Alesini, David

Cardelli, Fabio

Magi, Marco

Palumbo, Luigi

Pellegrino, F.

Pettinacci, Valerio

English

CARDELLI, FABIO

MAGI, Marco

PALUMBO, Luigi

PETTINACCI, VALERIO

ALESINI, DAVID

Archivio della ricerca- Università di Roma La Sapienza

DESIGN OF A LINAC WITH THE NEW GASKETS CLAMPINGFABRICATION TECHNIQUEF. Cardelli1∗, M. Magi, L. Palumbo1, F. Pellegrino, V. Pettinacci, INFN-Roma1, Roma, ItalyD. Alesini, INFN-LNF, Frascati, Italy1 also at Universitá di Roma "La Sapienza", Roma, ItalyAbstractRecently, a new technique for the realization of high gra-dient accelerating structures based on the use of gasketswithout brazing processes has been successfully tested athigh power on a 1.6 cells RF gun (D. Alesini, et al, PRST18, 02001, 2015). The new technique developed at the Lab-oratories of Frascati of the INFN (Italy) in the framework ofthe SPARC_LAB project has been also adopted for the ELI-NP RF gun. The use of special gaskets that simultaneouslyguarantee the vacuum seal and a perfect RF contact, allowto avoid the brazing process, strongly reducing the cost, therealization time and the risk of failure. Moreover, withoutcopper annealing due to the brazing process, it is possible, inprinciple, to decrease the breakdown rate increasing, at thesame time, the maximum achievable gradient. The extensionof this new fabrication process to complex LINAC structuresis the next step on the application of this new technique onparticle accelerators. In the paper, we discuss how to extendthis process to S-band and C-band Travelling Wave acceler-ating structures illustrating their electromagnetic design andtheir mechanical realization.INTRODUCTIONThere is a growing demand from the research and indus-trial communities for compact and high gradient RF acceler-ating structures. These structures are, in general, fabricatedby a brazing process of pre-machined parts. The brazing pro-cess requires a large vacuum furnace so it is very expensiveand implies a not negligible risk of failure. Furthermore,breakdown-rate studies [1] indicated that, avoiding the cop-per annealing associated with the brazing, the BDR probabil-ity can be reduced. For these reasons it has been developpeda new technique that avoid this process using special RF-vacuum gaskets [2, 3] between the different machined partsthat are clamped together by screws. This technique hasbeen successfully applied for the realization of both the newSPARC_LAB 1.6 cells RF gun [3] and the ELI-NP-GBS one.The results obtained for these devices in term of reached cath-ode peak field open the possibility to apply this technique toother rf structures. In this paper we discuss the applicationof this technique to the realization of a linac section. Tothis purpose an S-band prototype with a reduced numberof cells has been designed and will be fabricated and testedin the next months to demonstrate the feasibility of suchapproach. The prototype is a 10 cell S-band TW disk loadedaccelerating structure, constant impedance with 2π/3 field∗ fabio.cardelli@roma1.infn.itphase advance per cell. In the following sections the designof this prototype is presented both from the electromagneticthan from the mechanical point of view.ELECTROMAGNETIC DESIGNThe electromagnetic design of the S-band TW prototypehas been done using the 3D electromagnetic code HFSS [4]and can be summarized in two main steps: single cell designand input/output coupler design. In the overall design acontinuous feedback between the electromagnetic designand the mechanical model has been necessary.Cell designThe main RF parameters of the single cell such as shuntimpedance, group velocity and attenuation, have been stud-ied as a function of the iris diameter, retuning the cell outerradius in order to have the 2π3phase advance at the 2.856GHz working frequency. One of the internal edges of the cellhas been rounded both to increase the quality factor (of abouta factor 10%) than to simplify the mechanical realization ofthe cell cups.Figure 1: Main structure figures of merit as a function ofthe iris aperture.Figure 1 shows the main cell parameters as a functionof the iris half aperture. Since the prototype is a constantimpedance (CI) section with equal cell irises an half apertureof 13 mm has been chosen as the best compromise between agood pumping speed and structure efficiency, while the cellProceedings of IPAC2016, Busan, Korea MOPMW00507 Accelerator TechnologyT06 Room Temperature RFISBN 978-3-95450-147-2403 Copyright©2016CC-BY-3.0andbytherespectiveauthorsto cell iris thickness has been fixed to 6 mm. The frequencysensitivity with respect to the cell dimensions are reportedin Table 1.Table 1: Frequency sensitivity with respect to the cell mainparameters.Parameter SensitivityCell radius −80 kHz/μmIris half aperture 22 kHz/μmCell length −5 kHz/μmIris thickness 10 kHz/μmTo decrease the peak surface electric field on the irisesan elliptical cross-section with an aspect ratio 4/3 has beenchosen. This guarantees a reduction of the surface field ofthe order of 10% with respect to the round profile, which isdesiderable for high gradient application.Couplers designThe input and output couplers have been designed in orderto implement the clamping technique on the several couplerparts and adjacent cells. In particular it was not possible,obviously, to adopt a standard z-coupling coupler [5] butwe had to design the coupler with a coupling aperture (seeFigure 2). Moreover one of the constraint in the design wasto keep as small as possible the transverse dimension w ofthe coupling aperture to reduce the distance between theclamping screws with the adjacent cell.Figure 2: Sketch of the front view (left) and lateral view(right) of the input/output coupler.For easier machining, the coupler cell has been designedwith a racetrack shape and with a dual feed to cancel thedipole field components as given in Fig. 2 where it is possibleto see the geometry of the couplers and its main parame-ters. In the final implementation of the system a splitter willbe inserted on the input coupler to guarantee the correctphase between the two ports while the output one will beconnected to two RF loads. The dimensions Rc, w and lhave been optimized to match the waveguide mode TE10 tothe accelerating one TM01-like. The design has been doneFigure 3: Azimuthal magnetic field at 5 mm from the centerof the coupler cell for different geometries of the cell.using the 3D electromagnetic code HFSS and the techniquediscussed in [6].To compensate the quadrupole field components inducedby the presence of the hole apertures a further modificationof the coupler profile has been instroduced. In particularthe two rounded parts of the coupler cell have been com-pressed displacing their center by a quantity equal to Δy.With a Δy = −2mm the quadrupolar component has beencompletely compensated, as given in Figure 3, where themagnetic field on a circle of 5 mm radius in the center of thecoupler has been reported for different Δy and the couplinghole has been also strongly rounded to reduce pulsed heatingon the surfaces [7].Figure 4: Amplitude and phase of the accelerating fieldalong a 5 cells, plus in/out couplers, prototype.Finally, Table 2 reports the main parameters for the 10-cellprototype.MECHANICAL DESIGNThe entire accelerating section will be fabricated withthe use of special RF-vacuum gaskets without brazing butsimply clamping all machined components by screws. Thespecial gaskets guarantee at the same time the RF contactMOPMW005 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-2404Copyright©2016CC-BY-3.0andbytherespectiveauthors07 Accelerator TechnologyT06 Room Temperature RFFigure 5: Mechanical drawings of the assembly procedure (left) and of the entire prototype in its final configuration (right).Designed with [8].Table 2: Prototype RF parameterFreq. 2856MHzPhase advance per cell 2π /3Number of cells 10Period 34.99mmIris radius 13mmUnloaded quality factor Q 14 133Group velocity (vg/c) 0.02Field attenuation 0.11 m−1Shunt impedance 11.61 MΩ/mSeries impedance 55 MΩ/mEacc/√Pin 3.4MV/m√MWPulsed heating (1μs pulselength) 5 °Cand a perfect vacuum sealing between each part of the struc-ture. The main challenge for this design was to find thebetter compromise between electromagnetic performancesand mechanical constraints. Figure 5 shows the details of thestructure assembly. The prototype structure is composed byan input coupler, ten accelerating cells and an output couplerwith the final closing cap. All the cells are mechanicallyidentical except those adjacent to the couplers that have adifferent profile to allow the connection with the couplersthemselves. The structure will be assembled starting fromthe input coupler stacking up one cell after the other, insert-ing the gaskets between them and fixing each part with theprevious one through screws, up to the output coupler andclosing cap. Figure 6 shows the detail of a single cell that hasthrough-holes and threaded holes for the connection with thetwo adjacent cells. Each cell has eight screw holes disposedat 45 degree to one another to provide an homogeneous forceon the gasket when the cells are clamped together. Two op-posite bi-directional tuners are inserted on each cell and onthe couplers. All cell gaskets have the same dimensions andthe same profile as given in [3], while the gaskets connectedto the input and output couplers have a different geometry tofollow the coupler cells profiles. Each part will be fabricatedfrom cilinders of Oxygen Free High Conductivity (OFHC)copper. The single cell will be machined as a cup using ahigh precision lathe. The input and output coupler insteadwill be realized with a computer controlled milling machine.Figure 6: Mechanical drawings of the single cell.CONCLUSIONIn the paper we have presented the electromagnetic andmechanical design of a 10 cells S-band TW structure imple-menting the gasket-clamping technique. The manufacturingwill be done at the mechanical workshop of INFN-Roma1.After the vacuum leak test and the low power measurementsand tuning, high power test will be performed to verify theeffectiveness of this technique in the realization of an en-tire accelerating structure. This preliminary test open newperspectives to further extensions of this technique to therealization of C/X-band structures.ACKNOWLEDGEMENTI would like to aknowledge F. Cioeta and V. Lollo for theusefull suggestions on the mechanical design.Proceedings of IPAC2016, Busan, Korea MOPMW00507 Accelerator TechnologyT06 Room Temperature RFISBN 978-3-95450-147-2405 Copyright©2016CC-BY-3.0andbytherespectiveauthorsREFERENCES[1] V.A. Dolgashev , et al., Proceedings of the 21st Particle accel-erator conference, Knoxville, TN, 2005 (IEEE, Piscataway,NJ,2005).[2] Int’l patent application PCT/IB2016/051464, D. Alesini etal.,"Process for manufacturing a vacuum and radio-frequencymetal gasket and structure incorporating it" assigned to INFN.[3] D. Alesini,et al., Physical Review Accelerators and Beams 18,02001, 2015.[4] ANSYS HFSS, http://www.ansys.com[5] The Stanford Two-Mile Accelerator, Neal R.B. (ed), W.A. Ben-jamin, Inc., New York, 1968.[6] D. Alesini,et al., Nuclear Instrumentation Methods PhysicsResults A 580 (2007) 1176-83.[7] D. Alesini, Power Coupling, CERN Yellow Report CERN-2011-007 [arXiv:1112.3201] (2007).[8] Autodesk Inventor http://www.autodesk.comMOPMW005 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-2406Copyright©2016CC-BY-3.0andbytherespectiveauthors07 Accelerator TechnologyT06 Room Temperature RF

Designofa LINAC with the new gaskets clamping fabrication technique

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Designofa LINAC with the new gaskets clamping fabrication technique

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