RF Design of the Re-buncher Cavities for the LIPAC ... - CERN

MOPC047Proceedings of IPAC2011, San Sebastián, SpainRF DESIG OF THE RE-BUCHER CAVITIES FOR THELIPACDEUTERO ACCELERATOR*A. Lara # , I. Podadera, F. Toral, CIEMAT, Madrid, Spain.Copyright c○ 2011 by IPAC’11/EPS-AG — cc Creative Commons Attribution 3.0 (CC BY 3.0)AbstractRe-buncher cavities are an essential component ofLIPAC (Linear IFMIF Prototype Accelerator), presentlybeing built at Rokkasho (Japan). The deuteron beamexiting from the RFQ (Radio FrequencyQuadrupole)structure has to be properly adapted to thesuperconducting RF (SRF) linac. Re-bunchers are placedin the Medium Energy Beam Transport (MEBT) line andtheir objective is to longitudinally focus the deuteronbeam. IFMIF re-bunchers must provide a 350 kV E 0 LT at175 MHz continuous wave (CW). The available lengthfor the re-buncher is limited by the generallayout of theMEBT. The high power dissipation derived from the higheffective voltage and the short available length is animportant design challenge. Four different normalconducting cavity designs were investigated: the pillboxtype, double gap coaxial resonators, and multi-gap quarterwave and H resonators. The performance ofthese cavitieswas studied with the numerical codes HFSSS and ANSYS.The fundamental frequency and field pattern of each re-presents thebuncher was investigated in HFSS. This workresults of such analyses.ITRODUCTIOTwo re-bunchers are located in the MEBTline [1]. TheMEBT is part of the Spanish contribution toLIPAC. Thepurpose of the MEBT is to transport the 125 mA/5 MeVdeuteron beam from the RFQ and match it, longitudinallyand transversally, for its injection into the SRF linac. Re-buncher specifications are described in Table1.Table 1: MEBT re-buncher specificationsParameterValueFrequency 175 MHzMaximum E 0 LT 350 kVBeam pipe radius 22 mmCOFIGURATIO CHOICEA number of cavity configurations have been studied.The first design was a pillbox cavity [2]. Due to the highdissipated power, other designs have been analysed, withan increasing number of gaps to decrease RF losses. RFcalculations are the starting point, but also cooling systemand engineering fabrication are arguments used to selector discard a cavity configuration. Table 2 summarizes theresults of this comparison.___________________________________________*Work partially supported by the Spanish Ministry of Science andInnovation under Project ENE2009-11230# alvaro.lara@ciemat.es184Pillbox re-buncherThe pillbox power consumption isrelatively close tothe maximum available power (110kW). Besides, thepower density is extremely high, which makes coolingvery difficult in practice. These resonant structuresconcentrate magnetic field near nosecones (see Fig. 1),that is the point where the cooling is more necessary, butthe available space for the conducts issmall.Figure 1: Pillbox re-buncher magnetic (left) [2] andelectric (right) field maps.The pillbox re-buncher manufacturing could be acomplex process, mainly because of the size and the highpower dissipation.Another pillbox re-buncher feature is its high capacitiveimpedance, concentrated at the bignose cones. Thiscauses high frequency sensitivity to deformations, whichare maximum at the center due to the atmosphericpressure, as the endplates are supported at the outer rim.Due to these disadvantages, and the proximity of the RFlosses value to the maximum available coming from theRF source, other re-buncher designs were evaluated.Double gap coaxial resonatorsBoth half wave (HWR) and quarter wave resonators(QWR) are cavities based on RF coaxial lines that aresuitable for low frequency operation [3]. These structurespresent higher mechanical stabilitythan the pillbox.Furthermore, shunt impedance ishigher and theefficiency increases.The most promising candidate, the quarter waveresonator, has been optimized at increasing inner diameterto analyze the RF power variation with that parameter.Low RF losses would ease the cooling design. In anycase, power density is still very high.In comparison with pillbox design, QWR drasticallydecreases not only the size, cost and manufacturingcomplexity, but also the power dissipation. In addition,magnetic field is negligible at lower endplate, where avacuum port can be easily placed,even without anyconducting grid.07 Accelerator TechnologyT06 Room Temperature RF

Proceedings of IPAC2011, San Sebastián, SpainMOPC047Table 2: Comparison results of the RFre-buncher cavities studied for LIPAC (all in copper).Configuration Pillbox HWR QWR 3-GAP QWR 4-GAP QWR 4-GAP IH5-GAP IH UnitsVacuum length (beam axis) 200 240 240 280 280 280Vacuum height 860 876 507 495 495 504Total power 75 41.7 25.8 12.2 9.4 8.5Central stem power - 30.5 18 4.0 1.7 2.2Side stem power - - - - 1990 1447Q 24279 11347 10773 11413 10148 11570E peak 24.3 20 20.6 21.0 14.8 16.0Power density peak 30.3 42 97.4 17.0 13.1 12.0Shunt impedance 0.82 1.46 2.36 4.99 6.51 7.19The HWR power consumption is higher than QWRone, but the cooling system design would be easierbecause its stem geometry allows a direct cooling channelgoing across the re-buncher. Nevertheless, the HWR stemcomplicates the location of a vacuum port. In addition,HWR height is double than QWR one, and RF losses are50% higher.The QWR design decreases power consumptioncompared with the pillbox and HWR cavities, but someimportant issues should be taken into account: Steering effects: there is an asymmetry in thefield maps due to the drift tube position. Multipactoring inherent to coaxial resonators.In order to relax these issues, it can be concluded thatthe study of coaxial resonators with increasing number ofgaps is a very interesting strategy. One expects that bothpower consumption and power density can be furtherdecreased, while the aforementioned drawbacks do notincrease.Multi-gap resonatorsThree to five gap resonators are studied inthis section:3 and 4-gap ones are depicted in Fig. 2, while the five gapone is shown in Fig. 3. A significant decrease of RFlosses and peak electric field is achieved with eachadditional gap. The available length is too small toallocate a 5-gap cavity, but due to the significant benefits,the MEBT layout has been changed, ncreasing thatlength up to 350 mm. The stem cooling is much easier inthe 5-gap design, which clearly compensates the higherfabrication cost.Figure 2: 3 and 4 gap resonators: 3-gap QWR(left), 4-gapQWR (middle), and 4-gap IH (right).07 Accelerator TechnologyOn the other side, RF performancee (shunt impedance)of IH type is better than QWR one. The deformationunder atmospheric pressure will behigher in the IHmodel, but acceptable with steel endplates.Multipactoring will happen mainly in regular orsymmetric geometries, therefore QWR will be moreprone to problems. Finally, an additional advantage isenvisaged, as due to the lower required power, bothcooling system and RF source requirements are relaxed.In short, it is reasonable to choose the 5-gap IH cavityas the best candidate for LIPAC rebunchers.DETAILED DESIGThe next step is the detailed design of the 5-gap IHcavity. A full model, including the fillet radii necessaryfor the fabrication (milling tools) andthe recesses for thestem bases, has been studied. Two CF100 pumping portsare drilled on the side wall.High order modesFigure 3: 5-gaps IH cavity.320 mm460 mm6.6 kW1.2 kW678 W11746 --13.9 MV/m8.9 W/cm29.29 MΩPerturbations due to high orderr modes in a lowfrequency linac are usually moderate. However, due tothe high beam power, the allowed beam losses are verysmall, and high order modes have been analysed. Table 3T06 Room Temperature RF 185Copyright c○ 2011 by IPAC’11/EPS-AG — cc Creative Commons Attribution 3.0 (CC BY 3.0)

MOPC047Proceedings of IPAC2011, San Sebastián, Spainshows the results. None of the modes is dangerous for thebeam.Table 3: High order modesOrderFrequency(MHz)Q Type earestbunchfrequency1 175.16 11284 2 238.12 9536 1753 285.08 10229 350Copyright c○ 2011 by IPAC’11/EPS-AG — cc Creative Commons Attribution 3.0 (CC BY 3.0)4 329.04 13699 3505 486.62 33422 0 5256 602.88 33089 TE 11 -like 5257 698.23 32825 +stems 7008 720.39 21412 +stems 7009 777.43 16972 +stems 700CouplerThe coupler will be a loop, that is, of magnetic type.The design is based on the Spiral-2 rebuncher coupler [4].A transition will adapt the size of the coaxial line comingfrom the RF source to the input port of the coupler. Aceramic window will keep the vacuum tightness. Beyondthe ceramic, the coaxial line diameter is decreasedsmoothly while keeping 50-ohm characteristicimpedance. The inner conductor will be internally watercooled. The magnetic coupler has been placed closed tothe stems, where the magnetic field is stronger for a goodcoupling, while the field perturbation is far away from thebeam axis. The impedance matching is properly achieved:S 11 is -29.8 dB.Figure 4: 5-gaps IH coupler: Spiral-2 model from [4].TunerOnly a magnetic-type tuner is considered. A capacitiveplunger is not a convenient solution, because it should beplaced at the medium plane: it would be very long,dissipate a significant power and interfere with thepumping ports. There are two plungers, one is manuallydriven, to compensate frequency mismatch due tofabrication errors (cold tuning), and the other one ispowered by a stepping motor controlled from the LLRFsystem. The motorized tuner shall compensate frequencyvariation in the order of few per mil of the nominalfrequency.186Figure 5: Magnetic field distribution on the tuner.Symmetric inductive plungers have been modeled ascylinders (see Fig. 5). The tuner size depends on themagnetic field at the plunger position. Therefore, it is agood practice to place them near the stems, but not toclose, to avoid excessive heating. The overall RF losses ofthe cavity increase about 3% when both plungers areinserted at maximum depth.Manufacturing considerationsThe simplest manufacturing approach is to use stainlesssteel with copper plating for the wall and endplates. Thestems and the drift tubes will be machined from bulk OFEcopper. The stems will be cooled with concentric circuits:cold water inside and warm water outside. The drift tubeswill have not internal cooling conduits. The endplates willbe cooled only in front of the stems.COCLUSIOThis paper has shown the comparison between differentcavity configurations to be used as LIPAC re-buncher.The five gap IH resonator structure is the most efficientstructure. Tuner and coupler will be of magnetic type.ACKOWLEDGEMETSSpecial thanks for fruitful discussions and advices to M.di Giacomo (GANIL), M. Vretenar (CERN), A. Facco(INFN), A. Mosnier (CEA) and A. Schempp (FrankfurtUniversity).REFERECES[1] I. Podadera et al., “The Medium Energy BeamTransport Line (MEBT) of LIPAC”, these proceedings.[2] M. Desmons and P-Y. Beauvais, “Preliminary studiesof the IFMIF/EVEDA matching section buncher”,CEA/Saclay Internal Report, 2008[3] A. Facco, “Low and Intermediate-β Cavity Design”,SRF09 - Dresden, 17/9/ 2009.[4] M. di Giacomo et al. “Design of the MEBTRebunchers for the SPIRAL2 Driver”, Proceedings ofLINAC08, Victoria, BC, Canada.07 Accelerator TechnologyT06 Room Temperature RF