Development of High-Gradient C-Band RF Guns

Construction and test of a high-gradient, high rep rate C-band RF Gun David Alesini INFN-LNF (Frascati, Italy)

IFAST TASK 7.4 STRUCTURE AND OBJECTIVES An R&D activity has been started few years ago on a C-band gun and full C-band injector (gun + TW C band structures) for the XLS Design Study and for the EUPRAXIA@SPARC_LAB injector This R&D program has been funded by the I.FAST (Horizon 2020) and TUAREG (INFN commission V) projects I.FAST goal: Design, realization and high-power test of two different C-band (5712 GHz) RF electron guns operating at very high gradient cathode peak field (>160 MV/m): Standing Wave gun (INFN(IT), COMEB (IT)) Travelling Wave gun (PSI (SW), VLD (NE)) Comparison of the performances, beam dynamics simulations to exploit the device potentialities design, brazing and low power characterization of the TW Gun hosting and setting up the facility to perform the high- power test Coordination Design and low power test of the SW Gun Solenoid design and procurement RF circulator procurement mechanical construction of the TW gun components mechanical construction and assembly of the SW gun TW GUN SW GUN

BRAZED FREE RF PHOTO-GUNS In the last 10 years different RF photo-injectors have been fabricated, tested and put in operation. All injector realizations are based on the new INFN technology w/o brazing. ELI-NP CLEAR (CERN) C BAND NEW SPARC_LAB (I.FAST/TUAREG) SPARC (Ver. 1) Ready to be tested @ PSI D. Alesini et al., PHYS. REV. ACCEL. BEAMS 21, 112001 (2018) V. Shpakov et al., et al 2022 JINST 17 P12022 (2022) In operation @SPARC_LAB Built and in operation @ CERN (2020) D. Alesini et al., Phys. Rev. ST Accel. 092001 (2015) Beams 18, A. Giribono PRAB 26, 083402, 2023 Tested at high power In operation @UCLA S Band C Band

TYPICAL ASSEMBLY PROCEDURE OF THE NEW GUNS D. Alesini et al., PRAB 18, 092001 (2015) D. Alesini et al., PRAB 21, 112001 (2018) The structure has been fabricated without brazing but using special gaskets developed @LNF V. Shpakov et al., JINST 17 P12022 (2022) -simplify the fabrication -reduce the cost and the risk of failure -reach in principle (because of the hard copper not annealed) higher accelerating field with lower BDR and lower conditioning time.

FABRICATION PROCESS BRAZING FREE [Int'l patent application PCT/IB2016/051464] [Int'l patent application PCT/IB2016/051464] The brazing free technology uses a novel process developed at LNF-INFN involving the use of special RF-vacuum gaskets (same Cu material of gun) that guarantee (simultaneously) the vacuum seal and a perfect RF contact when the structure is clamped. Cathode (with integrated cooling system) waveguide Cu Special gaskets Body of the gun (single piece of OFHC copper) Closing cup and pipe

RF GUN CONDITIONING PROCEDURE: DETAIL V. Shpakov, D. Alesini et al., et al 2022 JINST 17 P12022 (2022) 5x106pulses, total cumulated discharges (vacuum) <103 HARD COPPER showed extremely good properties in term of conditioning time! No discharges now in operation >110 MV/m

NEXT STEP: C BAND RF PHOTO-GUN Parameter Resonant frequency Ecath/ Pdiss[MV/(m MW0.5)] rf input power [MW] Cathode peak field [MV/m] Rep. rate [Hz] Quality factor Filling time [ns] Coupling coefficient rf pulse length [ns] Mode sep. - /2 [MHz] Esurf/Ecath Mod. Poy. vector [W/ m2] Pulsed heating [oC] Average diss. Power [W] Value The new C-Band RF gun, developed in the context of the European I.FAST and INFN Commission V projects, has been realized. It is a 2.6 cell standing wave cavity with a four-port mode launcher, designed to operate with short rf pulses (300 ns) and cathode peak field larger than 160 MV/m. In the context of the X-band linacs of the EuPRAXIA@SPARC_LAB projectand XLS design study the possibility to implement a full C-band injector is attractive for both the reachable beam parameters and compactness than for the possibility to operate at high repetition rate (up to 1 kHz). 5.712 (5.712) 51.4 18(19) 160 100-400 11900 (11900) 166 (147) 3 (3.5) 300 47 (48.3) 0.96 2.5 16 250-1000 D. Alesini et al., MOPOMS021, IPAC2022, Bangkok, Thailand, p. 679, 2022 D. Alesini et al., TUPA009, IPAC2023, Venezia, Italy, p. 1356, 2023 F. Cardelli et al., MOPOMS020, IPAC2022, Bangkok, Thailand, p. 675, 2022 A. Giribono et al., PRAB 26, 083402, 2023

C BAND GUN LAYOUT Laser injection chamber RF gun solenoid

C-BAND GUN ASSEMBLY Machined cells and RF and Vacuum gaskets ready for the assembly in a clean room Brazed mode launcher Mode launcher ready for brazing

RF MEASUREMENTS Cavity temperature during measurement = 23 C f0 = 5.7107 GHz in Nitrogen Cavity temperature = 28 C, f0 = 5.712 GHz in Vacuum Unit GHz Measured Value 5.712 11900 147 3.5 48.3 Parameter Resonant frequency Quality factor Filling time Coupling coefficient Mode sep. - /2 Design Value 5.712 (5.712) 11900 (11900) 166 (147) 3 (3.5) 47 (48.3) ns MHz Bead-drop mesurement /2 mode mode 0 mode

FINAL GUN ASSEMBLY AND TESTS The cells have been clamped and finally also the cathode is mounted. The assembly has been vacuum tested and then characterized and tested at low RF power. RF Power Source The bunker has been refurbished with the addition of a new wall and preparation for the high power tests of the gun have begun. The waveguide network and the C-band Gun has been installed in July 2023.

ALTERNATIVE FEEDING SCHEME W/O ISOLATOR The original gun feeding scheme foresaw the use of a new in-vacuum isolator. The delivery of the isolator from the Company experienced some delays due to the difficulties that have been encountered in its realization. For this reason and also to have a backup solution in case of difficulties in reaching, with this new device, the required performances in term of power, we have developed an alternative scheme using power dividers and a BOC-type pulse compressor already available at PSI for the test of the travelling wave gun in the context of the I.FAST project. 45 MW 2.5 s 9/10 LOAD KLY POWER DIVIDER PULSE COMPR. 1/10 GUN LOAD 4.5 MW 2.5 s

RF Design Details Output Waveguide Output coupling cell Regular cells (Constant impedance, 120 degree phase advance) Input Waveguide Input coupling cell Output Coaxial Input Coaxial Exchangeable cathode Waveguide-to-Coaxial Transition Coaxial-to-Waveguide Transition Magnetic Coupling Slot Figure 1: The travelling-wave RF Photogun s internal volume. Reference: T.G. Lucas et al. Towards a Brightness Upgrade to the SwissFEL: A High Gradient Traveling-Wave RF Photogun. Accepted to PRAB. Seite 13

Electromagnetic Fieldmaps Reference: T.G. Lucas et al. Towards a Brightness Upgrade to the SwissFEL: A High Gradient Traveling-Wave RF Photogun. Accepted to PRAB. Seite 14

Mechanical Design of TW RF Photogun Reference: T.G. Lucas et al. Towards a Brightness Upgrade to the SwissFEL: A High Gradient Traveling-Wave RF Photogun. Accepted to PRAB. Seite 15

BEAM DYNAMICS AND C BAND GUNS PERFORMANCES

CONCLUSIONS We are developing new C band RF guns in the framework of the I.FAST project: SW and TW The SW has been built and is ready for the high power test They will be tested at high power at PSI Beam dynamics performances have been performed to fully investigated the potentialities of these devices: strong impact in high rep. rate machines ( kHz) THANK YOU FOR YOUR ATTENTION ACKNOWLEDGEMENTS F. Cardelli, G. Di Raddo, R. Di Raddo, A. Vannozzi, A. Giribono, L. Faillace, A. Gallo, L. Pellegrino, L. Ficcadenti, G. Pedrocchi, G. Castorina, S. Bini, A. Liedl, V. Lollo, L. Piersanti, S. Pioli, C. Vaccarezza, M. Ferrario, L. Spallino, P. Chimenti, T. Lucas, P. Craievich, M. Pedrozzi, R. Zennaro. This project has received funding from the European Union s Horizon 2020 Research and Innovation programme under GA No101004730 and by INFN Commission n. V