CERN LIBRARIES, GENEVA P O. Cryogenics in CERN Accelerators. Ph. Lebrun on behalf of the CERN Cryogenics Group

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1 EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN LIBRARIES, GENEVA / CERN AT/94-08 (CR) Sw Ll UZ P O Cryogenics in CERN Accelerators Ph. Lebrun on behalf of the CERN Cryogenics Group Invited paper to Third International Conference KRYOGENIKA April 1994, Usti Nad Labem, Czech Republic Geneva, Switzerland 17 May 1994 OCR Output

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3 CRYOGENICS IN CERN ACCELERATORS Ph. Lebrun on behalf of the CERN Cryogenics Group CERN, Geneva, Switzerland OCR OutputINTRODUCTION Cryogenics has been for long an ancillary technology to high-energy physics experiments, from bubble chambers and large spectrometer magnets, to liquid hydrogen targets and liquid argon calorimeters. lts emergence as a key technology for particle accelerators and colliders is more recent, as it follows the development of applied superconductivity and the increasing use of superconducting devices (electro-magnets and accelerating RF cavities) in modem machines [1]. Today, almost all new high-energy accelerators are superconducting and thus require large, powerful helium cryogenic systems [2, 3]. In the following, we present the historical development of cryogenics in CERN accelerators, which led to the large-scale projects of today (LEP2) and tomorrow (LHC). Other applications of cryogenics at CERN have been reported elsewhere [4]. HISTORICAL DEVELOPMENT Although their potential had been acknowledged in the 1960's, superconducting devices were still considered not mature enough to equip the kilometer-size accelerators of the early 1970's. Applied superconductivity first appeared in isolated devices equipping extracted beam lines, e.g. bending magnets [5, 6] and RF separators [7]. Superconducting RF already appeared as a demanding user, operating in closed cycle with a dedicated K cryoplant [8]. In other cases, the moderate thermal budgets permitted intermittent refilling from 500 l dewars supplied by the CERN Central Liquefier. The vapourized helium was always recovered through a network of lines spanning the site, and purified off line before reliquefaction. ln cryogenics as in other fields, the Intersecting Storage Rings (ISR) showed pioneering spirit and technical excellence, with the installation and operation for physics in the early 1980's of the superconducting high luminosity insertion [9]. The eight industry-made superconducting quadrupoles were the first of their kind to operate routinely in an accelerator, fed from a dedicated 270 l/h helium liquefier through 50-in long distribution lines [10]. In the preceding years, the ultra-high vacuum system of the 1SR had operated liquid helium-cooled condensation cryopumps [ll, 12] and tested an experimental cold bore [13, 14] to explore vacuum behaviour of future fully superconducting accelerators. Also worth mentioning is an application of cryogenics to accelerators not driven by superconductivity, but by limitation of thermal noise in wide-band, high-gain power electronics: the stochastic cooling pick-ups of the Antiproton Collector (ACOL) had their preamplifiers refrigerated down to 20 K by means of closed cycle Gifford-McMahon cryocoolers, while their mobile electrodes were radiatively cooled from a thermal shield bridged to the upper stage of the cryocoolers. This technique avoided all distribution and handling of liquid cryogens in an accelerator with limited access and no cryogenic infrastructure [15]. The 1980's were at CERN the years of construction of LEP, a lepton collider unprecendented in size, with a circumference of 26.7 km. Although the main magnetic system of LEP is low-field and thus normal conducting, the physics yield of the machine, which started to operate in 1989, was strongly enhanced hy four high-luminosity insertions [16], based on slim superconducting quadrupoles integrated in the experimental detectors. Although part of the accelerator lattice, two of the LEP high-luminosity insertions are fed in liquid helium from the K mixed duty cryoplants serving the ALEPH and DELPHI superconducting spectrometer magnets. The other detector magnets at L3 and OPAL being normal conducting, their high luminosity insertions operate with dedicated K cryoplants [17]. The deep underground siting and intricate layout of the LEP experimental areas, as well as stringent operational and maintenance requirements OCR Output

4 - 2 on the detectors, have imposed technical compromises on the cryogenic systems, in particular long-distzuice controlled liquid helium transfer across large differences in elevation [18], using high perforrnance flexible lines [19]. In spite of their complexity, the LEP superconducting high luminosity insertions have shown an excellent operational record [20]. The ultimate energy of LEP is limited by the synchrotron power radiated from the circulated beams along the arcs. Exploiting the full energy and luminosity potential of the machine thus requires additional, efficient superconducting acceleration cavities. Prototypes of such cavities were developed in the 1980's and operated reliably in the SPS accelerator, to gather operating experience in real beam conditions and environment. Liquid helium was supplied first from a storage vessel at ground level, through a 100-m long transfer line running down a shaft, then by a dedicated helium refrigerator installed underground at tunnel level [21]. Today the SPS routinely operates with superconducting cavities, which have progressively replaced the normal conducting units [22]. LARGE-SCALE PROJECTS: FROM LEP2 TO LHC The energy ugrade of LEP in the 90 GeV range and beyond, requires the installation of at least 192 superconducting acceleration cavities, in four equidistant long straight sections of the machine tunnel [23]. The 352 Ml Iz cavities, made of niobium sheet or niobium-plated copper, operate in baths of saturated helium at 4.5 K [24]. They require large refrigeration capacity, supplied by four K cryoplants (upgradeable to 18 kw), and distributed by cryogenic lines spanning the 250-m length of the cavity strings on either side of each cryoplant [25, 26]. Cryogenic refrigeration for reception and accelerator testing of the superconducting cavities is performed by two compact K cryoplants. one of which is installed underground at tunnel level [27], and the other in a test hall [28], The final K cryoplants, although also of compact design, cannot be fully located underground and are thus split in upper and lower cold boxes, connected together by compound lines ntnning vertically along the access shafts. The splitting temperature level of 20 K adequately limits the therrrtodynamic cost of transfer line heat loads, while avoiding excessive hydrostatic head in the 50 to 150 m-deep shafts. In view of their large capacity and heavy duty cycle, the cryoplants were designed for high efficiency: they feature power factors as low as 220 W/W [29]. lt is foreseen to reuse the four large refrigerators of LEP2, after suitable adaptation and upgrade, for the operation of CERN's next large project, the LHC. The Large Hadron Collider (LHC) is a high-energy, high-luminosity machine, to be installed in the existing LEP tunnel. lt will accelerate and bring into collision intense beams of protons and ions, to explore physics issues in the TeV per constituent energy range [30, 31]. For this purpose, the LHC makes use of high-tield, twin-aperture superconducting magnets with a bending field of about 9 T, operated in pressurized superfluid helium at 1.9 K [32]. Besides the boosting of current density in NbTi superconductor, this allows to take advantage of the high thermal conductivity and low viscosity of superfluid helium for stabilisation. Due to its unprecedented size, and in view of the specificities of superfluid helium operation, the LHC cryogenic system will face considerable challenges, such as the limitation of heat loads, staging of temperature levels and control of thertnal budgets in 24 km of magnet cryostats, the production of large, variable refrigeration duties in eight cryoplants installed around the machine circumference, and the distribution of the cooling power along the arcs under minute temperature difference and pressure drop. The main technical issues have been assessed and development of key technologies, e.g. low-pressure cryogenic helium compressors, efficient magnet eryostats [33], or two-phase flow of superfluid helium [34, 35], is well under way. CONCLUSION A leading laboratory in high-energy physics, CERN has also become a major center for large-scale helium cryogenics (Figure 1), with presently 21 cryoplants covering the whole spectrum of capacity (Figure 2), which perform with industrial-class reliability and totalize some 100'00O hours of integrated operation per year [36]. OCR Output

5 4.5 K] Needed _ z for LHC i~160 kw E ' ENeeded for} LEPZOO 67 kw L E L_. --- L-. L; LEP kw E 1000 ISR & RF tests Large Detector Magnets, Superccnductmg Separator H h-l ` I ng ummoszty nseruons NA 5 LEP! kw \ ALEPH. DELPHI \ (BEBC phased out) GMEGA, \. BEBC \ \x 1 J * Central Ilsuefler SPS RF Figure 1: Installed cryogenic capacity at CERN Figure 2: Unit size of CERN cryuplzuits ACKNOWLEDGEMENTS This brief review is an attempt to reflect the work of many colleagues from the CERN Cryogenics Group and Accelerator Technology Division, as well as of our partners in national laboratories and European industry. The reader is referred to the papers referenced below for more detailed information. REFERENCES [1] M. Wilson, Accelerators and Superconductivity: a Marriage of Convenience, CERN Report (1987). [2] G. Horlitz, Refrigeration of Large Scale Superconducting Systems for High-Energy Accelerators, Proc. ICEC14 Kiev, Cryogenics 32, ICEC Supplement, pp (1992). [3] Ph. Lebrun, Large-Scale Cryogenics for Particle Accelerators, Proc. XXVI lnt. Conf. on High-Energy Physics, Dallas 1992, AIP. PP (1993). [4] J.P. Dauvergne, D. Delikaris, W.K. Erdt, D. Giisewell, F. Haug, Ph. Lebrun, G. Passardi, J.M. Ricuhlztnd. J. Schmid & G. Winkler, Application of Liquid Helium Cryoplants at CERN, paper presented at CEC93, Albuquerque (1993). [5] Ph. Dow & G. Kesseler, Design and Construction of Two 10.8 T.m Superconducting Beam Bending Magnets, CERN Report (1978). OCR Output

6 - 4 [6] J. Perot & D. Leroy, The Superconducting Bending Magnets CESAR, Proc. MT-6, Bratislava pp (1978). [7] A. Citron, G. Dammertz, M. Grundner, L. Husson, R. Lehm, H. Lengeler, D.E. Plane & G. Winkler. First Operation of a Superconducting RF Particle Separator, Nucl. Instr. Meth. 155, pp (1978). [8] AJ. Steel, S. Bmzzi & M.E. Clarke, A 300 W 1.8 K Refrigerator and Distribution System for the CERN Superconducting RF Particle separator, Proc. ICEC6, IPC Science & Technology Press, pp (1978). [9] J. Billan, K.N. Henrichsen, H. Laeger, Ph. Lebrun, R. Perin, S. Pichler, P. Pugin, L. Resegotti, P. Rohmig, T. Tortschanoff, A. Verdier, L. Walckiers & R. Wolf, A Superconducting High-Luminosity Insertion in the lntersecting Storage Rings (ISR), Proc. PAC'79, IEEE Trans. Nucl. Sci. NS 26, 3, pp (1979). [10] H. Laeger & Ph. Lebrun, The Helium Cryogenic System for the Superconducting High-Luminosity Insertion at the CERN ISR, Adv. Cry0. Eng. 29, pp (1984). [11] C. Benvenuti, Characteristics, Advantages and Possible Applications of Condensation Cryopumping, J0urn. Vac. Sci. Technol. 11, 3, pp (1974). [12] C. Benvenuti & M. Firth, Improved Version of the CERN Condensation Cryopump, CERN Repart ISR VA/78-27 (1978). [13] C. Benvenuti, R. Calder & N. Hilleret, A Vacuum Cold Bore Test Section at the CERN ISR. Proc. PAC`77, IEEE Trans. Nucl. Sc. NS-24, 3, PP (1977). [14] C. Benvenuti & N. Hilleret, Cold Bore Experiments at the CERN ISR, Proc. PAC'79, IEEE Trans. Nucl. Sci. NS-26, 3, pp (1979). [15] Ph. Lebrun, S. Milner & A. Poncet, Cryogenic Design of the Stochastic Cooling Pick-Ups for the CERN Antiproton Collector (ACOL), Adv. Cry0. Eng. 31, pp (1986). [16] T.M. Taylor, Technological Aspects of the LEP Low Beta lnsertions, Proc. PAC'85, IEEE Trans. Nucl. Sci. NS 32, 5. PP (1985). [17] J.P. Dauvergne, M. Firth, A. Juillerat, Ph. Lebrun & l.m. Rieubland, Helium Cryogenics at the LEP Experimental Areas, Adv. Cryo. Eng. 35B, pp (1990). [18] H. Blessing, W. Erdt, Ph. Lebrun, L. Walckiers & W. Weingarten, Controlled Downward Transfer of Saturated Liquid Helium Across Large Differences in Elevation, Proc. ICEC12, Butterworth, pp (1988). [19] H. Blessing, Ph. Lebrun & K. Schippl, Very Low-loss Liquid Helium Transfer with Flexible Cryogenic Lines, Adv. Cry0. Eng. 35B, pp (1990). [20] Ph. Lebrun, O. Pagano, T.M. Taylor & L. Walckiers, Operational Experience with the LEP Low-Beta Superconducting Quadrupoles, Proc. 3rd EPAC Berlin, 2, pp (1992). [21] G. Passardi, The Cryogenic System for the Superconducting Cavities in the CERN SPS Accelerator, Proc. ICEC13, Cryogenics 30, September Supplement, pp (1990). OCR Output

7 - 5 [22] G. Passardi, N. Delruelle, A. Juillerat & J. Tischhauser, Long-Term Operational Experience and Upgrade of the Cryogenic System for the Superconducting Cavities in the CERN SPS Accelerator, Proc. ICEC14, Cryogenics 32, ICEC Supplement. PP (1992). [23] C. Wyss, Progress of the LEP Energy Upgrade Project, paper presented at PAC'93, Washington (1993). [24] G. Cavallari, C. Benvenuti, P. Bemard, D. Bloess, E. Chiaveri, F. Genesio, E. Haebel, N. Hilleret, J. Tuckmantel & W. Weingarten, Superconducting cavities for the LEP Energy Upgrade, paper presented at PAC'93, Washington (1993). [25] M. Barranco-Luque, J.P. Dauvergne, W. Erdt, P. Frandsen, D. Gusewell, F. Haug, A. Juillerat, G. Passardi, J. Schmid & G. Winkler, The Refrigeration System for the LEP Energy Upgrade, Proc. ICECI3. Cryogenics 30, September Supplement pp (1990). [26] M. Barranco-Luque, S. Claudet, W.K. Erdt, P.K. Frandsen, Ph. Gayet, D. Gusewell, K.P. Hoffmann..I. Schmid, N. Solheim, G. Winkler & C. Wyss, Status of Cryogenics t`or the LEP200 Energy Upgrade Project at CERN, Proc. ICEC14, Cryogenics 32, ICEC Supplement, pp (1992). [27] K. Kurtcuoglu, K. Lohlein, U. Wagner & B. Ziegler, Plant Performance of the High Ellicient 6 kw Helium Refrigerator for the LEP2OO Project at CERN, paper presented at CEC'93, Albuquerque (1993). [28] G.M. Gistau & M. Bonneton, A 6kW at 4.5 K Helium refrigerator for CERN's Cryogenic Test Station, paper presented at CEC'93, Albuquerque (1993). [29] D. Giisewell, M. Bananco-Luque, S. Claudet, W.K. Erdt, P. Frandsen, Ph. Gayet, J. Schmid, N. Solheim, Ch. Titcomb & G. Winkler, Cryogenics for the LEP2OO Superconducting Cavities at CERN, paper presented at PAC'93, Washington (1993). [30] The LHC Study Group, Design Study of the Large Hadron Collider: A Multiparticle Collider in the LEP Tunnel, CERN Report (1991). [31] G. Brianti on behalf of the LHC Machine Group, LHC Progress and Status, paper presented at PAC'93, Washington (1993). [32] R. Perin & J. Vlogaert for the LHC Magnet Team, Magnets for the Large Hadron Collider, paper presented at EUCAS'93, Gottingen (1993). [33] J.C. Brunet, J. Kerby, Ph. Lebnin, P. Rohmig, B. Szeless & L.R. Williams, Design ol LllC Prototype Dipole Cryostats, Proc. ICEC 14, Cryogenics 32, ICEC Supplement, pp (1992). [34] J. Casas, A. Cyvoct, Ph. Lebrun, M. Marquet, L. Tavian & R. van Weelderen, Design Concept and First Experimental Validation of the Superfluid Helium System for the Large Hadron Collider Project at CERN. Proc. ICEC14, Cryogenics 32, ICEC Supplement, pp (1992). [35] A. Bézaguet, J. Casas-Cubillos, Ph. Lebrun, M. Marquet, L. Tavian & R. van Weelderen, The Supertluid Helium Model Cryoloop for the CERN Large Hadron Collider, paper presented at CEC`93, Albuquerque (1993). [36] D. Delikaris, J.P. Dauvergne, P.K. Frandsen, F. Haug, G. Passardi, J.M. Rieubland & J. Schmid, Long Term Experience with Cryoplant operation for Superconducting Magnets and RF cavities at CERN, paper presented at PAC'93, Washington (1993). OCR Output

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