  Citation: Spataro, B.; Behtouei, M.; Cardelli, F.; Carillo, M.; Dolgashev,V.; Faillace, L.; Migliorati, M.; Palumbo, L. A Hard Copper Open X-Band RF Accelerating Structure Made by Two Halves. Instruments 2022, 6, 5. https://doi.org/10.3390/instruments 6010005 Academic Editors: Nicolas Delerue and Maud Baylac Received: 29 October 2021 Accepted: 12 January 2022 Published: 15 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). instruments Communication A Hard Copper Open X-Band RF Accelerating Structure Made by Two Halves Bruno Spataro 1, * , Mostafa Behtouei 1 , Fabio Cardelli 1 , Martina Carillo 2 , Valery Dolgashev 3 , Luigi Faillace 1 , Mauro Migliorati 2,4 and Luigi Palumbo 2,4 1 Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, Via Enrico Fermi 54, 00044 Frascati, Italy; Mostafa.Behtouei@lnf.infn.it (M.B.); fabio.cardelli@lnf.infn.it (F.C.); Luigi.Faillace@lnf.infn.it (L.F.) 2 Dipartimento di Scienze di Base e Applicate per l’Ingegneria (SBAI), Università degli Studi di Roma “La Sapienza”, Via Scarpa 14, 00161 Rome, Italy; martina.carillo@uniroma1.it (M.C.); mauro.migliorati@uniroma1.it (M.M.); luigi.palumbo@uniroma1.it (L.P.) 3 SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA; dolgash@slac.stanford.edu 4 INFN/Roma1, Istituto Nazionale di Fisica Nucleare, Piazzale Aldo Moro, 2, 00185 Rome, Italy * Correspondence: Bruno.Spataro@lnf.infn.it 1. Introduction This communication focuses on the technological developments aiming to show the viability of novel welding techniques [1,2], and related applications, in order to benefit from the superior high-gradient performance of accelerating structures made of hard-copper alloys. The technological activity of testing high-gradient RF sections is related to the investigation of breakdown mechanisms, which limit the high gradient performance of these structures. In this content, our activity consists of the design, construction and high- power experimental tests of standing-wave (SW) 11.424 GHz (X-band) accelerating cavities with different materials and methods. The goal is to assess the maximum sustainable gradients with extremely low probabil- ity of RF breakdown in normal-conducting high-gradient RF cavities. The most common bonding techniques, used worldwide, are high-temperature brazing and diffusion bonding. Brazing and diffusion bonding are performed inside a high-temperature furnace. On the other hand, experimental results with hard copper cavities, conducted at SLAC, CERN and KEK [3,4] have shown that hard materials sustain higher accelerating gradients for the same breakdown rate. Therefore, it would be better to avoid high-temperature processing of the cavities to benefit from superior high-gradient performance of hard copper alloys. In this framework, we conduct experiments that involve the Electron Beam Welding (EBW) and Tungsten Inert Gas (TIG) processes, which allow us to build practical, multi-cell structures made with hard copper alloys in order to increase their RF performance against soft ones. For this purpose, open structures made of two halves have been investigated and fabricated. Details on the fabrication procedure of hard copper X-band structure by using the TIG method are given in our previous paper [1]. In this paper, we present RF characterization and low-power RF tests of a two-halves split hard-copper structure [5,6] that will be consequently TIG welded and employed for high-gradient tests and for the study of the RF breakdown physics. To achieve this aim, the structure geometry that we propose (shown in Figure 1a, cavity design) allows getting a high longitudinal shunt impedance Rsh of the accelerating mode, increases the mode separation frequencies, and improves the operating vacuum level. In addition, intense beam currents and multibunch operation are essential features, for example, for increasing the luminosity of a linear collider, but beam current wakefields and coupled-bunch mode instabilities, which mostly arise from the parasitic modes of the accelerating structures, can limit the accelerator performance. Hence, our main interest is also to detune the cavity in order to reduce the beam instability using a novel simpler Instruments 2022, 6, 5. https://doi.org/10.3390/instruments6010005 https://www.mdpi.com/journal/instruments