To keep the ambitious Super Heavy Element (SHE) physics program at GSI competitive a superconducting (sc) continuous wave (cw) high intensity heavy ion LINAC is currently under progress as a multi-stage R&D program of GSI, HIM and IAP*. The baseline linac design consists of a high performance ion source, a new low energy beam transport line, an (cw) upgraded High Charge State Injector (HLI), and a matching line (1.4 MeV/u) which is followed by the new sc-DTL LINAC for post acceleration up to 7.3 MeV/u. In the present design the new cw-heavy ion LINAC comprises constant-beta sc Crossbar-H-mode (CH) cavities operated at 217 MHz. The advantages of the proposed beam dynamics concept applying a constant beta profile are easy manufacturing with minimized costs as well as a straightforward energy variation**. An important milestone will be the full performance test of the first CH cavity (Demonstrator), in a horizontal cryo module with beam. An advanced demonstrator setup comprising a string of cavities and focussing elements is proposed to build from 10 short CH-cavities with 8 gaps. The corresponding simulations and technical layout of the new cw heavy ion LINAC will be presented

Amberg, Michael

Aulenbacher, Kurt

Barth, Winfried

Basten, Markus

Busch, Marco

Dziuba, Florian

Gettmann, Viktor

Heilmann, Manuel

Mickat, Sascha

Miski-Oglu, Maksym

Mäder, Dominik

Podlech, Holger

Schwarz, Malte

GSI Repository

R&D STATUS OF THE NEW SUPERCONDUCTING CW HEAVY IONLINAC@GSIM. Basten1,∗, M. Amberg1,3, K. Aulenbacher3,4, W. Barth2,3, M. Busch1, F. Dziuba1, V. Gettmann3,M. Heilmann2, D. Mäder1, S. Mickat2,3, M. Miski-Oglu3, H. Podlech1, M. Schwarz11IAP University of Frankfurt, 60438 Frankfurt am Main, Germany2GSI Helmholtzzentrum, 64291 Darmstadt, Germany3Helmholtz-Institut Mainz (HIM), 55099 Mainz, Germany4KPH Mainz University, 55128 Mainz, GermanyAbstractTo keep the ambitious Super Heavy Element (SHE)physics program at GSI competitive a superconducting (sc)continuous wave (cw) high intensity heavy ion LINAC iscurrently under progress as a multi-stage R&D programof GSI, HIM and IAP [2]. The baseline linac design con-sists of a high performance ion source, a new low energybeam transport line, an (cw) upgraded High Charge StateInjector (HLI), and a matching line (1.4 MeV/u) which isfollowed by the new sc-DTL LINAC for post accelerationup to 7.3 MeV/u. In the present design the new cw-heavyion LINAC comprises constant-beta sc Crossbar-H-mode(CH) cavities operated at 217 MHz. The advantages of theproposed beam dynamics concept applying a constant betaprofile are easy manufacturing with minimized costs as wellas a straightforward energy variation [6]. An important mile-stone will be the full performance test of the first CH cavity(Demonstrator), in a horizontal cryo module with beam. Anadvanced Demonstrator setup comprising a string of cavitiesand focussing elements is proposed to build from 10 shortCH-cavities with 8 gaps. The corresponding simulationsand technical layout of the new cw heavy ion LINAC willbe presented.INTRODUCTIONAt the moment the Demonstrator for the sc cw-LINAC atGSI is under construction and its successful beam operationwill be the first milestone realizing the new sc cw-LINACat GSI [1]. The advanced Demonstrator which is presentedhere will be the second milestone for the sc LINAC at GSI.The Demonstrator will be the first cavity for the sc LINACfollowed by 5 additional cryomodules with 2 CH-cavities percryomodule [2,3]. The design of the advanced Demonstratorwill be used for all cavities in the sc cw-LINAC after theDemonstrator. The 8 cell cavity is designed and optimizedfor high power applications and has a design gradient of5 MV/m. Its frequency is the second harmonic of the HighCharge Injector (HLI) at GSI, Darmstadt. Table 1 shows themain parameters and figure 1 shows the Layout of the sc217 MHz CH-cavity.∗ Basten@iap.uni-frankfurt.deFigure 1: Layout of the sc 217 MHz CH-cavity.Table 1: Main Parameters of the 217 MHz CH-cavityParameter Unitβ 0.069Frequency MHz 215Accelerating cells 8Length (βλ-definition) mm 381.6Cavity diameter (inner) mm 392.3Cell 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) <10G Ω 50Ra /Q0 Ω 1070MOPB066 Proceedings of SRF2015, Whistler, BC, CanadaISBN 978-3-95450-178-6258Copyright©2015CC-BY-3.0andbytherespectiveauthorsNew ProposalsB02-Hadron LinacsMULTIPACTINGThe phenomenon of multipacting is a crucial point for scaccelerating structures. To find possible areas where mul-tipacting can occur several simulations with CST particlestudio [4] have been performed. Critical areas where multi-pacting can occur are the tuners. The space between tunersand stems is narrow and can provide resonance conditionsfor multipacting. Also the gaps of the bellow tuner provideareas where multipacting can build up. To investigate if mul-tipacting conditions are fulfilled in these areas the secondaryemission yield was simulated with particle sources in CSTparticle studio [4] for the different parts of the cavity. Thesecondary emission yield is defined as:SEY =current emit ted due to secondary electronscurrent absorbed due to par t icle coll isionsThis ratio indicates if electron multiplication is presentor not. If the secondary emission yield is above 1, thenmore electrons are produced than absorbed which resolves inmultipacting. If it is below 1, no electron avalanche can buildup and no multipacting will occure. Of course multipactingcan occure on different energies stored in the cavity. So thesecondary emission yield has to be simulated for increasingaccelerating gradients. The simulation results [4] for the twodynamic tuners and the cavity walls are shown in figure 2.Figure 2: Simulated secondary emission yield for dynamictuners and cavity walls.It can be seen, that multipacting occurs only on the cavitywalls. Shown also by the particle trajectories in Figure 3.Probably no multipacting will arise in the dynamic tunersitself or between the tuners and the stem, assuming a tunerdiameter of 35 mm. Simulations have been performed alsofor larger diameters. The simulations show multipactingresonances for accelerating gradients below 1 MV/m. Toavoid these critical conditions the tuner diameter is fixed on35 mm for the final design.The secondary emission yield obviously depends on thematerial properties. To compare the influence of the materialcharacteristics the secondary emission yield was evaluatedFigure 3: Particle trajectories generated by CST ParticleStudio [4].for niobium with three different surface preparations. Theworst preparation scheme is a ”wet treatment” of the niobiumsurfaces which are only high pressure rinsed. Additionallysurface prepared cavities as ”300°C bakeout” and ”argondischarged” have been simulated. The different secondaryemission yields as function of the accelerating gradients forthe cavity walls are shown in figure 4.Figure 4: Simulated secondary emission yield for differentcavity preparation schemes.The impact of the surface preparation on the secondaryemission yield and the multipacting can clearly be seen.The expected condition for the final sc 217 MHz CH-cavityshould be at least 300°C bakeout so that multipacting on thecavity walls may only arise while increasing the acceleratinggradient. For the desired maximum field gradient of 5MV/mthe estimated secondary emission yield does not indicaterisks of multipacting..Proceedings of SRF2015, Whistler, BC, Canada MOPB066New ProposalsB02-Hadron LinacsISBN 978-3-95450-178-6259 Copyright©2015CC-BY-3.0andbytherespectiveauthorsTUNING CONCEPTDue to the fact that superconducting cavities have a verysharp resonance curve it is essential that the desired fre-quency can be reached and kept constant during operation.A tuning system with static tuners and dynamic tuners isrequiered for stable operation. The sc 217 MHz CH-cavityhas 3 static and 2 dynamic tuners which have been basicallydesigned and optimized with CST Microwave Studio [4].Due to the limited tuner diameter of 35 mm the tuning rangefor the static as well as for the dynamic tuners was increasedby tuner heads. To guarantee save operation at 217 MHzthe dynamic tuners should cover at least a tuning range of250 kHz. The maximum elongation of the dynamic tunersis 1 mm. A further elongation is hazardous concerning dam-aging the tuners (wall thickness of 1 mm only). To achievea frequency range of 250 kHz per mm different tuner headdiameters (35 mm to 50 mm) have been simulated. Theresults are shown in figure 5.Figure 5: Frequency change per mm for different tuner heads.The important values of the sc 217 MHz CH-cavity arethe Ep /Ea and Bp /Ea ratios. The impact of the differenttunerhead diameters on the Ep /Ea ratio can bee seen infigure 6.The Ep /Ea ratio gets better with increasing tuner headdiameter for a fixed frequency change per mm of 250 kHz.This is due to the fact that the necessary tuner height de-creases with increasing tuner head diameter. The maximumelectric fields between the dynamic tuners and the stemsare decreasing. On the other hand the tuner heads are get-ting closer to the stems with increasing tuner head diameterwhich is raising the maximum magnetic fields. In the finaldesign the tuner head diameter for the dynamic tuner wasset to 45 mm. Thermal shrinkage of the cavity as well asbuffered chemical polishing increase the frequency of the sc217 MHz-cavity. The static tuners should provide 70% to80% of the tuning range to decrease the frequency and 30%to 20% to increase the frequency. The frequency can also beincreased with additional steps of chemical polishing. Thetunerhead for the static tuners was set to 50 mm to guaranteeFigure 6: Ep /Ea ratio dependent on frequency changer permm.a sufficient tuning range of 2,3 MHz. The tuning ranges forthe different tunerhead diameters is depicted in figure 7.Figure 7: Tuning range static tuner.NEW BELLOW TUNER DESIGNThe performed simulations on the risk of multipactinglimited the tuner diameter to 35 mm. To decrease the num-ber of fins needed for a tuner stroke of 1 mm the design ofthe dynamic tuners was revised. For the old tuner design(diameter of 35 mm) at least 4 fins with a gap distance of3 mm where necessary to achieve a tuner stroke of 1 mmwithout raising the Von-Mises-Stress above 0.25 GPa. Al-though the multipacting simulations show no discrete multi-pacting conditions in the planar areas of the dynamic tunerit is desirable to have as less fins as possible. To achievea tuner stroke of 1 mm with less fins the design of the dy-namic bellow tuner was changed so that the diameter of thefins is the same as the tunerhead. This is possible becausethe neighbouring stems are getting smaller from the cavitywalls to the beam axis so that the space between fins andstems is still uncritical for construction and multipacting. Acomparison between the old and the new design is shown infigure 8.MOPB066 Proceedings of SRF2015, Whistler, BC, CanadaISBN 978-3-95450-178-6260Copyright©2015CC-BY-3.0andbytherespectiveauthorsNew ProposalsB02-Hadron LinacsFigure 8: New (left) and old (right) design of the dynamicbellow tuner.The old design needs at least 4 fins to achieve a tunerstroke of 1 mm and the Von-Mises-Stress arises to 0.24 GPa.In the new design only 3 fins are necessary to performe atuner stroke of 1 mm and the maximum Von-Mises-Stress isin the range of 0.19 GPa only. The improved design providesmore safety for operation. It is possible to performe a tunerstroke of more than 1 mm without damaging the tuner. Theyield strenght for niobium in cold conditions is in the rangeof 0.47 GPa. But in general the Von-Mises-Stress shouldnot exceed 0.25 GPa.Due to the fact that the dynamic tuners are made of 1 mmthick niobium they are vulnerable to vibrations. It is neces-sary to investigate the eigenfrequencies of the tuner itself.This was done with the simulation program AnsysWB [5].The tuner height is determined by the tuning range neededfor safe operation, the tuner diameter is determined by themultipacting simulations and the number of fins and the gapdistance between two fins is determined by the necessarytuner stroke. The only parameter to influence the eigenfre-quency is the position of the fins on the tuner. The heightfrom the bottom of the tuner to the first fin was increased andthe change in the first 10 eigenfrequencies was simulated.The results for the first eigenmode of the old design and thefirst eigenmode of the new design is depicted in figure 9.The operation of vacuum pumps or tuner drives attachedto the cavity may cause interfering vibrations. All thesedisturbances are in the range of up to a few hundred Hz. Thenext eigenmodes of the dynamic tuners are above 500 Hz andnot interesting for further investigations. The eigenfrequencyis increasing when the fins are attached near the tunerhead.The new design provides higher eigenfrequencies furtheraway from critical low frequencies (see figure 9).CONCLUSIONThe simulations show that in critical regions of the cav-ity like the dynamic tuners multipacting should not occur.Multipacting should only arise on the cavity walls whiletheaccelerating gradient is increasing. The tuning concept withFigure 9: Eigenfrequencies for the rst two eigenmodes.3 static and 2 dynamic tuners assures to reach the designfrequency and keep it stable. The tunerheads have been op-timised concerning the tuning range and parameters like theEp /Ea ratio. The design of the dynamic tuners has beenrevised to decrease the number of fins and the maximumVon-Mises-Stress. The new design provides higher eigen-mode frequencies and three fins only for a tuner stroke of 1mm or even more. The basic design of the presented shortCH-cavity could also be used for the 17 MeV MYRRHAinjector of the MAX-project, adjusted for a frequency of176.1 MHz [7].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] ANSYS is a registered trademark of SAS IP Inc.,www.ansys.com[6] M. Schwarz et al., Beam Dynamics for the sc cw Heavy IonLinac at GSI, in Proc. of IPAC’15, Richmond, VA, USA (2015).[7] 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 SRF2015, Whistler, BC, Canada MOPB066New ProposalsB02-Hadron LinacsISBN 978-3-95450-178-6261 Copyright©2015CC-BY-3.0andbytherespectiveauthors

R&D Status of the New Superconducting CW Heavy Ion LINAC@GSI

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R&D Status of the New Superconducting CW Heavy Ion LINAC@GSI

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