The LHC RF system, which must handle high intensity (0.5 A d.c.) beams, makes use of superconducting single-cell cavities, best suited to minimizing the effects of periodic transient beam loading. There will be eight cavities per beam, each capable of delivering 2 MV (5 MV/m accelerating field) at 400 MHz. The cavities themselves are now being manufactured by industry, using niobium-on-copper technology which gives full satisfaction at LEP. A cavity unit includes a helium tank (4.5 K operating temperature) built around a cavity cell, RF and HOM couplers and a mechanical tuner, all housed in a modular cryostat. Four-unit modules are ultimately foreseen for the LHC (two per beam), while at present a prototype version with two complete units is being extensively tested. In addition to a detailed description of the cavity and its ancillary equipment, the first test results of the prototype will be reported

Boussard, Daniel

Chiaveri, Enrico

Häbel, E

Kindermann, H P

Losito, R

Marque, S

Rödel, V

Stirbet, M

CERN Document Server

THE LHC SUPERCONDUCTING CAVITIESD. Boussard*, E. Chiaveri, E. Haebel, H.P. Kindermann,R. Losito, S. Marque, V. Rödel and M. Stirbet†, CERN, Geneva, Switzerland                                                          *  Daniel.Boussard@cern.ch†  On leave from Institute of Atomic Physics, Bucharest, RomaniaAbstractThe LHC RF system, which must handle high intensity(0.5 A d.c.) beams, makes use of superconducting single-cell cavities, best suited to minimizing the effects ofperiodic transient beam loading. There will be eightcavities per beam, each capable of delivering 2 MV(5 MV/m accelerating field) at 400 MHz. The cavitiesthemselves are now being manufactured by industry,using niobium-on-copper technology which gives fullsatisfaction at LEP. A cavity unit includes a helium tank(4.5 K operating temperature) built around a cavity cell,RF and HOM couplers and a mechanical tuner, all housedin a modular cryostat.  Four-unit modules are ultimatelyforeseen for the LHC (two per beam), while at present aprototype version with two complete units is beingextensively tested. In addition to a detailed description ofthe cavity and its ancillary equipment, the first test resultsof the prototype will be reported. 1 INTRODUCTIONCavities having a large stored energy (low R/Q, highvoltage) are best suited for the LHC to minimize theeffects of transient beam loading due to the long gaps (upto 3 µs) in the high intensity (0.56 A) proton beams [1].This leads naturally to single-cell superconducting (SC)cavities with large beam tubes very similar to thosedesigned for the high current e+e- factories. There will beeight 400 MHz cavities per beam, grouped by four in eachof two cryomodules.  Each cavity is connected inside acryomodule to its neighbours by large diameter(∅ = 300 mm) beam tubes and (unshielded) bellows. Theoperating voltage of the cavities (2 MV during storage,i.e. 5 MV/m) is quite low by to-day’s standards, but givesample margin to boost the LHC RF voltage and reducebunch length in the future, if necessary.A variable coupler will equip the LHC cavities in order togive maximum flexibility for the various modes ofoperation of the cavities during injection, ramping andstorage. The cryomodules are located on either side of along straight section (point 4) where the distance betweenthe two beams has been increased from 195 mm to420 mm. Additional room-temperature 200 MHz cavitieswill be installed in the LHC for optimum capture ofbeams having a large emittance (~ 1 eV.s). 2 CAVITY MANUFACTUREThe cavity technology is similar to that used successfullyon a large scale for LEP2 [2]; it is based on niobium filmon copper cavities operating at 4.5 K and on a modularcryostat with easy lateral access. Bare cavities areproduced by spinning and electron-beam welding and arecoated with a thin (1 to 2 µm) thickness) film of niobiumby magnetron sputtering. The series production of 21 barecavities is now being carried out by industry; sevencavities have already been accepted at CERN. Theirtypical performance is displayed in Figure 1 together withthe acceptance curve. The copper wall thickness resultsfrom a compromise between tuning force and mechanicalstability against buckling. With a thickness of 2.8 to3 mm, the cavity axial spring constant is about20 kN/mm.Figure 1: Typical cavity performance and acceptancecurveThe helium tank of each cavity is made of 2 mm thickstainless steel. Its cross-section is cylindrical around thecavity cell and octagonal at the location of the ports. Thefour helium tanks within a cryomodule are interconnectedat the liquid and gas levels in such a way that a commonhelium feed and a common gas return are sufficient(Figure 2). Individual safety exhaust pipes with rupturedisks are, however, provided for each cavity. As in LEP,each cavity cradle is suspended inside the cryostat toallow for contraction during cooldown. The longitudinalfixed point corresponds to the main coupler position toavoid stresses on the double-walled tube. Neither amagnetic shield nor a heat shield is necessary. Themeasured static losses of the prototype cryomodule(having only two cavities, no couplers and no secondbeam tube) amount to about 25 W.0 .11100 2 4 6 8 10 12Eacc [MV/m]Q/109L IM ITED BY AMPLIFIER POW ER0-7803-5573-3/99/$10.00@1999 IEEE. 946Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Figure 2: Partial view of the prototype cryomodulewith two cavities (a series cryomodule has fourcavities)A modular construction was adopted for the vacuumtanks of the cryomodule (Figure 2). Each tank is astainless steel cylinder, without any welds, with largelateral openings to permit easy access to the cavity.The four cavities are connected together with the widebellows in a clean room; this assembly is then rolledinside the complete vacuum tank. The main couplersare mounted last, again in a clean room. It is alsopossible to disassemble and reinstall a single cavity inthe middle of a cryomodule, without moving itsneighbours. The second beam tube (cold) is attached tothe side of the cavity cradle. 3 TUNERContrary to the case of LEP cavities [2], the LHCcavity tuner is purely mechanical in order to providethe large tuning range at full speed required tocompensate beam loading. The cavity cradle, with itstwo thick end plates joined by four columns forms arigid assembly, free of harmful resonances, which cantake the strong tuning forces (~ 20 kN) withoutexcessive deformation. The cavity end plate and thecradle end plate are pulled together via thin (0.75 mm)stainless-steel blades and torsion shafts with long leverarms (Figure 3). This arrangement provides a leveraction (ratio 14:1) without sliding parts or backlash.The internal stresses (290 Mpa) inside the blades staybelow the elastic limit.The axial force and movementat the extremities of the two lever arms, inside the coldcradle, are transferred to the outside via two thin-walled stainless-steel cylinders acting as counter-rotating torsion shafts The latter are driven by stainlesssteel cables (∅ = 3 mm) providing again a transmissionwithout friction or backlash.Furthermore, this system allows displacement of thecradle  during cooldown and provides a low heatconductance. A slightly different version of this tunerFigure 3: Tuner mechanismwas successfully tested on the prototype cryomodule;the achieved tuning range and speed (limited by thestepping motor) were 180 kHz and 9 kHz/srespectively. The resolution is too small to bemeasurable. 4 POWER COUPLERThe LHC variable coupler is an upgraded version ofthe LEP2 fixed coupler [2][3] (Figure 4). An open-ended 75 Ω coaxial line is coupling to the cavity. Theouter conductor (not represented in Figure 4) is madeof copper-plated stainless-steel (double-walled) andcooled with 4.5 K helium gas, while the innerconductor (antenna) is a copper tube cooled by forcedair. A cylindrical ceramic window, with massivecopper rings brazed on its edges is placed in thewaveguide-to-coaxial transformer. A reduced heightwaveguide provides directly the matching to thecoaxial line, avoiding the usual “doorknob”. In order tosuppress multipactor during operation, a d.c. bias of3 kV is applied to the antenna, isolated from groundwith a coaxial capacitor mounted in the waveguide. Aircooling is provided on the window and other criticalelements of the coupler. A vacuum gauge and anelectron pick-up antenna are located close to thewindow.947Proceedings of the 1999 Particle Accelerator Conference, New York, 1999The antenna is moved (60 mm stroke) by making useof bellows about λ/4 long. A low impedance (7Ω) λ/4line transformer brings the current in the bellows tolow enough values, not requiring copper plating of thisstainless steel part. The displacement of the antenna isguided by a (motor driven) high precision device.Figure 4: The LHC variable couplerTwo prototype LHC couplers have been manufactured,assembled and vacuum-tested. Technical problemsoccurred during electron-beam welding of the ceramicwindow to the copper body and during titaniumcoating of the vacuum side of the ceramic, resulting inthe breaking of some ceramic windows. Solutions toavoid these failures have been found. We also sufferedfrom the bad quality of the OFE copper material usedfor the copper body, which developed vacuum leaksafter baking out at 200°C for 24 h. In the future forgedOFE copper will be used.High-power RF tests at room temperature were donewith two couplers, mounted horizontally on a 400 MHzcopper test cavity. One coupler is connected to a500 kW 400 MHz klystron [4], via a circulator, thesecond to either a 1 MW load or to a mobile shortcircuit. In travelling-wave mode, and with a pressurelimit of 2×10-7 mbar, the RF power could be rampedbetween 15 and 500 kW, crossing several multipactorlevels, for which d.c. bias was effective. Below 15 kWmultipactor occurred inside the 7Ω λ/4 line, with noelectrons picked up on the antenna and no influence ofd.c. bias. (An additional bias on the λ/4 line is beingconsidered.) The maximum power could be sustainedfor long periods (400 kW for 150 hours, 500 kW for 50hours) with no sign of damage inside the coupler.At full reflection, any phase and any coupling thecoupler sustains a 500 kW forward power (2 MWequivalent power) provided it is pulsed (50 ms on, dutycycle 10%) to avoid local overheating. 5 HOM COUPLERSDuring the design of the LHC cavity its higher ordermode (HOM) characteristics were carefully studied [5].Due to the large iris diameter and a cell length smallerthan  λ/2 , only two of the HOMs do not propagate intothe intercell beam tubes. These are the two dipolemodes TE111 and TM110 which also have the highestcoupling impedance of all transverse modes. Clearlyfor damping them beam tube couplers near the cells areneeded. These are of a doubly resonant type and tunedto provide equal shunt impedances (R/Q)*Q of about1.5 kΩ for the two modes with one coupler mountedfor each polarization.In addition two broadband couplers are installed percell. Derived from LEP design they can each carry aHOM power of 800 W. They have response peaks nearthe frequencies of the high (R/Q) mode TM011(770 MHz) and the trapped mode [5] TM012(1240 MHz) respectively. On a model cavity externalQs of 500 have been measured for these modesensuring that even if mode excitation by the LHC beamis resonant the RF power coupled out will not exceed500 W. 6 ACKNOWLEDGEMENTSThe authors wish to thank the many persons whocontributed efficiently to the design, construction andtests of the LHC cavities and their ancillary equipment. 7 REFERENCES[1] “The Large Hadron Collider — Conceptual Design”,CERN/AC/95-05 LHC[2] LEP Design Report, Vol. III, LEP2, CERN/AC/96-01 (LEP2)[3] H.P. Kindermann, E. Haebel, M. Stirbet, V. Vescherevitch,“Status of RF Couplers for SC Cavities at CERN”, EPAC,Sitges 1996[4] H. Frischholz, W.R. Fowkes, C. Pearson, “Design andConstruction of a 500 kW CW, 400 MHz Klystron to be usedas RF Power Source for LHC/RF Component Tests”, PAC,Vancouver 1997.[5] E. Haebel, V. Rödel, F. Gerick, Z.T. Zhao, “The Higher-Order Mode Dampers of the 400 MHz SC LHC Cavities”,CERN-SL-98-008 RFAir cooling inlet Air inlet antennaAir pressureinterlockMotor driveWaveguide75 Ω Coaxial line(under vacuum)Antenna (copper)CeramicwindowCapacitorfor DC bias7 Ω Coaxial line(under vacuum)Displacement    mechanism (60 mm stroke)RF screen andHV protection Bellows948Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

The LHC superconducting cavities

The LHC superconducting cavities

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