We illustrate the RF design of the X-band linac for the upgrade of the SPARC_LAB facility at INFN-LNF (EuPRAXIA@SPARC_LAB). The structures are travelling wave (TW) cavities, working on the 2π/3 mode, fed by klystrons with pulse compressor systems. The tapering of the cells along the structure and the cell profiles have been optimized to maximize the effective shunt impedance keeping under control the maximum value of the modified Poynting vector, while the couplers have been designed to have a symmetric feeding and a reduced pulsed heating. In the paper we also present the RF power distribution layout of the accelerating module and a preliminary mechanical design

Alesini, David

Alessandro, Gallo

Alexej, Grudiev

Andrea, Ghigo

Bellaveglia, Marco

Buonomo, Bruno

Cardelli, Fabio

Diomede, Marco

Enrica, Chiadroni

Ferrario, Massimo

Gianluca Di Raddo,

Giribono, Anna

Nuria Catalán Lasheras,

Piersanti, Luca

Roberto Di Raddo,

Spataro, Bruno

Vaccarezza, Cristina

Valerio, Lollo

Walter, Wuensch

English

Chiadroni, Enrica

Di Raddo, Gianluca

Di Raddo, Roberto

Gallo, Alessandro

Ghigo, Andrea

Lollo, Valerio

Catalán Lasheras, Nuria

Grudiev, Alexej

Wuensch, Walter

CERN Document Server

RF Design of the X-band Linac for the EuPRAXIA@SPARC_LAB Project

DIOMEDE, MARCO

David Alesini

Marco Bellaveglia

Bruno Buonomo

Fabio Cardelli

Nuria Catalán Lasheras

Enrica Chiadroni

Gianluca Di Raddo

Roberto Di Raddo

Massimo Ferrario

Alessandro Gallo

Andrea Ghigo

Anna Giribono

Alexej Grudiev

Valerio Lollo

Luca Piersanti

Bruno Spataro

Cristina Vaccarezza

Walter Wuensch

Archivio della ricerca- Università di Roma La Sapienza

RF DESIGN OF THE X-BAND LINAC FOR THEEUPRAXIA@SPARC_LAB PROJECTM. Diomede∗1, D. Alesini, M. Bellaveglia, B. Buonomo, F. Cardelli, E. Chiadroni, G. Di Raddo,R. Di Raddo, M. Ferrario, A. Gallo, A. Ghigo, A. Giribono, V. Lollo, L. Piersanti, B. Spataro,C. Vaccarezza, INFN-LNF, Via E. Fermi 40, Frascati, ItalyN. Catalan Lasheras, A. Grudiev, W. Wuensch, CERN, CH-1211 Geneva-23, Switzerland1also at Sapienza University, Piazzale Aldo Moro 5, Roma, ItalyAbstractWe illustrate the RF design of the X-band linac for theupgrade of the SPARC_LAB facility at INFN-LNF (Eu-PRAXIA@SPARC_LAB). The structures are traveling wave(TW) cavities, working on the 2pi/3 mode, fed by klystronswith pulse compressor systems. The tapering of the cellsalong the structure and the cell profiles have been optimizedto maximize the effective shunt impedance keeping undercontrol the maximum value of the modified Poynting vector,while the couplers have been designed to have a symmetricfeeding and a reduced pulsed heating. In the paper we alsopresent the RF power distribution layout of the acceleratingmodule and a preliminary mechanical design.INTRODUCTIONEuPRAXIA@SPARC_LAB [1] is the foreseen upgradeof the SPARC_LAB test facility at INFN-LNF (Frascati,Italy) [2]. It might be one of the possible candidates tohost EuPRAXIA (European Plasma Research Acceleratorwith eXcellence In Applications) [3]. The purpose of theproject is to bring together, for the first time, novel accelera-tion schemes, modern lasers, the latest correction/feedbacktechnologies and large-scale user areas.The preliminary EuPRAXIA@SPARC_LAB linac layoutis based on an S-band Gun, three S-band TW structures andan X-band booster with a bunch compressor. The X-bandtechnology allows to reach a high accelerating gradient anda high facility compactness, which are some of the goals ofthe projects. The accelerating structures are TW cavities fedby klystrons and pulse compressor systems. The operatingmode is the 2pi/3 mode at the frequency of 11.9942GHz.Two different scenarios are foreseen: acceleration withand without plasma stage. In the second case (named fullRF), two average gradients are foreseen: 57MV/m (high-gradient case, HG) and 80MV/m (very high-gradient case,VHG). The second value has been considered for the boosterdesign and optimization.In this paper we illustrate the electromagnetic (e.m.) de-sign of the single cell, the numerical optimization of theX-band booster and the preliminary results of the input cou-pler design. In the second part, the preliminary mechanicaldesign of the single cell and layout of the accelerating mod-ule are shown.∗ marco.diomede@lnf.infn.itELECTROMAGNETIC DESIGNThe X-band booster design has been performed followingtwo main steps: e.m. design of the single cell and numericaloptimization of the booster on the basis of the single cellresults. The input coupler has then been designed. Thedesigns of single cell and coupler have been performed usingthe 3D e.m. code ANSYS Electronics Desktop [4].Cell DesignThe design aimed to minimize the modified Poyntingvector normalized to the average accelerating gradientScmax/E2acc [5] and to calculate the other cell parameters(shunt impedance per unit length R, quality factor Q andthe group velocity vg/c) as a function of the iris aperturea. A sketch of the cell is shown in Fig. 1. According tobeam dynamics simulations and beam break-up limits [6],an average iris radius of 3.2mm has been chosen.Figure 1: Sketch of the single cell with main parametrizeddimensions.An elliptical shape of the irises has been considered andthe modified Poynting vector as a function of the iris elliptic-ity r1/r2 has been calculated (Fig. 2). The minimum valueof Scmax/E2acc has been obtained for r1/r2 = 1.3. The cellparameters are summarized in Table 1. The other mentionedcell parameters, as a function of the iris radius a, have thenbeen calculated (Fig. 3).Structure DesignThe analytical formulas of TW structure [7] and SLEDpulse compressor [8] have been implemented in a numericaltool able to calculate the main structure parameters for a9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPMK058THPMK0584422ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.02 Photon Sources and Electron AcceleratorsA08 Linear Accelerators1 1.2 1.4 1.6 1.8 2r1/r23.33.43.53.63.73.8S c max/Eacc2[A/V]10-4Figure 2: Modified Poynting vector as a function of the irisellipticity (a = 3.2 mm).Table 1: Single Cell Parameters for a = 3.2 mmIris radius a [mm] 3.2Iris thickness t [mm] 2.0Iris ellipticity r1/r2 1.3Edge rounding radius r0 [mm] 2.5Outer radius b [mm] 10.452Cell length d [mm] 8.332R [MΩ/m] 117vg/c [%] 1.77Q 7036Sc max/E2acc [A/V] 3.3 · 10−42 2.5 3 3.5 4 4.5 550100150200R [M/m]02468vg/c [%]Rvg/c2 2.5 3 3.5 4 4.5 5a [mm]69007000710072007300Q00.51S c max/Eacc2[A/V]10-3QSc max/Eacc2Figure 3: Single cell parameters as a function of the irisradius.given structure length and an arbitrary cell-by-cell iris mod-ulation along the structure itself [9]. In particular, we haveconsidered linear iris tapered structures as sketched in Fig.4.Figure 4: Sketch of the cell-by-cell iris tapering.The effective shunt impedance Rs of the structure [10]and the peak value of the modified Poynting vector havebeen calculated as a function of the iris tapering angle θfor different structure lengths. As an example, in Fig. 5we report Rs for three different lengths, while in Fig. 6 itis shown Sc max/E2acc of Ls = 0.5 m and for an averageaccelerating field equal to 80 MV/m. In the same plot, wehave also reported the theoretical value that, according to[11], corresponds to a breakdown rate (BDR) of 10−6 bpp/m.0 0.05 0.1 0.15 0.2 [°]370380390400410420Rs [M/m]Ls=0.4 mLs=0.5 mLs=0.667 mFigure 5: Effective shunt impedance as a function of the iristapering angle for three different structure lengths.0 0.05 0.1 0.15 0.2 [°]33.544.555.5S c max [W/m2 ]Structure peak value10-6 bpp/mFigure 6: Peak value of modified Poynting vector as a func-tion of the iris tapering angle for Ls = 0.5 m (VHG case).As a reference design of the structure, we have finallyconsidered 0.5 m long structures with a tapering angle of0.1° as a good compromise between modularity, Rs and iristapering. In fact, this gives a total number of structures equalto 32 that can be easily divided into groups of 8 structures fedby 1 klystron in the HG case and 2 klystrons in parallel in the9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPMK05802 Photon Sources and Electron AcceleratorsA08 Linear AcceleratorsTHPMK0584423ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.VHG case. In both cases the RF pulse will be compressedby means of 1 SLED [9]. The X-band linac main parametersare shown in Table 2.Table 2: X-band Linac ParametersFrequency of operation [GHz] 11.9942Peak klystron power [MW] 50Available klystron power [MW] ≈ 40Klystron pulse length tk [µs] 1.5Structure filling time t f [ns] 100Unloaded Q-factor Q0 of SLED 180000External Q-factor Qe of SLED 19300a first-last cell [mm] 3.636 − 2.764vg/c first-last cell [%] 2.77 − 1.02Rs [MΩ/m] 410Structure length Ls [m] 0.5Booster active length Lt [m] 16No. of structures Ns 32Average gradient 〈G〉 [MV/m] 57 80Energy gain Wgain [MeV] 912 1280Total required RF power PRF [MW] 127 250No. of klystrons Nk 4 8Input coupler pulsed heating [°C] <26Input Coupler Preliminary DesignAs a first case, we have considered a z-type coupler be-cause of its compactness with respect to the waveguide andmode-launcher types. We have calculated, in particular, thepulsed heating [12, 13] in the VHG case. In Fig. 7, thegeometry of the coupler and the surface magnetic field arereported. We have considered a dual feeding in order to can-cel the dipole field components introduced by the couplingholes. The calculated pulsed heating on the input coupler is<26 °C, while the obtained reflection coefficient is <−30 dB.Racetrack geometries will be considered in order to com-pensate for the residual quadrupole field components.(a)(b)Figure 7: Sketch of the front view of the input coupler (a),surface magnetic field distribution of the input coupler (b).PRELIMINARY MECHANICAL DESIGNAND RF MODULE LAYOUTA mechanical design activity has been started. The me-chanical 3D model of the single cell is given in Fig. 8. Thecells will integrate four symmetric cooling channels. Bymeans of a prototype activity, we are exploring the possibil-ity to avoid the tuning of the cells.Figure 8: Mechanical drawings of the single cell.The preliminary layout of the RF module with the powerdistribution waveguide network is shown in Fig. 9 in theHG case. A low-loss circular waveguide will connect theRF power sources (in klystron gallery) to the acceleratingmodule (in the experimental hall). Downstream the circularwaveguide, the SLED will compress the RF pulses while asystem of seven 3 dB hybrid splitter will distribute the RFpower to the 8 structures of the module itself. The modulewill also integrate the vacuum pumping system, the magnetsand the diagnostics. Calculations on the final waveguidedistribution attenuation and vacuum pressures are still inprogress.Figure 9: Layout of a single accelerating module.CONCLUSIONIn this paper we have illustrated the preliminary RF de-sign of the EuPRAXIA@SPARC_LAB X-band linac. Thesingle cell geometry has been optimized by e.m. simulations,minimizing the modified Poynting vector normalized to theaverage accelerating field. A numerical optimization of thewhole structure has been performed (maximization of theRF efficiency and minimization of the expected BDR). Apreliminary e.m. design of the input coupler has also beenperformed. Finally, a preliminary mechanical design of thecell and the layout of the RF module have been presented.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPMK058THPMK0584424ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.02 Photon Sources and Electron AcceleratorsA08 Linear AcceleratorsREFERENCES[1] M. Ferrario et al., ŞEuPRAXIA@SPARC_LABDesign studytowards a compact FEL facility at LNF,ŤNucl. Instrum. Meth.A, 2018, in press. doi: https://doi.org/10.1016/j.nima.2018.01.094[2] M. Ferrario et al., ŞSPARC_LAB present and future,Ť Nucl.Instrum. Meth. B, vol. 309, pp. 183Ű188, 2013.[3] P. Walker et al., ŞHORIZON 2020 EuPRAXIA DesignStudy,Ť in Proc. of IPAC2017, TUOBB3, Copenhagen, Den-mark, 2017.[4] ANSYS Electronics Desktop website, http://www.ansys.com/products/electronics/ansys- electronics-desktop[5] A. Grudiev, S. Calatroni, andW. Wuensch, ŞNew local Ąeldquantity describing the high gradient limit of acceleratingstructures,Ť Physical Review Special Topics-Acceleratorsand Beams, vol. 12, p. 102 001, 2009.[6] C. Vaccarezza et al., ŞEUPRAXIA@SPARC_LAB: Beamdynamics studies for the X-band Linac,Ť Nucl. Instrum. Meth.A, 2018, in press. doi: https://doi.org/10.1016/j.nima.2018.01.100[7] A. Lunin, V. Yakovlev, and A. Grudiev, ŞAnalytical solu-tions for transient and steady state beam loading in arbitrarytraveling wave accelerating structures,Ť Phys. Rev. ST Accel.Beams, vol. 14, p. 052 001, 5 May 2011.[8] Z. Farkas, H. Hogg, G. Loew, and P. Wilson, ŞSLED: Amethod of doubling SLACŠs energy,Ť in Proc. of 9th Int.Conf. on High Energy Accelerators, SLAC, 1974, p. 576.[9] M. Diomede et al., ŞPreliminary RF design of an X-bandlinac for the EuPRAXIA@SPARC_LAB project,Ť Nucl. In-strum. Meth. A, 2018, in press. doi: https://doi.org/10.1016/j.nima.2018.01.032[10] R. B. Neal, ŞDesign of Linear Electron Accelerators withBeam Loading,Ť Journal of Applied Physics, vol. 29, no. 7,pp. 1019Ű1024, 1958.[11] K. Sjobak, E. Adli, and A. Grudiev, ŞNew Criterion forShapeOptimization of Normal-ConductingAccelerator Cellsfor High-Gradient Applications,Ť in Proc. of LINAC2014,MOPP028, Geneva, Switzerland, 2014.[12] D. Pritzkau, ŞRF pulsed heating,Ť PhD thesis, 2001.[13] V. Dolgashev, ŞHigh magnetic Ąelds in couplers of X-bandaccelerating structures,Ť in Proceedings of the 2003 ParticleAccelerator Conference, IEEE, vol. 2, 2003, pp. 1267Ű1269.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPMK05802 Photon Sources and Electron AcceleratorsA08 Linear AcceleratorsTHPMK0584425ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.

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RF Design of the X-band Linac for the EuPRAXIA@SPARC_LAB Project

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