Martinez-Reviriego, P., Esperante, D., Grudiev, A., Gimeno, B., Blanch, C., Gonzalez-Iglesias, D., et al. (2024). Dielectric assist accelerating structures for compact linear accelerators of low energy particles in hadrontherapy treatments. Front. Physics, 12, 1345237–12pp.
Abstract: Dielectric Assist Accelerating (DAA) structures based on ultralow-loss ceramic are being studied as an alternative to conventional disk-loaded copper cavities. This accelerating structure consists of dielectric disks with irises arranged periodically in metallic structures working under the TM02-pi mode. In this paper, the numerical design of an S-band DAA structure for low beta particles, such as protons or carbon ions used for Hadrontherapy treatments, is shown. Four dielectric materials with different permittivity and loss tangent are studied as well as different particle velocities. Through optimization, a design that concentrates most of the RF power in the vacuum space near the beam axis is obtained, leading to a significant reduction of power loss on the metallic walls. This allows to fabricate cavities with an extremely high quality factor, over 100,000, and shunt impedance over 300 M omega/m at room temperature. During the numerical study, the design optimization has been improved by adjusting some of the cell parameters in order to both increase the shunt impedance and reduce the peak electric field in certain locations of the cavity, which can lead to instabilities in its normal functioning.
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Martin-Luna, P., Gimeno, B., Gonzalez-Iglesias, D., Esperante, D., Blanch, C., Fuster-Martinez, N., et al. (2023). On the Magnetic Field of a Finite Solenoid. IEEE Trans. Magn., 59(4), 7000106–6pp.
Abstract: The magnetostatic field of a finite solenoid with infinitely thin walls carrying a dc current oriented in the azimuthal direction is calculated everywhere in space in terms of complete elliptic integrals by direct integration of the Biot-Savart law. The solution is particularized near the solenoid axis and in the midplane perpendicular to the axis obtaining expressions that agree with some typical approximations that are made in introductory courses of electromagnetism or in the technical literature. The range of validity of these approximations has been studied comparing them with the obtained general expression.
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Gonzalez-Iglesias, D., Gimeno, B., Esperante, D., Martinez-Reviriego, P., Martin-Luna, P., Fuster-Martinez, N., et al. (2024). Non-resonant ultra-fast multipactor regime in dielectric-assist accelerating structures. Results Phys., 56, 107245–12pp.
Abstract: The objective of this work is the evaluation of the risk of suffering a multipactor discharge in an S-band dielectric-assist accelerating (DAA) structure for a compact low-energy linear particle accelerator dedicated to hadrontherapy treatments. A DAA structure consists of ultra-low loss dielectric cylinders and disks with irises which are periodically arranged in a metallic enclosure, with the advantage of having an extremely high quality factor and very high shunt impedance at room temperature, and it is therefore proposed as a potential alternative to conventional disk-loaded copper structures. However, it has been observed that these structures suffer from multipactor discharges. In fact, multipactor is one of the main problems of these devices, as it limits the maximum accelerating gradient. Because of this, the analysis of multipactor risk in the early design steps of DAA cavities is crucial to ensure the correct performance of the device after fabrication. In this paper, we present a comprehensive and detailed study of multipactor in our DAA design through numerical simulations performed with an in-house developed code based on the Monte-Carlo method. The phenomenology of the multipactor (resonant electron trajectories, electron flight time between impacts, etc.) is described in detail for different values of the accelerating gradient. It has been found that in these structures an ultra-fast non-resonant multipactor appears, which is different from the types of multipactor theoretically studied in the scientific literature. In addition, the effect of several low electron emission coatings on the multipactor threshold is investigated. Furthermore, a novel design based on the modification of the DAA cell geometry for multipactor mitigation is introduced, which shows a significant increase in the accelerating gradient handling capabilities of our prototype.
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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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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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Gonzalez-Iglesias, D., Aksoy, A., Esperante, D., Gimeno, B., Latina, A., Boronat, M., et al. (2021). X-band RF photoinjector design for the CompactLight project. Nucl. Instrum. Methods Phys. Res. A, 1014, 165709–10pp.
Abstract: RF photoinjectors have been under development for several decades to provide the high-brightness electron beams required for X-ray Free Electron Lasers. This paper proposes a photoinjector design that meets the Horizon 2020 CompactLight design study requirements. It consists of a 5.6-cell, X-band (12 GHz) RF gun, an emittance-compensating solenoid and two X-band traveling-wave structures that accelerate the beam out of the space-charge-dominated regime. The RF gun is intended to operate with a cathode gradient of 200 MV/m, and the TW structures at a gradient of 65 MV/m. The shape of the gun cavity cells was optimized to reduce the peak electric surface field. An assessment of the gun RF breakdown likelihood is presented as is a multipacting analysis for the gun coaxial coupler. RF pulse heating on the gun inner surfaces is also evaluated and beam dynamics simulations of the 100 MeV photoinjector are summarized.
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Fuster, J., Irles, A., Melini, D., Uwer, P., & Vos, M. (2017). Extracting the top-quark running mass using t$(t)over-bar-$+1-jet events produced at the Large Hadron Collider. Eur. Phys. J. C, 77(11), 794–9pp.
Abstract: We present the calculation of the next-to-leading order QCD corrections for top-quark pair production in association with an additional jet at hadron colliders, using the modified minimal subtraction scheme to renormalize the top- quark mass. The results are compared to measurements at the Large Hadron Collider run I. In particular, we determine the top-quark running mass from a tit of the theoretical results presented here to the LHC data.
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Fuster, J., Garcia, I., Gomis, P., Perello, M., Ros, E., & Vos, M. (2015). Study of single top production at high energy electron positron colliders. Eur. Phys. J. C, 75(5), 223–7pp.
Abstract: The effect of single top production on the study of top quark pair production in future high energy electron-positron colliders is evaluated. The rate of the single top quark production process is sizeable throughout a large range of center-of-mass energies and the final state cannot easily be distinguished from the dominant pair production process. We discuss the impact on the top quark mass extraction from a scan through the pair production threshold and the determination of top quark form factors in the continuum. These results advocate for the exploration of the inclusive e(+) e(-) -> W(+)bW(-)b(-) process, that includes both top quark pair and single top quark production.
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DEPFET collaboration(Alonso, O. et al), Boronat, M., Esperante-Pereira, D., Fuster, J., Garcia, I. G., Lacasta, C., et al. (2013). DEPFET Active Pixel Detectors for a Future Linear e(+)e(-) Collider. IEEE Trans. Nucl. Sci., 60(2), 1457–1465.
Abstract: The DEPFET collaboration develops highly granular, ultra-transparent active pixel detectors for high-performance vertex reconstruction at future collider experiments. The characterization of detector prototypes has proven that the key principle, the integration of a first amplification stage in a detector-grade sensor material, can provide a comfortable signal to noise ratio of over 40 for a sensor thickness of 50-75 μm. ASICs have been designed and produced to operate a DEPFET pixel detector with the required read-out speed. A complete detector concept is being developed, including solutions for mechanical support, cooling, and services. In this paper, the status of the DEPFET R & D project is reviewed in the light of the requirements of the vertex detector at a future linear e(+)e(-) collider.
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DELPHI Collaboration(Abdallah, J. et al), Costa, M. J., Ferrer, A., Fuster, J., Garcia, C., Oyanguren, A., et al. (2010). Study of the dependence of direct soft photon production on the jet characteristics in hadronic Z (0) decays. Eur. Phys. J. C, 67(3-4), 343–366.
Abstract: An analysis of the direct soft photon production rate as a function of the parent jet characteristics is presented, based on hadronic events collected by the DELPHI experiment at LEP1. The dependences of the photon rates on the jet kinematic characteristics (momentum, mass, etc.) and on the jet charged, neutral and total hadron multiplicities are reported. Up to a scale factor of about four, which characterizes the overall value of the soft photon excess, a similarity of the observed soft photon behavior to that of the inner hadronic bremsstrahlung predictions is found for the momentum, mass, and jet charged multiplicity dependences. However for the dependence of the soft photon rate on the jet neutral and total hadron multiplicities a prominent difference is found for the observed soft photon signal as compared to the expected bremsstrahlung from final state hadrons. The observed linear increase of the soft photon production rate with the jet total hadron multiplicity and its strong dependence on the jet neutral multiplicity suggest that the rate is proportional to the number of quark pairs produced in the fragmentation process, with the neutral pairs being more effectively radiating than the charged ones.
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