A revision of the physics needs and recent progress in the technology of superconducting (SC) RF cavities have triggered major changes in the design of a SC H-linac at CERN. With up to 5MW beam power, the SPL can be the proton driver for a next generation ISOL-type radioactive beam facility (âEURISOLâ) and/or supply protons to a neutrino () facility (conventional superbeam + beta-beam or -factory). Furthermore the SPL can replace Linac2 and the PS booster (PSB), improving significantly the beam performance in terms of brightness, intensity, and reliability for the benefit of all proton users at CERN, including LHC and its luminosity upgrade. Compared with the first conceptual design, the beam energy is almost doubled (3.5GeV instead of 2.2 GeV) while the length is reduced by 40%. At a repetition rate of 50 Hz, the linac reuses decommissioned 352.2MHz RF equipment from LEP in the low-energy part. Beyond 90MeV the RF frequency is doubled, and from 180MeV onwards high-gradient SC bulkniobium cavities accelerate the beam to its final energy of 3.5GeV. This paper presents the overall design approach, together with the technical progress since the first conceptual design in 2000

Baylac, M

Bellodi, G

Benedico-Mora, E

Body, Y

Caspers, Friedhelm

Chel, S

De Conto, J M

Duperrier, R

Froidefond, E

Garoby, R

Gerigk, F

Hanke, K

Hill, C

Hori, M

Inigo-Golfin, J

Kahle, K

Kroyer, T

Küchler, D

Lallement, J B

Lindroos, M

Lombardi, AM

López-Hernandez, A

Magistris, M

Meinschad, T

Millich, Antonio

Noah-Messomo, E

Pagani, C

Palladino, V

Paoluzzi, M

Pasini, M

Pierini, P

Rossi, C

Royer, Jean Pierre

Sanmartí, M

Sargsyan, E

Scrivens, R

Silari, M

Steiner, T

Tückmantel, Joachim

Uriot, D

Vretenar, M

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN  ⎯  AB DEPARTMENT  CERN-AB-2006-081         The SPL (II) at CERN, a Superconducting 3.5 GeV H¯ Linac   F. Gerigk, G. Bellodi, E. Benedico Mora, Y. Body, F. Caspers, R. Garoby, K. Hanke, C. Hill, J. Inigo-Golfin, K. Kahle, T. Kroyer, D. Kuechler, J.-B. Lallement, M. Lindroos, A.M. Lombardi, A. Lopez Hernandez, M. Magistris, T.K. Meinschad, A. Millich, E. Noah Messomo, M. Paoluzzi, M. Pasini, C. Rossi, J.P. Royer, M. Sanmarti, E. Sargsyan, R. Scrivens, M. Silari, T. Steiner, J. Tückmantel, M. Vretenar, CERN, Geneva, M. Baylac, J.M. De Conto, E. Froidefond, LPSC Grenoble, S. Chel, R. Duperrier, D. Uriot, CEA Saclay, M. Hori, CERN/Tokyo University, C. Pagani, P. Pierini, INFN Milan, V. Palladino, INFN Naples    Abstract  A revision of the physics needs and recent progress in the technology of superconducting (SC) RF cavities have triggered major changes in the design of a SC H¯ linac at CERN. With up to 5MW beam power, the SPL can be the proton driver for a next generation ISOL-type radioactive beam facility (”EURISOL”) and/or supply protons to a neutrino (ν) facility (conventional superbeam + beta-beam or ν-factory). Furthermore the SPL can replace Linac2 and the PS booster (PSB), improving significantly the beam performance in terms of brightness, intensity, and reliability for the benefit of all proton users at CERN, including LHC and its luminosity upgrade. Compared with the first conceptual design, the beam energy is almost doubled (3.5GeV instead of 2.2 GeV) while the length is reduced by 40%. At a repetition rate of 50 Hz, the linac reuses decommissioned 352.2MHz RF equipment from LEP in the low-energy part. Beyond 90MeV the RF frequency is doubled, and from 180MeV onwards high-gradient SC bulkniobium cavities accelerate the beam to its final energy of 3.5GeV. This paper presents the overall design approach, together with the technical progress since the first conceptual design in 2000.   Paper presented at the 2006 Linear Accelerator Conference, Knoxville, TN, USA, 21-25 August 2006    Geneva, Switzerland 4/10/2006 THE SPL (II) AT CERN, A SUPERCONDUCTING 3.5 GEV H− LINACF. Gerigk, G. Bellodi, E. Benedico Mora, Y. Body, F. Caspers, R. Garoby, K. Hanke, C. Hill,J. Inigo-Golfin, K. Kahle, T. Kroyer, D. Kuechler, J.-B. Lallement, M. Lindroos, A.M. Lombardi,A. Lopez Hernandez, M. Magistris, T.K. Meinschad, A. Millich, E. Noah Messomo, M. Paoluzzi,M. Pasini, C. Rossi, J.P. Royer, M. Sanmarti, E. Sargsyan, R. Scrivens, M. Silari, T. Steiner,J. Tu¨ckmantel, M. Vretenar, CERN, Geneva, M. Baylac, J.M. De Conto, E. Froidefond, LPSCGrenoble, S. Chel, R. Duperrier, D. Uriot, CEA Saclay, M. Hori, CERN/Tokyo University,C. Pagani, P. Pierini, INFN Milan, V. Palladino, INFN NaplesAbstractA revision of the physics needs and recent progress inthe technology of superconducting (SC) RF cavities havetriggered major changes in the design of a SC H− linac atCERN. With up to 5 MW beam power, the SPL can bethe proton driver for a next generation ISOL-type radioac-tive beam facility (”EURISOL”) and/or supply protons to aneutrino (ν) facility (conventional superbeam + beta-beamor ν-factory). Furthermore the SPL can replace Linac2 andthe PS booster (PSB), improving significantly the beamperformance in terms of brightness, intensity, and relia-bility for the benefit of all proton users at CERN, includ-ing LHC and its luminosity upgrade. Compared with thefirst conceptual design, the beam energy is almost doubled(3.5 GeV instead of 2.2 GeV) while the length is reducedby 40%. At a repetition rate of 50 Hz, the linac reuses de-commissioned 352.2 MHz RF equipment from LEP in thelow-energy part. Beyond 90 MeV the RF frequency is dou-bled, and from 180 MeV onwards high-gradient SC bulk-niobium cavities accelerate the beam to its final energy of3.5 GeV. This paper presents the overall design approach,together with the technical progress since the first concep-tual design in 2000.INTRODUCTIONThe first SPL design (SPL I) [1] was driven by the idea toreuse decommissioned LEP RF equipment (klystrons, SCcavities) for the construction of a SC proton driver for aCERN-based ν-factory. Five years later a design revision[2] was triggered by the evolving design parameters for ν-facilities, the possibility of using the SPL as a EURISOL[3, 4] driver and the progress in the performance of SC bulkniobium cavities. The new design is devised to have max-imum flexibility for the adaption to the needs of various(still evolving) neutrino schemes [5]. Without any hard-ware changes in the linac itself, the SPL can provide suit-able beams for the β-beam/superbeam scenario and (possi-bly in a 2nd stage) to a full-blown ν-factory [6]. It thereforeallows a staged approach to ν-physics which cannot be re-alised easily with other types of proton drivers such as rapidcycling synchrotrons. Furthermore the SPL beam can betime-shared with other high-power users (e.g. EURISOL),reducing the initial investment per user considerably.The construction of the SPL is foreseen in three stages:i) currently approved and under construction: a 3 MeVtest stand to characterise the performance and beam qual-ity of the H− front-end including the beam chopper. Thissection is crucial for low-loss injection of the high-powerbeam into subsequent circular machines; ii) awaiting ap-proval end 2006: construction of Linac4 [7], the normalconducting (NC) part of the SPL up to 160 MeV. Operatingat low duty cycle, it will replace the existing proton Linac2(50 MeV) and inject into the PSB; iii) relocation of Linac4,extension of the NC section to 180 MeV and the additionof a SC linac to reach an energy of 3.5 GeV. The approvalfor the last stage will depend on the European post-LHCphysics road map (LHC upgrades, neutrino physics, linearcollider, ...) and is not expected before 2010.SPL HIGH-POWER USERSThe present design foresees two modes of high-poweroperation: i) ν-operation: 4 MW chopped H− pulses of0.57 ms are accumulated in a subsequent ring via charge-exchange injection. The injection losses are minimised bycutting gaps into the bunch train corresponding to the tran-sition time between RF (ring) buckets. In the referencescheme three out of eight bunches are removed by the low-energy beam chopper. ii) EURISOL operation: a 5 MW un-chopped beam is sent directly onto a high-power target andone (or more) 100 kW beams are sent to low-power targets.Sending beam to different users within the same pulse canbe done by either using the chopper to create 0.1 ms gapsin the bunch train allowing for the rise time of a switchingmagnet, or by using magnetic and/or laser stripping to re-move a fraction of the beam for a low-power target. The 2ndoption is under study by EURISOL and – given its success– would be preferable, since one can create longer pulsesand thus reduce thermal stress in the target.The energy of 3.5 GeV is an optimum for a beta-beambased neutrino facility on the CERN site, which is also rea-sonably compatible with the needs of EURISOL [8]. Whilethe repetition rate of 50 Hz is well suited for neutrinos, it isalso considered high enough for a EURISOL proton driver,even though a CW machine is the preferred driver in a“green field” scenario. Table 1 lists the main parametersof SPL I and of SPL II for ν or EURISOL operation.In parallel to the high-power users the SPL will provide   	 ﬁﬀﬂﬃ  !"! #$!%&('*) +&(' )-,.$/ .' 10243&6587,293;:58&$/&<' ):=&' )7-%' )$/ ,?> )@A&-%/%B' 10Figure 1: Schematic layout of the SPLa 1 Hz H− beam to the CERN PS, replacing the PSB , andenabling a beam out of the PS that is compatible with allforeseen LHC luminosity upgrade scenarios.DESIGN AND IMPROVEMENTS TO SPL IThe layout of the SPL NC section (< 180 MeV) is thatof a classical high-power proton front-end: an RFQ [9] op-erating at 352.2 MHz accelerates the beam to 3 MeV. Afterthe beam chopper a Drift Tube Linac (DTL) takes the beamto an energy of 40 MeV, followed by a Cell-Coupled DTL(CCDTL) up to 90 MeV. A Side Coupled Linac (SCL) at704 MHz then accelerates the beam to 180 MeV (see also[7, 10]). Two families of SC five-cell bulk-niobium ellip-tical cavities then cover the energy range from 180 MeVto 3.5 GeV. The electric gradients of 19 and 25 MV/m, re-spectively, are based on electric and magnetic peak surfacefields of Epeak = 50 MV/m and Bpeak = 100 mT, valueswhich are consistent with reported performances of bulkniobium structures [11, 12]. Figure 1 shows a block dia-Table 1: Main linac parameters for SPL I and SPL II oper-ating as driver for ν production or EURISOLSPL I SPL IIν ν EURISOLenergy [GeV] 2.2 3.5 3.5length [m] 690 430 430av. beam power [MW] 4 4 5av. RF power† [MW] 24 17 21av. cryo power [MW] 9.6 3.6 4.4repetition rate [Hz] 75 50 50beam pulse length [ms] 2.2 0.57 0.71 +0.014Iav, pulse ‡ [mA] 11 40 40Ipeak ‡ [mA] 18.4 64 40beam duty cycle ‡ [%] 16.5 2.9 3.6chopping ratio [%] 75 62 –εt, (r.m.s.) [pimm mrad] 0.4 0.36 0.36εl, (r.m.s.) [pimm mrad] 0.76 0.5 0.5inj. turns (into ISR) 660 176 –peak RF power [MW] 32 162 162tetrodes* 79 3 3LEP klystrons** 44 14 14new klystrons*** – 44 44cryo temperature [K] 4.5 2 2† without 30% margin for Lorentz detuning, ‡ > 3 MeV,* 0.1 MW, 352 MHz, ** 1 MW, 352 MHz, *** 5 MW,704 MHzTable 2: Main parameters of the accelerating sectionsSection E Cavit. PˆRF Klystr. l[MeV] [MW] [m]Source 0.095 – – – 3RFQ 3 1 1.0 1 6Chopper 3 3 0.1 – 3.7DTL 40 3 3.8 5 13.6CCDTL 90 24 6.4 8 25.5SCL 180 24 15.1 5 34.9β = 0.65 643 42 18.5 7 86β = 1.0 3560 136 116.7 32 256Total 3560 233 161.6 58 429gram of the SPL layout and Table 2 lists the main parame-ters of each accelerating section.With respect to SPL I the power consumption was re-duced by ≈ 30% for the RF system and by > 60% forthe cryogenic system. This is mainly due to i) the use ofsmaller SC cavities at 2 K (rather than 4.5 K), and ii) theraise in average pulse current. Both choices reduce the fill-ing time of the cavities and thus reduce the ratio of RF pulselength over beam pulse length, resulting in a reduced RFduty cycle. The higher currents also increase the accelera-tion efficiency in the NC section by increasing the ratio ofbeam power over power dissipated in the copper structures.On the other hand more RF power at 704 MHz needs to beinstalled to cover the increased RF peak power.The cryogenic system is based on the design of theTESLA/ILC cryo-modules [13, 14] taking into account theCERN LHC project experience [15]. All cryogenic pipingas well as the SC quadrupoles are contained inside of thecryo-modules to reduce the amount of static losses and tominimise the overall length of the linac. Using long inter-connected modules containing 6 to 8 high-gradient cavitieseach, it becomes possible to cover the energy range from180 MeV to 3.5 GeV in less than 350 m.For neutrino operation the SPL has to be complementedwith circular machines to modify the pulse and bunch timestructure: for the superbeam scenario an accumulator ringhas to be added and for a ν-factory an additional com-pressor ring is needed. First the linac burst is shortenedto a length given by the circumference of the accumulator(≈ µs) and then a compressor ring can be used to reducethe length of the single bunches to the ns range. Due tothe increased bunch current and higher energy of SPL II,the linac pulse length was reduced from 2.2 ms to 0.57 ms.The shorter pulses have the double benefit of decreasingthe number of injection turns into any subsequent circularmachine of a given size (e.g. the CERN PS) and of reduc-ing the size of accumulator and/or compressor rings. Fur-thermore, the injection at higher energy reduces the spacecharge tune shift in the rings and allows for greater flexibil-ity in the bunch structure that can be created with such anaccelerator chain (compare [6]).BEAM DYNAMICSTracking studies have influenced the structural layoutof the SPL from the first conception stage onwards. Themain guidelines were the minimisation of r.m.s. emittance(ε) growth, halo development and losses by: i) keeping allzero-current tunes below 90◦, ii) smoothing the tunes permetre across all transitions, and iii) avoiding ε-exchangeby keeping the longitudinal to transverse full-current tuneratio between 0.5 and 0.8 (adequate for the SPL emittanceratio of εl/εt ≈ 1.4). However, the concept of smoothtransitions cannot be maintained in the chopper line, wherea drastic change in the length of the focusing periods is un-avoidable in order to house the deflecting plates. As a con-sequence the largest “local” ε-growth occurs in the chopperline. It is limited to ≈ 20% by scraping the beam with aconical collimator which also acts as a beam dump for thechopped beam [16].End-to-end simulations have been carried out startingfrom the RFQ input (95 keV) using a matched distributionwith an r.m.s. relative energy spread of 0.5%, a value thatwe expect from the source extraction voltage jitter. Thetransverse ε for the 70 mA input beam is 0.25pimm mradand multi-particle simulations have been performed us-ing uniform and Gaussian distributions. The beam perfor-mance is expected to be bound between these two caseswhich are reported in Table 3.A large number of error runs has been used to establishthe effects of statistical errors (quadrupoles: misalignment,displacement, rotation, gradient; RF fields: phase, ampli-tude) and to impose limitations on the machine tolerances(see [2, 17]). Using a Gaussian distribution the combinedeffect of all errors is expected to yield an additional 40-50%ε-growth in the transverse plane and ≈ 35% in the longi-tudinal plane. For this simulation no losses are observedapart from the beam scraping in the chopper line.Table 3: R.m.s. emittance growth in end-to-end simulationswith uniform and Gaussian input distributionsuniform in [%] Gaussian in[%]∆εx ∆εy ∆εz ∆εx ∆εy ∆εzRFQ 9.3 8.4 – 8.5 10.6 –Chopper 19.5 5.8 14.7 29.7 1.1 9.9DTL 3 11 8.2 -2.4 20.7 11.9CCDTL -0.4 2 0.5 0.7 1.5 0.3SCL 1.5 7.7 1.3 0.8 8.8 0.9SC 0.3 0.4 4 0.3 0.4 4total 36 39.7 26.4 40 49 25UPGRADE OPTIONSIn case of changing requirement for ν-production or incase of additional users the SPL II is designed to offerample scope for upgrades. The klystrons and accelerat-ing structures are designed for a maximum duty cycle of10% while the present design only uses half of this capa-bility. Extending the beam pulse length but keeping a re-alistic safety margin a power upgrade to 8 MW seems fea-sible without major investment and is only limited by theavailability of a suitable H− source. Another upgrade op-tion is an increase in beam energy which can be achieved bysimply adding more SC modules. For each≈ 75 m of addi-tional SC modules the beam energy can be raised by 1 GeV.This upgrade path is somewhat limited by the increaseddifficulty to avoid H− stripping via bending magnets andblackbody radiation in the transfer lines (see [2, 6, 18]).However, a factor of two gain in energy still seems feasi-ble.REFERENCES[1] M. Vretenar (Ed.), Conceptual design of the SPL, CERN-2000-012.[2] F. Gerigk (Ed.), Conceptual design of the SPL II, CERN-2006-006.[3] EURISOL Design Study, http://eurisol.org[4] E. Noah et al., EURISOL target stations operation and im-plications for its proton driver beam, CERN-AB-2006-055,EPAC06, Edinburgh.[5] BENE Steering Group, Beams for European neutrino exper-iments (BEBE), CERN-2006-005.[6] F. Gerigk, R. Garoby, Operational flexibility of the SPL asproton driver for neutrino and other applications, CERN-AB-2006-020, ICFA HB 2006, Tsukuba.[7] R. Garoby et al., Linac4, a new injector for the CERN PSbooster, CERN-AB-2006-027, EPAC06, Edinburgh.[8] J.E. Campagne and A. Caze, The θ13 and δCP sensitivitiesof the SPL-Fre´jus project revisited, LAL-2004-102 (2004).[9] P.Y. Beauvais, Installation of the French high-intensity in-jector at Saclay, LINAC06, Knoxville.[10] M. Vretenar et al., Design and development of RF structuresfor Linac4, LINAC06, Knoxville.[11] L. Lilje, High gradients in multi-cell cavities, 11th Work-shop on RF Superconductivity, Travemu¨nde 2003.[12] P. Pierini, Analysis of gradients for future high-current pro-ton accelerators, ICFA HPSL 2005, Naperville.[13] TESLA Technical Design Report (2001): http://tesla.desy.de/new pages/TDR CD/start.html[14] http://www.interactions.org/linearcollider/gde[15] LHC Design Report Volume 1, CERN-2004-003-V-1.[16] A.M. Lombardi et al., End-to-end beam dynamics forCERN Linac4, ICFA HB 2006, Tsukuba.[17] M. Baylac et al., Statistical simulations of machine errorsfor Linac4, ICFA HB 2006, Tsukuba.[18] W. Chou et al. 8 GeV H− ions: transport and injection, PAC2005, Knoxville.

The SPL (II) at CERN, a Superconducting 3.5 GeV H- Linac

A revision of the physics needs and recent progress in the technology of superconducting (SC) RF cavities have triggered major changes in the design of a SC H- linac at CERN. With 4 - 5 MW beam power, the SPL can be the proton driver for a next generation ISOL-type radio-active beam facility ("EURISOL") and/or supply protons to a neutrino facility (conventional superbeam + beta-beam or neutrino factory). Furthermore the SPL can replace Linac2 and the PS booster, improving significantly the beam performance in terms of brightness, intensity, and reliability for the benefit of all proton users at CERN, including LHC and its luminosity upgrade. Compared with the first conceptual design, the beam energy is almost doubled (3.5 GeV instead of 2.2 GeV) while the length is reduced by 40%. At a repetition rate of 50 Hz, the linac re-uses decommissioned 352.2 MHz RF equipment from LEP in the low-energy part. Beyond 90 MeV the RF frequency is doubled, and from 180 MeV onwards high-gradient SC bulk-niobium cavities accelerate the beam to its final energy of 3.5 GeV. This paper presents the overall design approach, together with the technical progress since the first conceptual design in 2000

The SPL (II) at CERN, a Superconducting 3.5 GeV H- Linac

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