The upgrade of the ISOLDE machine at CERN foresees a superconducting linac
based on two gap independently phased Nb sputtered Quarter Wave Resonators
(QWRs) working at 101.28MHz and producing an accelerating field of 6MV/m on
axis. A careful study of the fields in the cavity has been carried out in order
to pin down the crucial e-m parameters of the structure such as peak fields,
quality factor and e-m power dissipated on the cavity wall. A tuning system
with about 200kHz frequency range has been developed in order to cope with
fabrication tolerances. In this paper we will report on the cavity simulations.
The tuning plate design will be described. Finally the frequency measurements
on a cavity prototype at room temperature will be presented.Comment: 5 pages, SRF09 Conference in Berli

D'Elia, A.

Jones, R. M.

Pasini, M.

English

arXiv

The upgrade of the ISOLDE machine at CERN foresees a superconducting linac based on two gap independently phased Nb sputtered Quarter Wave Resonators (QWRs) working at 101.28MHz and producing an accelerating field of 6MV/m on axis. A careful study of the fields in the cavity has been carried out in order to pin down the crucial e-m parameters of the structure such as peak fields, quality factor and e-m power dissipated on the cavity wall. A tuning system with about 200kHz frequency range has been developed in order to cope with fabrication tolerances. In this paper we will report on the cavity simulations. The tuning plate design will be described. Finally the frequency measurements on a cavity prototype at room temperature will be presented.The upgrade of the ISOLDE machine at CERN foresees a superconducting linac based on two gap independently phased Nb sputtered Quarter Wave Resonators (QWRs) working at 101.28MHz and producing an accelerating field of 6MV/m on axis. A careful study of the fields in the cavity has been carried out in order to pin down the crucial e-m parameters of the structure such as peak fields, quality factor and e-m power dissipated on the cavity wall. A tuning system with about 200kHz frequency range has been developed in order to cope with fabrication tolerances. In this paper we will report on the cavity simulations. The tuning plate design will be described. Finally the frequency measurements on a cavity prototype at room temperature will be presented

Jones, R.M.

CERN Document Server

arXiv:0910.0409v1  [physics.acc-ph]  2 Oct 2009HIE-ISOLDE HIGH BETA CAVITY STUDY AND MEASUREMENTSA. D’Elia and R. M. Jones, Cockroft Institute, UK, University of Manchester, Manchester, UKM. Pasini, CERN, Geneva, Switzerlandand Instituut voor Kernen Stralingsfysica, K. U. Leuven, Leuven, BelgiumAbstractThe upgrade of the ISOLDE machine at CERN foreseesa superconducting linac based on two gap independentlyphased Nb sputtered Quarter Wave Resonators (QWRs)working at 101.28MHz and producing an accelerating fieldof 6MV/m on axis. A careful study of the fields in the cav-ity has been carried out in order to pin down the crucial e-mparameters of the structure such as peak fields, quality fac-tor and e-m power dissipated on the cavity wall. A tuningsystem with  200kHz frequency range has been developedin order to cope with fabrication tolerances. In this paperwe will report on the cavity simulations. The tuning platedesign will be described. Finally the frequency measure-ments on a cavity prototype at room temperature will bepresented.INTRODUCTIONThe upgrade of the ISOLDE machine will consist inboosting the energy of the machine from 3MeV/u upto 10 MeV/u with beams of mass-to-charge ratio of2.5A/q 4.5 and in replacing part of the existing normalconducting linac [1].The new accelerator will employ superconductingQWR’s based on Nb sputtered on Cu substrate technol-ogy [2] with a working frequency of 101.28MHz and witha nominal accelerating field of 6MV/m on beam axis.Two cavity geometries, low and high , will be used tocover the whole energy range. As the first part of the up-grade will consist in the realization of the high energy sec-tion, the R&D effort have been focused on the study andproduction of a prototype of the high  cavity.The electromagnetic study and realization of the cavityprototype has been carried out in different steps. The def-inition of the cavity main parameters derives directly frombeam dynamics studies (beam aperture, gap to gap dis-tance, RF field asimmetry) [3], from the upstream linac (RFfrequency) and from manufacturing technique (Nb sput-tering). Given the above constraints, the electromagneticstudy has been performed in order to minimize the surfacepeak electric and magnetic field, maximizing the R sh=Qand the g factor. In addition a particular attention has beenpayed to the study of the frequency sensitivity of the dif-ferent geometric parameters, in order to evaluate a suitabletuning range. During the fabrication of the cavity proto-type, after each significant step (machining, welding, deep Alessandro.Delia@cern.chpressing) an RF measurement has been performed and re-sults are reported after in the paper. Given the limited fre-quency range of the tuner ( 200kHz), the final dimensionof the external conductor has been set only after the fre-quency measurement performed with the cavity still at thenumerical control machine.Figure 1 shows the prototype built at CERN and,presently, under sputtering tests.In the following we will show the main electromagneticparameters for the optimized geometry of the cavity andwe will compare them to the ones of similar structures asin TRIUMF and SPIRAL2. The tuner system will be pre-sented. Finally the results of the frequency tests at roomtemperature at different steps of the machining will be re-ported.(a) the cavity during the cleaningprocess(b) internal view of the cavityFigure 1: The prototype of the high  cavity produced atCERN.ELECTROMAGNETIC SIMULATIONSThe electromagnetic design of the cavity aims to min-imize the surface peak fields, both electric and magnetic.Nb sputtering process will reduce the number of the use-ful possible shapes of the cavity: between the all possiblegeometries, the better suitable for Nb sputtering has to bechosen. Therefore the optimum is a compromise betweenRF performances and sputtering requirements [2].In particular beam ports on the external conductor havebeen shaped in order to avoid any hidden edges on surfaceto the Nb cathode. Similarly, the region of the maximummagnetic field has been rounded in order to have a betterhomogeneity of the Nb film and this shape gives also a reg-ular surface current paths minimizing the magnetic fieldson these positions.The electric peak field, located at the bottom part of theresonator antenna, can be varied by changing the distanceof the bottom part of the antenna from the bottom plate ofthe cavity (tipgap). The tipgap has been chosen of 70mmbut the cavity, at the beginning, has been manufacturedwith a tipgap of 90mm. This gives the opportunity of tuneup the frequency at the end of the machining procedures ifneeded. The simulations presented in the following havebeen carried out with the nominal value of 70mm. Figure 2shows a cut-view of the CAD model and the positions ofthe maximum electric and magnetic field.Figure 2: Cut-view of the cavity showing the internal ge-ometry and also coupler and pick-up seats; in the middle,the regions of the maximum electric and magnetic fieldsare shown.Because of the cavity geometry is not azimuthal sym-metric, 3D electromagnetic codes need to be used to eval-uate its RF properties. In order to pin down reliable re-sults, a calibration of the simulation tools have been per-formed. A simpler geometry without beam ports has beendesigned in order to get a comparison between AnsoftHFSS c [ 5], CST Microwave c [ 6] and POISSON SU-PERFISH (Fig. 3). For this kind of structures, we considerthe data from SUPERFISH as the reference one.Considering the azimuthal symmetry of this simplerstructure, only one quarter has been simulated in CST andthe mesh refinement has been used. The higher step in therefinement corresponds to about 1.000.000 of meshcells.In HFSS, only a slice of one sixteenth has been simulated.The results have been obtained by considering a surface re-finement of the meshes of 2m, the total number of tetrahe-dra is about 110.000. The mesh stepsize for SUPERFISHis of 2.5mm (finer meshing does not change the results).The comparison terms have been the maximum electricand magnetic field and the Q 0. The results for peak fieldsare shown in Fig. 4.The fields are normalized to give 1J stored energy in theFigure 3: Models used for simulations: a) MWS, b) SU-PERFISH, c) HFSS.1 2 3 4 5 6 7 8 91010.51111.51212.513# of StepsMaximum E−Field (MV/m) CST (Max Evaluation)HFSS=11.6MV/mSuperfish=11.38MV/m(a) Max E-field1 2 3 4 5 6 7 8 91616.216.416.616.81717.217.417.617.818# of StepsH−Field (kA/m) CST Maximum H field HFSS (Color Label)=16.9kA/mSuperfish=16.71kA/m(b) Max H-fieldFigure 4: Peak surface fields.cavity (CST normalization). The plots show consistent re-sults between the three codes. A summary of the results,included Q 0 values are listed in Table 1.When the beam ports are added to the structure, the re-sults are expected to stay consistent with the ones foundpreviously. For this structure, Superfish is no longer pos-sible to use and then the results are extracted only fromHFSS and CST. Actually E peak and H peak are consis-tent between HFSS and CST and also with the previousplots, but the value of Q 0 found by CST is overestimated,about 14000, and not consistent with the HFSS simula-Table 1: Comparison table.Superfish CST HFSS C ST   SF%H FSS   SF%Frequency(MHz) 101.674 101.666 101.674 - -H p eak(kA/m) 16.711 16.733 16.763 0.1 0.3E p eak(MV/m) 11.38 11.57 11.6 1.7 1.9Q 0 11795 11844 11746 0.4 -0.4tions, about 11700. Probably this is due to some geometryproblem in the CST model around the noses. In fact, whenthe racetrack shape of the noses is changed to a simpler cir-cular geometry, the value becomes compatible with othersimulations.Finally, the electromagnetic cavity parameters are shownin Table 2 in comparison with TRIUMF and SPIRAL2.Table 2: Cavity parameters in comparison with TRIUMFand SPIRAL2.ISOLDE TRIUMF SPIRAL2Frequency (MHz) 101.28 141.4 88 (%) 11.4 11.2 12E acc (MV/m) 6 6 6.5Lnorm (mm) 300 180 410E peak=E acc 5.4 4.9 4.9B peak=E acc[G/(MV/m)] 96 99 90R sh=Q 0 ( ) 554 545 518g=R s  Q0 ( ) 30.34 25.6 37.5Pcav (W) 7 7a 10baMeasurements on a cavity prototype showed results exceeding thedesign parameters: Q 0=7108 for E acc=8.5MV/m with Pcav=7W [7].bMeasurements on a cavity prototype showed: Q 0=109 forE acc=6.5MV/m with Pcav=10W [8].THE TUNER PLATEFor the tuning system it has been decided to follow theconcept that has been developed at TRIUMF [9]. An oil-can shaped diaphragm of CuBe has been hydroformed witha pressure up to 120 bar. All radial slots necessary for theelongation and contraction of the diaphragm are performedwith a laser beam. The same plate can be mounted directlyon the low  cavity or welded to a flange in the case of thehigh  cavity. The actuator is designed to have no backlash.A pictorial view of the tuner is presented in Fig. 5.The useful stroke of the tuner plate is of 20mm. From themanufacturing position the plate can be pushed up towardsthe central resonator of 5mm (position +5) and down, inthe other direction, of 15mm (position -15). In Table 3 theresults of the simulations are listed for the nominal value ofthe tipgap of 70mm and for a tipgap of 90mm.The coarse range of the tuner plate for tipgap=70mm isforeseen to be of 245kHz for a moveable range of 20mmFigure 5: Tuner plate with its actuator.Table 3: Simulated frequency values of the tuning platewhen all up (Position +5) or all down (Position -15); inyellow the values for the nominal tipgap value.Tipgap 70mm Tipgap 90mmPosition +5 100.684 MHz 101.235 MHzPosition -15 100.929 MHz 101.339 MHz tunerplate 12.25 kHz/mm 5.2 kHz/mmgiving an average  tunerplate  12.25kHz/mm. The valuefor tipgap=90mm is of 5.2kHz/mm and the correction isless efficient as expected (coarse range of only 140kHz).However, both coarse ranges largely cope with frequencydetuning coming from machining errors, listed in Table 4,considering that mechanical tolerances are in the order ofthe tenth of mm.Table 4: Simulations results of the frequency detuning dueto the main possible machining errors.Type of error  freq (kHz/mm)Cavity diameter  65Resonator length  160Nose length  50Another important aspect to take into account is the ra-diation pressure acting on the tuner plate. Because of thehigh number of cuts and the reduced thickness of the di-aphragm, a strong force could irreversibly modify the tunershape leading to an unrecoverable frequency shift. The ra-diation pressure can be calculated by evaluating both elec-tric and magnetic field on the plate surface by the followingequation:P =14 0jH j2  0jEj2The result is shown in Fig. 6 in the case of a flat plate:the total force is quite low, equal to -1.77N.Figure 6: Radiation pressure on a flat plate.FREQUENCY MEASUREMENTSThe working frequency of the cold cavity has to be101.28MHz. Taking into account the scaling factors dueto the superconducting mode of operation (shortening ofthe length of the central resonator, skin depth variation,etc...) and due to the vacuum-air environment change,the hot frequency at room temperature in air should be 100.900MHz.As described before we have left the value of the tipgapas a free parameters in order to compensate possible errorscoming from the machining process. During the welding ofthe top part of the cavity, a problem occurred and the resulthas been a change of the length of the internal resonator,shortened of about 0.4mm, with a foreseen increasing ofthe frequency of about 65kHz. The measured variation isof 77kHz. In Table 5 the measured values before and af-ter welding the inner conductor to the outer are shown. Itis also shown the value of Cu Q 0 which is significantlyimproved after welding as expected. Furthermore the mea-sured value is consistent with simulations giving a value ofabout 11700.The last step is to finally define the tipgap size. An inter-mediate cut of the bottom of the cavity has been defined of75mm and directly in the mechanical atelier a set of mea-surements with two different coupler insertion lengths havebeen done to cross-check the simulated values. The resultsare shown in Table 6: the values both of the simulations andTable 5: Measurements before and after welding the innerconductor to the outer with tipgap=90mm.Before welding After weldingFrequency 101.147 101.224Q 0 5908 11380of the measurements have been properly scaled (values inbold) to get a comparison. The effect of the lengthening ofthe central resonator can be accepted and the cut has beendone at the nominal tipgap length of 70mm.Table 6: Set of measurements for different cut of the tipgapand different coupler insertion; bold values are scaled toget a comparison.Coupler in=64mm and Pickup in=22mmTipgap 90Simulation101.233MHz(-32kHz air)101.201MHzMeasurement101.246MHz(-77kHz res)101.169MHzTipgap 75Simulation101.013MHz(-32kHz air)100.981MHzMeasurement101.000MHz(-77kHz res)100.923MHzTipgap 70Simulation100.899MHz(-32kHz air)100.867MHzMeasurement100.916MHz(-77kHz res)100.839MHzCoupler in=22mm and Pickup in=22mmTipgap 90Simulation101.410MHz(-32kHz air)101.378MHzMeasurement101.483MHz(-77kHz res)101.406MHzTipgap 75Simulation101.191MHz(-32kHz air)101.159MHzMeasurement101.240MHz(-77kHz res)101.163MHzTipgap 70Simulation101.083MHz(-32kHz air)101.051MHzMeasurement101.150MHz(-77kHz res)101.073MHzFinally the measurement with the tuning plate has beendone. The value is shown in Table 7 in comparison withsimulation results opportunely scaled and goal frequencyand all the values are fully compatible.Table 7: Frequency measurement at room temperature withtuner plate in rest position.Measured frequency 100.885 MHzSimulation100.816 MHz(-32kHz air)(+77kHz res)100.861MHzGoal frequency  100.900 MHzCONCLUSIONThe high  cavity for ISOLDE upgrade has been fullydesigned and built. The cavity parameters have been de-rived showing values comparable to other similar struc-tures (TRIUMF and SPIRAL2). The foreseen Q 0 shouldbe 6:6  108 with a surface resistance R s=46n giving apower dissipation on the cavity wall of 7W.A prototype tuner plate has been built. The total coarserange in simulation is of 245kHz for a total course of 20mmgiving 12.25kHz/mm.The frequency measurement at room temperature showsa perfect agreement with the designed frequency: the mea-sured frequency is 100.885MHz, the design frequency atroom temperature should be  100.900MHz (the simula-tions give 100.861MHz).Presently the cavity and the tuner plate are under sput-tering tests.ACKNOWLEDGEMENTSThanks to Cockroft Institute for the financial support.Thanks to Philippe Perret who took charge of the mechan-ical design of the tuning plate.REFERENCES[1] M. Pasini “HIE-Isolde: The Superconducting RIB Linac atCERN”, these proceedings.[2] G. Lanza et al., “The HIE-ISOLDE Superconducting Cav-ities: Surface Treatment and Niobium Thin Film Coating”,these proceedings.[3] M. A. Fraser et al., “Compensation of Transverse FieldAsymmetry in the High-Beta Quarter-Wave Resonator of theHie-Isolde Linac at CERN”, these proceedings.[4] S. Calatroni et al., “The HIE-ISOLDE Superconducting Cavi-ties: Mechanical Design and Fabrication”, these proceedings.[5] http://www.ansoft.com/products/hf/hfss/[6] www.cst.com/[7] V. Zvyagintsev et al., “Development, Production And TestsOf Prototype Superconducting Cavities For The High BetaSection Of The Isac-ii Heavy Ion Accelerator At Triumf”,RuPAC 2008, Zvenigorod, Russia.[8] G. Olry et al., “Tests Results Of The Beta 0.12 Quarter WaveResonators For The Spiral2 Superconducting Linac”, LINAC2006, Knoxville, Tennessee USA[9] T. Ries et al., “A mechanical Tuner for the ISAC-II Quar-ter Wave Superconducting Cavities”, PAC03, Portland, USA,May 2003.

Hie-Isolde High Beta Cavity Study and Measurements

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