This paper describes the basic parameters and the machine layout of the Low-Power SPL (LP-SPL), a 4GeV superconducting H− linac. In the first stage this machine is only designed to replace the PS booster (PSB) and to inject into a new proton synchrotron (PS2) at CERN. At the same time the components are designed such that the machine can be upgraded to become a high-power proton driver (5GeV, > 4MW) for future radioactive ion beam (RIB) facilities or for various neutrino production schemes at CERN. The consequences for the low-power design are explained together with the possible upgrade paths

Gerigk, F

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THE LOW-POWER SPLF. Gerigk for the SPL study group, AB/RF, CERN, Geneva, SwitzerlandAbstractThis paper describes the basic parameters and the ma-chine layout of the Low-Power SPL (LP-SPL), a 4 GeVsuperconducting H− linac. In the first stage this machineis only designed to replace the PS booster (PSB) and toinject into a new proton synchrotron (PS2) at CERN. Atthe same time the components are designed such that themachine can be upgraded to become a high-power protondriver (5 GeV, > 4MW) for future radioactive ion beam(RIB) facilities or for various neutrino production schemesat CERN. The consequences for the low-power design areexplained together with the possible upgrade paths.INTRODUCTIONThe aim of the LP-SPL [1] (together with PS2) is i) toremove any reliability concerns of the ageing LHC injec-tor chain, specifically to replace Linac2, PSB, PS and ii)to provide a proton beam that is suitable for all foreseenLHC upgrade scenarios (compare [2]). After a recent costand performance comparison [3] between an RCS (RapidCycling Synchrotron) based PS2 injector and the LP-SPL,the CERN management endorsed the SPL despite the 30%higher cost of the installation. Since then the LP-SPL to-gether with the PS2 are considered as the baseline scenariofor an upgrade of the CERN proton injector chain.PS2 requires the injection of 1.5 × 1014 particles perpulse with a 1 Hz repetition rate at an energy of 4 GeV.This beam will be provided by the LP-SPL as a 1.2 ms longpulse with an average pulse current of 20 mA. In case of theHigh-Power SPL (HP-SPL) the average pulse current in thelinac will be doubled to 40 mA, reducing the PS2 injectionperiod to 0.6 ms. At the same time the repetition rate willbe increased to 50 Hz.THE SPL PROJECT AT CERNThe HP-SPL has to deliver beams with different timestructures to a variety of users. This necessary flexibilityis achieved by augmenting the linac with a combinationof an accumulator and compressor ring. These first com-press the pulse length of the linac pulse from the ms to theµs range and then rotate the bunches to lengths in the nsrange. Since the requirements for the time structure of afuture neutrino driver are still evolving, we foresee the fol-lowing construction stages (taken from [5]), which can bematched to a growing number of applications and to evolv-ing time structure requirements for future users:1. Construction of Linac4 [6]: the 160 MeV normalconducting front-end of the SPL. This machine is ap-proved and is expected to be operational in 2013. Itwill replace the present Linac2 (50 MeV) and will in-ject at 160 MeV into the PSB. It is also the first steptowards reaching the full luminosity potential of theLHC. The location of Linac4 on the CERN site is suchthat a straight prolongation of Linac4 is tangential tothe SPS, with enough space between the two machinesto construct the SPL and a new proton synchrotron(PS2), which will replace the aging PS machine. Thislayout allows to use the Linac4 beam for the commis-sioning of SPL and PS2, while maintaining the oper-ation of the present LHC proton injector chain (PSB- PS - SPS). This approach minimises any interrup-tion to LHC operation until the new injector chain isfully operational. Figure 1 shows a block diagram ofLinac4/SPL and Fig. 2 shows the layout of the newinjector chain on the CERN site.2. Low-power SPL (LP-SPL): installation of a 4 GeVsuperconducting linac, producing 200 kW of beampower with a repetition rate of 2 Hz. Two families ofsuperconducting cavities (β = 0.65 and β = 1.0) areused to accelerate the beam to its top energy. This ma-chine will replace the PSB and will inject directly intoPS2. The R&D for this machine is partly covered bythe recently approved ”white paper”, an initiative byCERN director general, which also provides the fund-ing for Linac4.3. High-power SPL (HP-SPL): extension of the LP-SPL to 5 GeV and increase of the repetition rate to50 Hz, producing 4 MW (or more) of proton beampower. At this stage the beam can be used for the pro-duction of neutrinos via beta-beams [7] and to drive apulsed RIB facility [8].4. Accumulator ring: in this configuration the SPL candrive a beta-beam facility and produce a so-called Su-perbeam at the same time, which is considered to be apromising combination for neutrino physics [9]. Thecapability to drive a RIB facility and the LHC injectorchain remains unchanged.5. Compressor ring: this enables the SPL to producebunches in the nanosecond range, which are nowa-days recommended for a neutrino factory target. Incase further target studies prove the need for higherproton energies it seems realistic to extend the SPL toBEAM’07 PROCEEDINGS1433 MeV 50 MeV 102 MeV352.2 MHz643 MeV 4/5 GeV704.4 MHz180 MeVFigure 1: Block diagram of Linac4 and SPL.Figure 2: Layout of the SPL on the CERN site.the 10 GeV range. A similar approach is proposed atFNAL in the 8 GeV proton driver project [10]. Ener-gies beyond 10 GeV do not seem practical, since H−stripping due to magnetic fields and black-body radi-ation becomes a serious problem [11, 12, 13].It should be noted that the optimum energy and timestructure for neutrino factory targets has not yet been ex-perimentally determined. In this context the flexibility ofthe linac-based solution may ease the task of adapting theproton driver time structure to an evolving set of input pa-rameters for a neutrino factory. The main parameters ofLinac4, LP-SPL, and HP-SPL are summarised in Table 1.Table 1: Parameter list for the machine evolution from LP-SPL to HP-SPL.LP-SPL HP-SPLEnergy [GeV] 4 5Beam power [MW] 0.192 > 4Repetition rate [Hz] 2 50Av. pulse current [mA] 20 40Chopping ratio [%] 62 62Beam pulse [ms] 1.2 0.4 - 0.6Protons p. pulse [1014] 1.5 1.0-1.5Filling time PS2 [ms] 1.2 0.6Beam duty cycle [%] 0.24 2.0No. SC cavities 194 234No. klystrons (352 + 704 MHz) 19+28† 19+57RF peak power [MW] 100 219Av. power consumption [MW] 4.5 38.5Length [m] 459 534SUPERCONDUCTING LINAC DESIGNThe superconducting section of the SPL is based on onlytwo families of cavities with a geometrical beta of 0.65 and1.0, respectively (see Table 2). The accelerating gradientsof 19 and 25 MV/m have been chosen after considering thepeak surface fields already achieved in tests with variouscavities, which are summarised in Fig. 3. The chosen gradi-ents correspond to a peak surface field of 50 MV/m, whichis challenging but which seems realistic assuming that onecan invest several years of R&D in the engineering of cav-ities and cryo-modules. Another conclusion to be drawnfrom Fig. 3 is that the maximum gradient does not seem tohave a strong frequency dependance. Figure 4 shows a sim-ilar graph for the maximum surface magnetic field, whichwas limited to 100 mT in the case of the SPL.0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1010203040506070=1 @ EACC=28 MV/mFE onset @ 22 MV/mCEA/CNRSSNS prototypes (upper values)production (lower values)  Surface electric field [MV/m] Maximum gradient reached Field emission onset5 cavitiesTRASCO/RIAFigure 3: Peak surface electric fields corresponding to themaximum gradients reached in cavity tests (down triangle)and to the onset field emission (up triangle). From leftto right: TRASCO/RIA (704/805 MHz), SNS (805 MHz),CEA/CNRS (704 MHz), SNS, TTF (1.3 GHz).The cryo-module design will be based on the ILC/XFELapproach, using long interconnected cryo-modules withsuperconducting quadrupoles to minimise the number ofcold-warm transitions. An appropriate R&D effort to re-design the ILC/XFEL cryo-modules for the SPL is planneduntil 2012, when a decision regarding the construction ofthe LP-SPL is expected. Until this date the SPL studygroup is preparing a technical design report, which will bethe basis for a decision of the CERN management on con-struction.BEAM’07 PROCEEDINGS1440.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10102030405060708090100110120130140CEA/CNRS5 c avities TRASCO/RIA=1 @ 28 MV/mSNS prototypes (upper values)production (lower values)  Surface magnetic field [mT]Figure 4: Peak surface magnetic fields corresponding tothe maximum gradients reached in cavity tests. From leftto right: TRASCO/RIA (704/805 MHz), SNS (805 MHz),CEA/CNRS (704 MHz), SNS, TTF (1.3 GHz).Table 2: Characteristics of the two families of supercon-ducting cavities.section β = 0.65 β = 1.0accelerating gradient [MV/m] 19 25el. peak surface field [MV/m] 50mag. peak surface field [mT] 100R/Q [Ω] 290 570quality factor at 2 K ≥ 1010cells per cavity 5cavities per module 6 8focusing periods per module 2 1module length [m] 11.45 14.26focusing period length [m] 6.13 15.06aperture radius [mm] 42.5 45CIVIL ENGINEERINGWhile Linac4 is installed horizontally, the SPL tunnelswill have a slope of ≈ 1.6%, to avoid crossings with exist-ing tunnels and to ensure sufficient distance to tunnel areaswhich need to be accessed during operation. Furthermorethe slope is necessary to reach the depth for the injectioninto the PS2, which is approximately 50 m under ground.The civil engineering effort for the LP-SPL is made in viewof the full power operation of the machine. In particularthis means:i) keeping a distance of ≥ 8m of earth between the SPLaccelerating tunnel and areas that must be accessibleduring the operation of the machine. This distanceensures a radiation dose suitable for public access forinstance for overground parking areas or basements ofbuildings.ii) providing enough tunnel space for the HP-SPL infras-tructure, comprising klystron, klystron modulators,cooling water pipes, air conditioning, etc.Based on the above points a first layout of the tunnelcross sections is shown in Fig. 5. The klystron gallery andthe accelerating tunnel have diameters of 6 m and 4.5 m, re-spectively. The distance between the tunnels is 9 m to en-sure safe access to the klystron gallery during the SPL oper-ation. The size of the klystrons is based on estimates usingexisting devices with similar specifications. The dimen-sions of klystron modulators was estimated by the CERNPower Group, which is now developing the modulators forLinac4, and the requirements of the services have beentaken from [1].Figure 5: Cross-section of the accelerator tunnel (right) andklystron gallery (left).As one can see from Fig. 6 most of the klystron tun-nel volume is taken by the klystron modulators, despite thefact that approximately half of the modulator equipment isalready housed on the surface. In the accelerating tunnelconsiderable space is taken by the RF distribution network,which splits the power from the klystrons to the single cav-ities.Figure 6: 3D layout of accelerator tunnel (right) andklystron gallery (left).RCS VERSUS LINACFollowing a request of the CERN Scientific ProgramCommittee (SPC) a comparison was made between aBEAM’07 PROCEEDINGS145synchrotron-based and linac-based proton driver for PS2(see also [5]). The goal was to outline the pros and consof each solution and to compare with the findings of theFNAL proton driver study II [10] which compared a 8 GeVfull-energy superconducting linac with a 600 MeV linacplus an 8 GeV synchrotron. The FNAL aim was to achieveinitially 0.5 MW beam power and to have the possibility toupgrade the power to 2 MW. It was found that the linac-based solution is approximately 30% more expensive thanthe synchrotron-based solution. Nevertheless the linac so-lution was preferred due to: i) its upgrade potential andits adaptability to future proton needs at FNAL, and ii) thepossibility to construct a test bench for the InternationalLinear Collider (ILC), making use of TESLA-style SC cav-ities and cryo-modules.The CERN study compared the low-power SPL (4 GeV,0.2 MW) with a rapid cycling synchrotron [3]. Both ma-chines have to provide 1.5 · 1014 particles per pulse with a1 Hz repetition rate for injection into PS2 with 4 GeV in-jection energy. In accordance with the FNAL study it wasfound that the linac solution demands a 28% higher initialinvestment. However, due to its upgrade potential and itsexpected performance advantages, the linac solution wasendorsed by the management and represents now the baseline for the planned upgrade of the CERN proton injectorchain. The relative merits of each solution are summarisedin Table 3.Table 3: Relative merits of RCS and SPL options for theinjection into the proposed CERN PS2, see [3].SPL RCS AdvantageFilling time PS2 0.6 ms 1.3 s SPLTime struct. LHC inherent different SPLRel. proton rate 2.5 1 SPLFixed target phys. ideal acceptable SPLIons acceptable ideal RCSUpgrade potential high low SPLRelative cost† 1.28 1 RCS† the relative cost considers only the items that differ be-tween both solutionsSUMMARYThe normal conducting part of the SPL (Linac4) is ap-proved and is planned to deliver protons in 2013. The LP-SPL together with PS2 is now the baseline scenario for up-grading the LHC proton injector chain. A new layout forwhole proton injector complex has been found which al-lows the staged construction of Linac4 and SPL with min-imum interruption of LHC operation. The first stage of theSPL, the LP-SPL is designed such that it can be upgradedto a multi MW proton linac for a reasonable cost. A civilengineering feasibility study including a cost estimate forthe construction of the SPL is in preparation.ACKNOWLEDGEMENTSThis paper results from the work of the SPL study group,the TS Civil Engineering group, the Safety Commission,and our collaborators in CEA Saclay (France), INFN Mi-lano (Italy), and LPSC Grenoble (France). We also whishto thank Thales, CPI, Toshiba, AFT, and ACCEL for pro-viding valuable data for the estimation of component per-formance and characteristics.REFERENCES[1] F. Gerigk (Ed.), Conceptual design of the SPL II, a high-power superconducting H− linac at CERN, CERN-2006-006.[2] R. Garoby, LHC injector upgrade plan, this workshop.[3] R. Garoby, M. Benedikt, A. Fabich, and F. Gerigk, et al.,Comparison of options for the injector of PS2, CERN-AB-2007-014-PAF.[4] F. Gerigk and R. Garoby, Operational flexibility of the SPLas proton driver for neutrino and other applications, HB’06,Tsukuba, Japan.[5] F. Gerigk, Future high-intensity proton accelerators,SRF’07, Beijing, China.[6] R. Gerigk and M. Vretenar (Eds.), Linac4 technical designreport, CERN-AB-2006-084.[7] P. Zucchelli, A novel concept for a ν¯e/νe neutrino factory,Phys. Lett B 532 (2002) 166.[8] E. Noah et al., EURISOL target stations operation and im-plications for its proton linac driver beam, EPAC’06, Edin-burgh, UK.[9] J.E. Campagne and A. Caze, The θ13 and δCP sensitivitiesof the SPL-Fre´jus project revisited, CERN-NUFACT-NOTE142 (2004).[10] G.W. Foster, W. Chou, and E. Malamud (Eds.), Proton driverstudy II, FERMILAB-TM-2169 (2002).[11] L.R. Scherk, An improved value for the electron affinity ofthe negative hydrogen ion, Can. J. Phys. 57 (1979) 558.[12] W. Chou et al., 8 GeV H− ions: transport and injection,PAC’05, Knoxville, USA.[13] H.C. Bryant and G.H. Herling, Atomic physics with a rela-tivistic H− beam, J. Modern Optics 53 (2006) 45.BEAM’07 PROCEEDINGS146

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