Amberg, M.

Aulenbacher, K.

Barth, Winfried

Basten, M.

Busch, M.

Dziuba, F.

Mickat, Sascha

Mäder, D.

Podlech, H.

GSI Repository

DEVELOPMENT OF A 217-MHz SUPERCONDUCTINGCH-STRUCTURE∗M. Basten†, M. Busch, F. Dziuba, D. Mäder, H. Podlech,IAP Frankfurt University, Frankfurt am Main, GermanyM. Amberg, K. Aulenbacher, Helmholtz-Institut Mainz (HIM), Mainz, GermanyW. Barth, S. Mickat, GSI-Helmholtzzentrum, Darmstadt, GermanyAbstractTo compete in the production of Super Heavy Elements(SHE) in the future a 7.3 AMeV superconducting (sc)continuous wave (cw) LINAC is planned at GSI. Thebaseline design consists of 9 sc Crossbar-H-mode (CH)cavities operated at 217 MHz. Currently an advanced cwdemonstrator is under design at the Institute for AppliedPhysics (IAP) at Frankfurt University. The purpose of theadvanced demonstrator is to investigate a new conceptfor the superconducting CH structures. It is based onshorter CH-cavities with 8 equidistant gaps without girdersand with stiffening brackets at the front and end cap toreduce pressure sensitivity. One major goal of the advanceddemonstrator is to show that the new design leads to higheracceleration gradients and smaller Ep /Ea values. In thiscontribution first simulation results and technical layoutswill be presented.INTRODUCTIONAt the moment the demonstrator for the sc cw-LINACat GSI is under construction and its successful beamoperation will be the first milestone realizing the newsc cw-LINAC at GSI [1]. The advanced demonstratorwhich is presented here will be the second milestone forthe sc LINAC at GSI. The sc LINAC will consist of thedemonstrator as first cavity and 5 additional cryomoduleswith 2 CH-cavities per cryomodule [2, 3]. The design ofthe advanced demonstrator will be used for all 10 cavitiesin the sc cw-LINAC after the demonstrator. The cavityis designed and optimized for high power applications,consists of 8 accelerating cells and has a design gradientof 5 MV/m. Its frequency is the second harmonic of theHigh Charge Injector (HLI) at GSI, Darmstadt. Table 1shows the main parameters of the sc 217 MHz CH-cavityand Figure 1 shows the Layout of the sc 217MHz CH-cavity.DRIFT TUBE DESIGNThe superconducting cavity for the advanced demonstra-tor has to reach high electric fields and high acceleratinggradients. Without drift tubes the electric field on the beamaxis is dropping rapidly in the first and last gap (Figure 2).To increase the field in the first gaps several simulations with∗ Work supported by HIM, GSI† Basten@iap.uni-frankfurt.deFigure 1: Layout of the sc 217 MHz CH-cavity.CST-Microwave-Studio have been performed to determinethe optimum drift tube length for the different gaps [4].Figure 2 shows the electric field on the beam axis withoutdrift tubes and with optimized drift tubes. The electric fieldin the first and last two gaps could be increased significantlywich results in higher accelerating gradients and betterEp /Ea values.The optimized drift tubes in gap 1 and 8 are 7 mm and ingap 2 and 7 2.5 mm long as shown in Figure 3BUFFERED CHEMICAL POLISHINGOne of the most important conditions for sc cavities is agood surface preparation without contamination wich canbe achieved by the so called buffered chemical polishing(BCP). Two BCP treatments of at least 100 µm are plannedfor the sc CH-cavity after fabrication. Each processingstep will change the frequency of the cavity so several CST-Proceedings of LINAC2014, Geneva, Switzerland TUPP06002 Proton and Ion Accelerators and Applications2E Superconducting StructuresISBN 978-3-95450-142-7563 Copyright©2014CC-BY-3.0andbytherespectiveauthorsTable 1: Specifications of the 217 MHz CH-cavityParameter Unitβ 0.069Frequency MHz 215.5Accelerating cells 8Length (βλ-definition) mm 381.6Cavity diameter mm 412Cell length mm 47.7Aperture diameter mm 30Static tuner 3Dynamic bellow tuner 2Wall thickness mm 3-4Accelerating gradient MV/m 5Ep /Ea 5.2Bp /Ea mT/(MV/m) 8.5G Ω 51Ra /Q0 1045Figure 2: Field on the beam axis with and without drifttubes.Figure 3: Optimized drift tubes for the sc CH-cavity.simulations have been performed to estimate the frequencychange of the cavity per 100 µm erosion through BCP.Experiences so far are showing an inhomogeneous effectthrough BCP treatment on the cavity. For example the in-terior of the cavity like stems and tuners are often moreaffected by the BCP than the cavity itself. To estimate theparts of the cavity which have been affected the most dueto BCP treatment several CST-simulations have been per-formed were the BCP effect at the stems was increased by afactor of 1.5 or 2 in respect to the cavity. Additional simula-tions have been performed where only the stems or the cavitywere treated with BCP to estimate the frequency influenceof each part of the cavity. The results of these simulationsare shown in Figure 4.Figure 4: Frequency change through BCP erosion with vari-ous strength on various parts of the cavity.The results of these investigations show that 100 µmerosion at the stems increase the frequency by ca. 470kHz whereas 100 µm erosion at the cavity decrease thefrequency by ca. 110 kHz.BELLOW TUNER DEVELOPMENTPrevious CH-cavities have so called girders in which thestems are welded. This leads to high fabrication costs andextended fabrication duration. Additionally the girders re-duce the mechanical stability of the cavity caused by a breakof the cylindrical symmetry. The desgin of the advanceddemonstrator without girders is cylindrically symmetricalwhich leads to significantly higher stability. Additionallythe stiffening brackets at both ends of the cavity increasethe mechanical stability of the cavity so that the pressuresensitivity is under 5 Hz/mbar.Usually the dynamic tuning range for sc CH-cavities iscovered by bellow tuners which are welded into the girders.For the design of the advanced demonstrator without girdersa new type of bellow tuner has to be developed. The bellowtuner has to cover the whole area from the cavity wall to theregion with high electric fields where the small tuner strokeof at most 1 mm can achieve enough frequency change forstable operation.Several CST-simulations have been performed where pa-rameters like the number of cells, cell length and gap distancehave been optimized concerning Von-Mises-Stress and forceneeded for a tuner stroke of 1 mm. To investigate the attain-able frequency change per mm several types of bellow tunersTUPP060 Proceedings of LINAC2014, Geneva, SwitzerlandISBN 978-3-95450-142-7564Copyright©2014CC-BY-3.0andbytherespectiveauthors02 Proton and Ion Accelerators and Applications2E Superconducting Structureswith different diameters were simulated. The attainable fre-quency change from the different bellow tuners can be seenin Figure 5 whereas the different types of bellow tuners areshown in Figure 6.Figure 5: Frequency change through different bellow tuners.Figure 6: Layout of the different bellow tuners.After these simulations concerning the attainablefrequency change by the different bellow tuners severalparticle tracking simulations concerning multipacting havebeen performed with CST to investigate where multipactingmight appear. It was found, that multipacting betweenthe bellow tuner and the stem occured on several fieldlevels with a tuner diameter of 40 mm or more. So thediameter was reduced to 35 mm to decrease the risk ofmultipacting between the stem and the tuner. To decreasethe risk of multipacting in the cells of the bellow tuneritself the gap distance of each cell was set to 2 mm. Bythat the voltage per cell was increased to reach the uppervoltage limit for multipacting at lower field levels than inthe desired operation mode. So the only parameters toinfluence the needed force for a tuner stroke of 1 mm andthe Von-Mises-Stress was the number of cells and the celllength. The simulation results shown in Figure 7 resultin 5 cells with a cell length of 8 mm with a maximumVon-Mises-Stress of ca. 0.25 GPa and a force of ca. 480 N.Figure 7: Von-Mises-Stress and force needed for a tunerstroke of 1 mm.CONCLUSIONThe short sc CH-cavities for the sc cw-LINAC are mini-mizing the technical risk for the operation at GSI and canalso minimize the costs and construction effort. It was shownthat the installation of drift tubes in the first and last twogaps increases the electric field on the beam axis signifi-cantly. The simulations concerning the influence of BCPon the different parts of the cavity are providing the possi-bility to estimate the frequency change per 100 µm erosion.The dynamic tuner concept without girders as a base forthe tuner was investigated and optimized to operate at lowrisk and high frequency stability. The basic design of thepresented short CH-cavity will also be used for the 17 MeVMYRRHA injector of the MAX-project with an adjustedgeometry operating at a frequency of 176.1 MHz [5].ACKNOWLEDGEMENTThis work has been supported by Helmholtz-InstitutMainz (HIM) and Gesellschaft für Schwerionenforschung(GSI).REFERENCES[1] F. Dziuba, M. Amberg, K. Aulenbacher,W. Barth, M. Busch, H.Podlech, U. Ratzinger, S. Mickat, Superconducting CH Cavi-ties for Heavy Ion Acceleration, in Proc. of IPAC’13, Shanghai,China, p. 3794-3796 (2013)[2] W. Barth, K. Aulenbacher, F. Dziuba, M. Amberg, V.Gettmann, S. Mickat, H. Podlech, U. Ratzinger, Further R&Dfor a New Superconducting cw Heavy Ion Linac@GSI, in Proc.of IPAC’14, Dresden, Germany, p. 3211-3213 (2014)[3] H. Podlech, Proposal for a Superconducting 217 MHz CH-Cavity, institute report: IAP-ACCC-01122013 (2013)[4] CST Microwave Studio, http://www.cst.com.[5] D. Mäder, M. Basten, D. Koser, H. C. Lenz, N. F. Petry,H. Podlech, A. Schempp, M. Schwarz, M. Vossberg, R&Dof the 17 MeV MYRRHA Injector, MOPP064, in Proc. ofLINAC’14, Geneva, Switzerland, (2014)Proceedings of LINAC2014, Geneva, Switzerland TUPP06002 Proton and Ion Accelerators and Applications2E Superconducting StructuresISBN 978-3-95450-142-7565 Copyright©2014CC-BY-3.0andbytherespectiveauthors

Development of a 217-MHz Superconducting CH-Structure

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Development of a 217-MHz Superconducting CH-Structure

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