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Berenguer, A., Coves, A., Gimeno, B., Bronchalo, E., & Boria, V. E. (2019). Experimental Study of the Multipactor Effect in a Partially Dielectric-Loaded Rectangular Waveguide. IEEE Microw. Wirel. Compon. Lett., 29(9), 595–597.
Abstract: This letter presents the experimental study of the multipactor threshold in a partially dielectric-loaded rectangular waveguide, whose results validate a multipactor model recently developed by the authors, which includes the charge distribution appearing on the dielectric surface during the multipactor discharge. First, the variation of the multipactor RF voltage threshold has been theoretically analyzed in different waveguide configurations: in an empty waveguide, and also in the cases of a one-sided and two-sided dielectric-loaded waveguides. To reach this aim, an in-house Monte Carlo simulation tool has been developed. The Secondary Electron Yield (SEY) of the metallic and dielectric materials used in the numerical simulations have been measured experimentally. Finally, an aluminum WR-75 symmetric E-plane rectangular waveguide transformer has been designed and fabricated, in which several multipaction tests have been carried out to validate the in-house software tool, demonstrating an excellent agreement between the simulation results and the experimental data.
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Gonzalez-Iglesias, D., Esperante, D., Gimeno, B., Boronat, M., Blanch, C., Fuster-Martinez, N., et al. (2021). Analytical RF Pulse Heating Analysis for High Gradient Accelerating Structures. IEEE Trans. Nucl. Sci., 68(2), 78–91.
Abstract: The main aim of this work is to present a simple method, based on analytical expressions, for obtaining the temperature increase due to the Joule effect inside the metallic walls of an RF accelerating component. This technique relies on solving the 1-D heat-transfer equation for a thick wall, considering that the heat sources inside the wall are the ohmic losses produced by the RF electromagnetic fields penetrating the metal with finite electrical conductivity. Furthermore, it is discussed how the theoretical expressions of this method can be applied to obtain an approximation to the temperature increase in realistic 3-D RF accelerating structures, taking as an example the cavity of an RF electron photoinjector and a traveling wave linac cavity. These theoretical results have been benchmarked with numerical simulations carried out with commercial finite-element method (FEM) software, finding good agreement among them. Besides, the advantage of the analytical method with respect to the numerical simulations is evidenced. In particular, the model could be very useful during the design and optimization phase of RF accelerating structures, where many different combinations of parameters must be analyzed in order to obtain the proper working point of the device, allowing to save time and speed up the process. However, it must be mentioned that the method described in this article is intended to provide a quick approximation to the temperature increase in the device, which of course is not as accurate as the proper 3-D numerical simulations of the component.
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Coves, A., Maestre, H., Archiles, R., Andres, M. V., & Gimeno, B. (2022). Surface-Impedance Formulation for Hollow-Core Waveguides Based on Subwavelength Gratings. IEEE Access, 10, 18843–18854.
Abstract: A rigorous Surface Impedance (SI) formulation for planar waveguides is presented. This modal technique splits the modal analysis of the waveguide in two steps. First, we obtain the modes characteristic equations as a function of the SI and, second, we need to obtain the surface impedance values using either analytical or numerical methods. We validate the technique by comparison with well-known analytical cases: the parallel-plate waveguide with losses and the dielectric slab waveguide. Then, we analyze an optical hollow-core waveguide defined by two high-contrast subwavelength gratings validating our results by comparison with reported values. Finally, we show the potential of our formulation with the analysis of a THz hollow-core waveguide defined by two surface-relief subwavelength gratings, including material losses in our formulation.
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Menendez, A., Esperante, D., Garcia-Olcina, R., Torres, J., Perez-Soler, J., Marco, R., et al. (2022). RF Acquisition System Based on μTCA for Testing of High-Gradient Acceleration Cavities. Electronics, 11(5), 720–22pp.
Abstract: The radio frequency (RF) laboratory hosted in the Corpuscular Physics Institute (IFIC) of the University of Valencia is designed to house a high-power and high-repetition-rate facility to test normal conduction RF accelerator cavities in the S-Band (2.9985 GHz) in order to perform R & D activities related to particle accelerator cavities. The system, which manages the entire process of RF signal generation, data acquisition and closed-loop control of the laboratory, is currently based on a modular and compact PXI platform system. This contribution details the development of a platform with similar features, but which is based on open architecture standards at both the hardware and software level. For this purpose, a complete system based on the μTCA platform has been developed. This new system must be able to work with accelerator cavities at other operating frequencies, such as 750 MHz, as well as to explore different options at firmware and software levels based on open-source codes.
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Gonzalez-Iglesias, D., Esperante, D., Gimeno, B., Blanch, C., Fuster-Martinez, N., Martinez-Reviriego, P., et al. (2023). Analysis of the Multipactor Effect in an RF Electron Gun Photoinjector. IEEE Trans. Electron Devices, 70(1), 288–295.
Abstract: The objective of this work is the evaluation of the risk of suffering a multipactor discharge within an RF electron gun photoinjector. Photoinjectors are a type of source for intense electron beams, which are the main electron source for synchrotron light sources, such as free-electron lasers. The analyzed device consists of 1.6 cells and it has been designed to operate at the S-band. Besides, around the RF gun there is an emittance compensation solenoid, whose magnetic field prevents the growth of the electron beam emittance, and thus the degradation of the properties of the beam. The multipactor analysis is based on a set of numerical simulations by tracking the trajectories of the electron cloud in the cells of the device. To reach this aim, an in-house multipactor code was developed. Specifically, two different cases were explored: with the emittance compensation solenoid assumed to be off and with the emittance compensation solenoid in operation. For both the cases, multipactor simulations were carried out exploring different RF electric field amplitudes. Moreover, for a better understanding of the multipactor phenomenon, the resonant trajectories of the electrons and the growth rate of the electrons population are investigated.
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