Since the end of 1994 we have been operating a prototype half-cell of the machine lattice, accumulating more than 10,000 hours at superfluid helium temperatures and recovering from 150, mainly provoke d, magnet resistive transitions. The system has confirmed the validity of the basic design choices of the LHC cryogenic system. Furthermore, extensive testing on the response of the system to current ramp and discharge, and to magnet resistive transition, has provided sufficient information to enable a simplification of the cryogenic scheme that fulfils the LHC requirements. We report on the cryog enic operation, testing and long-term behaviour of the LHC Test String during the last 4 years of operation

Casas-Cubillos, J

Provenaz, P

Serio, L

Suraci, A

Van Weelderen, R

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsLarge Hadron Collider ProjectLHC Project Report 210Operation, Testing and Long Term Behaviour of the LHC Test String Cryogenic SystemJ. Casas-Cubillos, Ph. Provenaz, L. Serio, A. Suraci and R. van WeelderenSince the end of 1994 we have been operating a prototype half-cell of themachine lattice, accumulating more than 10,000 hours at superfluid heliumtemperatures and recovering from 150, mainly provoked, magnet resistivetransitions. The system has confirmed the validity of the basic design choices ofthe LHC cryogenic system. Furthermore, extensive testing on the response ofthe system to current ramp and discharge, and to magnet resistive transition, hasprovided sufficient information to enable a simplification of the cryogenicscheme that fulfils the LHC requirements. We report on the cryogenicoperation, testing and long-term behaviour of the LHC Test String during thelast 4 years of operation.Presented at ICEC 17, 14-17 July 1998, Bournemouth, UKAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandGeneva, 29 July 1998AbstractLHC Division2Operation, Testing and Long Term Behaviour of the LHC Test String Cryogenic SystemJ. Casas-Cubillos, Ph. Provenaz, L. Serio, A. Suraci and R. van WeelderenLHC Division, CERN, Geneva (Switzerland)Since the end of 1994 we have been operating a prototype half-cell of themachine lattice, accumulating more than 10,000 hours at superfluid heliumtemperatures and recovering from 150, mainly provoked, magnet resistivetransitions. The system has confirmed the validity of the basic design choices ofthe LHC cryogenic system. Furthermore, extensive testing on the response ofthe system to current ramp and discharge, and to magnet resistive transition, hasprovided sufficient information to enable a simplification of the cryogenicscheme that fulfils the LHC requirements. We report on the cryogenicoperation, testing and long-term behaviour of the LHC Test String during thelast 4 years of operation.1 INTRODUCTIONAs far as main accelerator systems - magnets, cryogenics and vacuum - are concerned, a major milestonein the validation of basic technical choices of the Large Hadron Collider (LHC) project [1] is the testingand operation of a full-scale superconducting magnet string, representing a half-cell of the machine lattice.The first version of the LHC test string was assembled, commissioned and successfully operatedfor the first time at the end of 1994 [2], confirming the validity of the basic design choices of the LHCcryogenic system [3].After three major shutdown periods [4] to exchange and install new magnets, components (e.g. anactively cooled beam screen) and instrumentation, and after four experimental runs (each lasting severalmonths), the experimental program of the string will terminate this year. A new test string based onsystems in their final version (e.g. 15-m long magnets and a cryogenic separate distribution line) and basedon a 107-m long full-cell hydraulic scheme will replace it.2 OPERATIONAL PERFORMANCEThe LHC test string has now accumulated more than 20,000 hours of operation. This consisted in steady-state operation and transients such as cool-down, warm-up, current ramping/de-ramping, magnet resistivetransitions, thermal and powering cycles. More than 10,000 hours were spent at superfluid heliumtemperatures, of which more than 2200 hours with the magnets powered.Continuous improvement of the process and the control system has made it possible for the 105 kgcold mass to be cooled down from 300 K to 1.9 K in about two days. The warming-up is limited by theavailable heating power (25 kW) and takes about four days. The string cryogenic system is now operatedcontinuously in fully automated mode during a non-experimental period, requiring only a cryogenic on-call operator who can be remotely alerted by the process control software in case the main systemparameters fall outside of their authorised range.3Fourteen thermal cycles were performed in order to get information about effect of thermalstresses (see figure 1).Figure 1  Thermal cyclesIn order to investigate eventual fatigue-related flaws of components, 10 years of operation of thefuture collider were simulated by executing 3275 powering cycles (from 800 A up to the nominal currentof 12.5 kA). The cryogenic system, during ramping and de-ramping that corresponds approximately indoubling of the heat load, was able to maintain the magnets well below their operating temperature of1.9 K with a total temperature excursion of less than 10 mK (figure 2).Figure 2  Powering cycles4We consider the time the cryogenic system has been operational sufficiently representative of thefuture machine lifetime operation. In order to complete the study, several thermal cycles and recoveriesfrom 150, mainly provoked, magnet resistive transitions were performed.All components have been operated in an industrial environment with no major interventionsrequired. The cryogenic valves have, in average, performed half of the 10,000 stroke cycles for whichthe manufacturer designs them. Some of them have gone as far as 30,000 stroke cycles.During this whole period there was no major incident excepted for the bursting of a recoveryballoon following a quench and the breaking of the glued glass-fibre stem of an industrial valve usedexperimentally as quench relief valve. The first incident was due to an operator mistake that left therecovery line open to the high flow of helium discharged from the cold mass. A mechanical restrictionfor the flow discharged to the balloon and an electrical interlock are now taking care of possible humanerrors. The second one was probably due to a faulty construction of the valve.3 EXPERIMENTSSince 1994 the main goals of the experimental runs were the validation of the basic technical choices ofthe LHC design and the exploration of possibilities to optimise the cryogenic distribution scheme.Cryogenic experiments were performed during each of the four runs and the required cryogenic nominaloperating conditions were ensured during non cryogenic-related experimental programs.Co- and counter-current flow operating modes of the superfluid helium cooling loop wereexplored. Flow capacity limitations have led us to abandon the counter-current mode. The intensivetesting on the superfluid helium cooling loop permitted to validate the performance and sizing rules ofthe bayonet heat exchanger for transporting linear heat loads in the W m-1 range over the distance ofseveral tens of meters [5].The introduction of two Pitot tubes in the quench recovery line as high flow flowmeters enabledus to confirm the results of the thermohydraulic experiments (figure 3) [6] and validated theirextrapolation for a discharge from a 100-m long magnet string, as well as confirming the basic designchoices of the quench recovery system [7]. The discharged flow of first superfluid, then normal liquidfollowed by supercritical helium is within 20 % of the calculated mass flow rate, as shown in figure 4.Figure 3  Pressure development in the cold mass Figure 4  Calculated and measuredafter a quench  discharged mass flowExperiments on thermohydraulic quench propagation experiments from dipole-to-dipole magnetsenabled us to identify the mechanism of heat propagation through the helium to dominate over the5quench propagation by solid conduction via the bus-bars [8]. Following the results obtained, we expectthe number of magnets to which a quench can propagate to be limited to few units, thus having nosignificant impact on the cryogenic recovery time, the helium inventory and exergy recovery.Furthermore, the thermohydraulic propagation through the helium can be slowed down by increasing theopening pressure of the quench relief valves.4 INSTRUMENTATION AND PROCESS CONTROLThe test string has also been a test bed for cryogenic instrumentation and process control [9]. The PLCprogram is adapted to the constant evolution of the string test, in particular new routines for automaticthermal cycling, quench recuperation and additional closed control loops have been implemented.Studies concerning aging of temperature sensors have been conducted. The temperature sensors that havebeen investigated include Allen-Bradley, CERNOX and Russian TVO. Within the accuracy of the in-situmeasurements it is difficult to conclude that a given type of thermometer has better long-term stabilitycharacteristics.5 CONCLUSIONSThe LHC cryogenic system and the collective behaviour of all regular arc LHC systems (cryogenic,magnet protection and vacuum) have been fully validated and their design optimised during the extensivetesting performed on the LHC test string facility. Valuable information has been gathered on theprocesses and on how to efficiently commission and operate a half-cell of the machine lattice.This version of a half-cell will be dismantled next year to be replaced by the new test stringfacility that incorporates the final version of the LHC systems. This new facility will be operational inthe year 2000 and will include a separate cryogenic distribution line external to the magnet cryostat tofeed every 107-m the cooling circuits of the string of magnets.6 REFERENCES1 Evans, L.R., The LHC, in: Proceedings ICEC16/ICMC Kitakyushu, Elsevier Science, Oxford(1997), 45-522 Bézaguet, A. et. al., The Superfluid Helium Cryogenic System for the LHC Test String: Design,Construction and First Operation, Adv.CryoEng. (1996) 41 A 777-7843 Lebrun, Ph., Superfluid Helium Cryogenics for the LHC Project, Cryogenics (1994) 34 1-84 Bézaguet, A. et. al., Cryogenic Operation and Testing of the Extended Prototype Magnet String, in:Proceedings ICEC16/ICMC Kitakyushu, Elsevier Science, Oxford (1997), 91-945 Lebrun, Ph. et al., Cooling Strings of Superconducting Devices Below 2 K: the Helium II BayonetHeat Exchanger, in: paper presented at CEC-ICMC97 Portland (USA), July 19976 Chorowski, M. et. al., Thermohydraulics of Resistive Transitions of the LHC Prototype MagnetString: Theoretical Modeling and Experimental Results, paper presented at CEC-ICMC97 Portland(USA), July 19977 Chorowski, M. et. al., Helium Recovery in the LHC Cryogenic System following Magnet ResistiveTransitions, paper presented at CEC-ICMC97 Portland (USA), July 19978 Rodriguez-Mateos, F., Schmidt, R., Serio, L., Thermohydraulic Quench Propagation at the LHCSuperconducting Magnet String, paper presented at this conference9 Flemsaeter, B., et. al., Applying Advanced Control Techniques for the Temperature Regulation ofthe LHC Superconducting Magnets, paper presented at this conference

Operation, Testing and Long Term Behaviour of the LHC Test String Cryogenic System

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