Crab crossing schemes have been proposed to re-instate luminosity degradation due to crossing angles at the interaction points in next generation colliders to avoid the use of sharp bending magnets and their resulting large synchrotron radiation generation, highly undesirable in the detector region. The rf dipole has been considered for a different set of applications in several machines, both rings and linear colliders. We present in this paper a study of the effects on the multipacting levels and location depending on geometrical variations on the design for a crabbing/deflecting application in a high current (3/0.5 A), high repetition (750 MHz) electron/proton collider, as a matter to provide a comparison point for similar applications of rf dipoles

Castilla, A.

Delayen, J. R.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsPhysics Faculty Publications Physics2014Multipacting optimization of a 750 MHz rf dipoleA. CastillaOld Dominion UniversityJ. R. DelayenOld Dominion UniversityFollow this and additional works at: https://digitalcommons.odu.edu/physics_fac_pubsPart of the Mechanical Engineering Commons, and the Plasma and Beam Physics CommonsThis Conference Paper is brought to you for free and open access by the Physics at ODU Digital Commons. It has been accepted for inclusion inPhysics Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contactdigitalcommons@odu.edu.Repository CitationCastilla, A. and Delayen, J. R., "Multipacting optimization of a 750 MHz rf dipole" (2014). Physics Faculty Publications. 317.https://digitalcommons.odu.edu/physics_fac_pubs/317Original Publication CitationDelayen, J. R., & Castillo, A. (2014). Multipacting optimization of a 750 MHz rf dipole. Paper presented at the 27th Linear AcceleratorConference, Geneva, Switzerland. http://accelconf.web.cern.ch/AccelConf/LINAC2014/papers/thpp112.pdfMULTIPACTING OPTIMIZATION OF A 750 MHz RF DIPOLE∗A. Castilla1,2,3†, J. R. Delayen1,21Center for Accelerator Science, Old Dominion University, Norfolk, VA 23529, USA2Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA3Universidad de Guanajuato (DCI-UG), Departamento de Fisica, Leon, Gto. 37150, MexicoAbstractCrab crossing schemes have been proposed to re-instateluminosity degradation due to crossing angles at the interac-tion points in next generation colliders to avoid the use ofsharp bending magnets and their resulting large synchrotronradiation generation, highly undessirable in the detector re-gion. The rf dipole has been considered for a different set ofapplications in several machines, both rings and linear col-liders. We present in this paper a study of the effects on themultipacting levels and location depending on geometricalvariations on the design for a crabbing/deflecting applicationin a high current (3/0.5 A), high repetition (750 MHz) elec-tron/proton collider, as a matter to provide a comparissonpoint for similar applications of rf dipoles.INTRODUCTIONMultipacting conditioning and processing is one of theprincipal limitations of the performance of rf structures. Itconsists of the emission of large amounts of secondary elec-trons by the rf cavity walls due to incident primary electrons.This can lead to localized and resonant trajectories sustainedby the primary electrons, reaching impact energies corre-spondent to a secondary emission yield (SEY) greater thanthe unit, causing a cascade effect that can incur severe prob-lems, such as low achievable gradients in the structure, bunchinstabilities, and quenches by thermal breakdown in the caseof superconducting structures [1]. Due to its complexityand random nature, this non-relativistic phenomenon hasbeen for a long time an important case of study that needsto be assessed in order to further improve the operation andperformance of current and future particle accelerators.The SEY changes for different material and is surface con-dition dependent [2], therefore the impact energy bandwith(EI I −EI ) to have multipacting condition (i. e. δ > 1) variesfrom case to case, see Fig. 1. In the case of pure and cleanNb, the range of impact energies for the multipacting condi-tion is EI > 150 eV, and EI I < 2000 eV. The multipactingbarriers are known to be either soft and easily processed andcleaned or hard multipacting barriers that cannot be removedby processing and may need redesigning the cavity geometryin order to properly operate.∗ Authored by Jefferson Science Associates, LLC under U.S. DOE ContractNo. DE-AC05-06OR23177. This research used resources of the NationalEnergy Research Scientific Center, supported by the Office of Scienceunder U.S. DOE contract No. DE-AC02-05CH11231.† acastill@jlab.orgFigure 1: Secondary emission yield as a function of theparticles’ impact energy.750 MHz RF DIPOLE DESIGNThe 750 MHz rf dipole was designed as a compact super-conducting crab cavity for a luminosity correction schemeat the interaction points (IPs) of Jefferson Lab’s MediumElectron-Ion Collider (MEIC) with crabbing voltages fromVT = 1.3MV to 1.8 MV for the case of electrons. However,multiple applications of such device in linear colliders suchas beam spreader or high polarization corrector are understudy and development [3,4]. The rf dipole operates as a de-flector/crabber in its lowest mode (TE-like) and the electricand magnetic oscillating fields for this mode are shown inFig. 2, and its parameters are enlisted in Table 1.Figure 2: 750 MHz rf dipole longitudinal cross section withthe electric (left), and magnetic (right) fields for the funda-mental mode.METHODOLOGYWe used the extrusion length of the endcaps (see Fig. 3)as a varying parameter to study the optimization of the mul-tipacting conditions for the 750 MHz rf dipole, keeping allthe other geometrical parameters fixed with exception ofthe cavity radius that was used to correct the resonant fre-Proceedings of LINAC2014, Geneva, Switzerland THPP11203 Technology3A Superconducting RFISBN 978-3-95450-142-71111 Copyright©2014CC-BY-3.0andbytherespectiveauthors€ -0 'u 1.5 >=~ = 0 ·;;: 1.0 "' ·s J:tj ~ 0.5 -0 C: 0 u V ~ - - - -·_,--r---I I I - ..t. - -- -I I I I I I I I I I I I I E1 E.srnax Eu Impact Energy (cV) Table 1: Parameters of 750 MHz Crab Cavity RF DipolePrototypeParameter 750 MHz Unitsλ/2 of pi mode 200.0 mmCavity length 341.2 mmCavity radius 93.7 mmBars width 63.0 mmBars length 200.0 mmBars angle 45 degAperture diameter -d 60.0 mmEndcaps’ extrusion -l 60.0 mmquency of the fundamental mode. This results in the changeof the total cavity length but keeping the parallel bars lengthconstant to the effective length of λ/2 = 0.2 m.Figure 3: Variation of the extrusion length of the endcaps tostudy the multipacting condition and its optimization feasi-bility for the 750 MHz rf dipole.MULTIPACTING ANALYSISOnce a set of models with different extrusion lengths fromlmin = 20 mm to lmax = 50 mm were generated, we usedthe Omega3P and the Track3P codes from the ACE3P Suitedeveloped by SLAC to obtain the eigenmodes and the multi-pacting conditions respectively. The resulting 3D locationsand impact energies (in color code) of the multipacting con-ditions for 9 of the studied models are presented in Fig. 4,where the value of the extrusion length is increasing (fromright to left and top to bottom respectively in Fig. 4). Theparticles’ resonant locations considerably clear out from thetop of the external conductor to populate slightly more theendcaps, where both the electric and magnetic fields arelower, this places them closer to the auxiliar ports, makingit easier for the vacuum systems to help in the processing ofmultipacting.We used the two extremal values of our optimization pa-rameter lmin = 20 mm, and lmax = 50 mm to comparethe multipacting levels obtained by the simulatons as seenin Fig. 5. It can be observed that the multipacting barri-(a)(b)(c)Figure 4: Comparison of the multipacting impact energyand resonant locations for increasing values of the extrusionlength of the endcaps, from 20 mm -right most in (a)- to 50mm -left most in (c)-.ers do not appear to have great differences between eachother, despite the fact that for the model with smaller end-cap’s extrusion (lmin = 20 mm, Fig. 5(a) right most) higherimpact energies are reached, which is not necesarily an im-portant feature since particles with impact energies > 2000eV do not contribute to multipacting in Nb. However, bothmultipacting barriers are well defined at the lower voltagelevels, keeping the operating levels virtually multipactingfree. What is hard to appreciate from the graphs is that thebarriers for the model with smaller extrusion legth are heav-ily denser than those for the model with longer extrusionlength.THPP112 Proceedings of LINAC2014, Geneva, SwitzerlandISBN 978-3-95450-142-71112Copyright©2014CC-BY-3.0andbytherespectiveauthors03 Technology3A Superconducting RFImpact Energy (eV) 000 1600 (a)(b)Figure 5: Multipacting levels as a function of the transversevoltage for the 2 extremal values of extrusion length of theencaps: 20 mm (a), and 50 mm (b).TESTING THE SIMULATIONSThe 750 MHz rf dipole prototype has been fabricated andtested in several occasions, both in Niowave, Inc. and Jef-ferson Lab’s vertical testing areas (VTA). Two multipactingbarriers at low transverse voltage levels (VT < 1 MV) wereobserved at both 4 K and 2 K cryotests, and easily processedand eliminated at 2 K. A comparison between the Q0 datawith multipacting events taken during the cryotest at Jeffer-son Lab’s [5] and the multipacting levels obtained from thesimulations, showing very good agreement, is presented inFig. 6.CONCLUSIONSIn the present work we studied the effects on the resonantlocations, and incident energies for multipacting conditionon the 750 MHz rf dipole due to variations of the geometri-cal parameter extrusion length of the cavity endcaps (l), fora range between 20 mm to 50 mm. We presented a subset ofthe studied models to illustrate the variations in the locationand density of resonant particles in the cavity structure usingFigure 6: Q0 curves at 4 K (solid black) and 2 K beforeprocessing (solid blue) and after processing (solid green) ofmultipacting compared to the multipacting levels obtainedfrom the TRACK3P-ACE3P simulations. The purple linesenclose the design crabbing kick range needed per cavity.the parameter l as tweaking knob. Even when the multipact-ing barriers appear at roughly the same field levels in allour simulations, the density of the resonant particles andtheir impact energy decrease, and also the resonant positionsrelocate to potentially less problematic areas of the structureas the parameter l increases. We compared the simulationresults with the experimental observation of multipactingevents for the 750MHz rf dipole prototype with considerablygood agreement. We conclude that by modifying the extru-sion length, the multipacting can be tweaked for the case ofthe rf dipole. It is necesary to carry out further studies todetermine if a different parameter can also be used to furtheroptimize the mutipacting in the rf dipole geometry to lowerthe multipacting levels, such as the blending radius of theendcaps, which consistently showed to be one of the mostsusceptible to multipacting locations. However, the experi-mental data has proven that multipacting is easily processedand is not a limiting factor in the rf dipole performance, notjust for the 750 MHz prototype, but for the 400 MHz andthe 499 MHz prototypes as well [6].REFERENCES[1] S.U. De Silva et.al., in Proceedings of PAC 2011, New York,New York, p. 1018 (2011).[2] H. Padamsee and A. Joshi, J. Appl. Phys., p. 1112 50 (1979).[3] D. W. Storey et.al., in Proceedings of PAC 2013, Pasadena,California, p. 1226 (2013).[4] S. U. De Silva et.al., in Proceedings of IPAC’14, Dresden,Germany, p. 2672 (2014).[5] A. Castilla et.al., in Proceedings of IPAC’14, Dresden, Ger-many, p. 2672 (2014).[6] S.U. De Silva and J.R. Delayen,Phys. Rev. ST Accel. Beams16, 012001 (2013).Proceedings of LINAC2014, Geneva, Switzerland THPP11203 Technology3A Superconducting RFISBN 978-3-95450-142-71113 Copyright©2014CC-BY-3.0andbytherespectiveauthors8000 Q 6000 L > ~ t >, E" (l) ~ 4000 u "' a. .§ 2000 3000 cl 2500 ~ 9 ~ 2000 >, g e> ~ 1500 t, :g_ § 1000 500 Multipacting for Low Slope Endcaps 0 1" Order 0 2"" 0rder 0 3rd Order 0 41h Order ---VT [MV] Multipacting for Angled Endcaps 0 1s1 Order 0 2nd Order 0 3'd Order 0 41h Order ........... 2 VT [MV] 1e+10 0 0 1e+9 1e+8 0.0 750 MHz Crab Cryotests with Multipacting Analysis 0.5 1.0 0 Multipacting 1st order 0 Multipacting 2nd order O Multipacting 3rd order O Multipacting 4th order e Oo4K • 0 0 2K 1st run • 0 0 2K2"" run 2.0 2.5 3000 2500 2000 > ~ >, 1500 [ UJ (.) "' 1000 I 500 3.0 

Multipacting optimization of a 750 MHz rf dipole

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Multipacting optimization of a 750 MHz rf dipole

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