validated.Fig.1 Cryogenic flow-scheme of extended and modified stringOPERATIONAL PERFORMANCEDuring 1995 and the beginning of 1996 the cryogenic system has reliably provided the nominal cryogenicconditions necessary to perform experiments concerning the main systems of the LHC, operating for morethan 1500 hours at 1.9 K and recovering from more than 50 resistive transitions.The temperature increase in the pressurised helium II bath was limited to 6 mK during nominalcurrent ramping (10 A/s) and 50 mK during nominal discharge (-130 A/s), with steady-state temperaturestability at 1.9 K better than a few mK, both in co-current and counter-current operation of the saturatedhelium II loop.In spite of the limitations reached in counter-current mode, the capability of cooling down the stringfrom ambient temperature to 1.9 K in just more than a week has been maintained.The operational experience gained during the first run has allowed us to define control strategiessuch that all stand-by and transient operations can be performed unattended in fully automatic mode. Inthe same way the magnet powering does not require presence of  skilled cryogenic operator.Opportunity was taken to perform other cryogenic test, such as the validation of basicinstrumentation for LHC and the assessment of heat loads at different temperature levels.   COUNTER-CURRENT FLOW OF TWO-PHASE SUPERFLUID HELIUMSince the superfluid helium cooling loop is now fed by overflow from the phase separator (figure 2),acting as a weir at the upper end, the neighboring section of the corrugated heat exchanger tube seescounter-current flow of liquid and vapor phases at saturation. As the heat load, and thus the mass flow-rates are gradually increased, the velocity of the vapor flow rises up to the point where it transferssufficient momentum to block the gravity-driven, counter-current liquid flow, and thus prevents wettingof the heat exchanger tube. As the conditions of flow blocking are approached, the liquid level in thephase separator must sharply rise to compensate for the decrease in liquid velocity, and an instabilitydevelops. This phenomenon has been extensively studied in vertical or quasi-vertical flows ofconventional boiling fluids, and is known as "flooding" in boiler tubes [4], but no experimental work hadbeen done on superfluid helium in quasi-horizontal geometries.Fig.2 Superfluid helium cooling loopIn order to assess limitations to the superfluid helium cooling loop brought by this effect,experiments were performed by increasing heat loads, while maintaining magnet temperatures stable, atdifferent values of saturation temperature (i.e. vapor density) between 4.5 and 1.8 K. The onset of flowblockage can be clearly diagnosed from the sharp rise in the phase separator level, and the subsequentdrift in the magnet temperatures (figure 3). The most critical conditions occur at low temperature, i.e.when the vapor density is lowest. On the 1.4 % slope, the 1.8 K liquid flow in the 4.15-cm inner-diametercorrugated tube is blocked at a vapor velocity of  6 m/s, corresponding to a heat load of 46 W, i.e. abovethe nominal demand of the LHC half-cells [3]. Although occurring at higher mass flow-rate between 4.5and 2.2 K, blocking of the liquid flow in the heat exchanger tube may increase time for cool-down of thestring from 4.5 K. Strategy for fast cool-down is to optimize the saturation conditions as a function ofmagnet temperature, in order to work with high-density, low velocity vapor, while preserving sufficientdriving temperature difference. In this fashion, cool-down from 4.5 to 1.8 K could be achieved in 6 hours,only slightly longer than Joule-Thomson valve sizing could permit.MAGNET RESISTIVE TRANSITIONSCoping with resistive transitions in superconducting magnets is a concern of main importance for the safeoperation of the LHC. When occurring, it provokes the discharge of the magnetic energy stored in thecoils (several MJ) into the static liquid helium bath, provoking rapid thermohydraulic transientsAt a current of 13.1 kA, corresponding to the LHC ultimate magnetic field of 9 T, rates of pressureincrease up to 140 bar/s and pressure peaks reaching 12 bar are commonly observed.A fast responding protection system was designed at CERN to trigger electrically the opening of twoquench relief valves after 120 ms, with full opening less than 170 ms after quench detection. During thelatest experimental run, it was shown that one quench relief valve, located either on the lower part andlow end of the cold mass (discharging liquid helium) or on its upper part and high end (dischargingmainly in gaseous or supercritical phase) is sufficient to prevent the 1000 litres of the magnet heliuminventory from reaching the design pressure of 20 bar.As shown in figure 4 the opening of the quench relief valve located on the upper part of the coldmass (the other being mechanically blocked) was delayed, in step of 10 ms, up to a total delay of 200 ms.In the dipoles (where the rate of pressure increase are the highest), the pressure peak increased by 2 barwith respect to no delay, still well below the 20 bar design pressure of the cold mass vessel. Whilekeeping the maximum delay time for the electrical trigger, we lowered the pressure force on the valve seatdown to 10 bar in order to force the valve to open on pressure. The pressure actuated opening gave a totaldelay time of 165 ms and a cold mass peak pressure of 13.5 bar.These tests were aimed at investigating the use of modified commercial valves in order to avoid theuse of specially designed, fast-opening prototype valves as the ones in use 5.The final tests were done using a standard and commercially available industrial valve, modified tofully open  in less than 300 ms, mounted on the upper part and high end of the cold mass as one of thetwo fast opening valves. The test of this valve showed comparable discharge rates and pressure peaks inthe cold mass.       Fig.3 Flow instability test performed at 1.8 K   Fig.4 Cold mass peak pressure versus opening timeCONCLUSIONThe basic design technical choices for LHC and the modifications introduced after the first experimentaltests have been validated, giving us confidence on the cryogenic system that is going to be built for LHC.The practical limitations of the counterflow system have been investigated and will give usefulinformation for the definition of the final design which respect operational constraints.The tests performed on the prototype quench relief valves have given decisive information for thespecification of the quench relief valves to be used in LHC. Analysis and simulation work is under way tounderstand the physics behind the process of energy transfer to the helium bath after a quench and itssubsequent discharge and recovery.The string is now being prepared to assure 3000 hours of uninterrupted and fully automatedoperation at 1.9 K with continuous ramping and de-ramping to and from nominal current, to gaininformation on long-term behaviour of key components over the complete lifetime of the future machine.ACKNOWLEDGEMENTSThe authors wish to thank the LHC String Team, B. Gaillard-Grenadier and Th. Goiffon for the valuablework performed during the operation and maintenance of the LHC test string.REFERENCES1. Evans, L.R., The Large Hadron Collider Project, paper presented at this conference2 BÈzaguet, A., Casas-Cubillos, J., Flemsaeter, B., Gaillard-Grenadier, B., Goiffon, Th., Guinaudeau,H., Lebrun, Ph., Marquet, M., Serio, L., Suraci, A., Tavian, L. and van Weelderen, R., Thesuperfluid helium cryogenic system for the LHC test string: design, construction and first operation,paper presented at CEC 1995, Columbus (1995)3.  Lebrun, Ph., Superfluid helium cryogenics for the Large Hadron Collider Project, Cryogenics ICECSupplement (1994) 34 1-84.  Wallis, G.B., One-dimensional two-phase flow, McGraw-Hill (1969)5.  Danielsson, H., Ferlin, G., Jenninger, B, Luguet, C., Milner, S.E. and Rieubland, J.M., Cryogenicperformance of a superfluid helium relief valve for the LHC superconducting magnets, paperpresented at CEC 1995, Columbus (1995)

Cryogenic operation and testing of the extended LHC prototype magnet string

Cryogenic operation and testing of the extended LHC prototype magnet string

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