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.

X-band RF photoinjector design for the CompactLight project

Abstract CompactLight is a Design Study funded by the European Union under the Horizon 2020 research and innovation funding programme, with Grant Agreement No. 777431. CompactLight was conducted by an International Collaboration of 23 international laboratories and academic institutions, three private companies, and five third parties. The project, which started in January 2018 with a duration of 48 months, aimed to design an innovative, compact, and cost-effective hard X-ray FEL facility complemented by a soft X-ray source to pave the road for future compact accelerator-based facilities. The result is an accelerator that can be operated at up to 1 kHz pulse repetition rate, beyond today’s state of the art, using the latest concepts for high brightness electron photoinjectors, very high gradient accelerating structures in X-band, and novel short-period undulators. In this report, we summarize the main deliverable of the project: the CompactLight Conceptual Design Report, which overviews the current status of the design and addresses the main technological challenges. 

The CompactLight Design Study

The synergistic effects of a hybrid nanocomposite (Fe3O4@NrGO) and their influence on electromagnetic pollution shielding (EMI SE) were explored. This is a current concern where the impacts need to be fully clarified and assertively addressed. Thus, the main focus of the paper was based on the physical and mechanical properties of porosity, surface area, flexibility and, mainly, electrical conductivity of the graphene aerogels, which do not require the use of a binder and are of great interest for the EMI SE area. The precursors (rGO, NrGO and Fe3O4), as well as the nanocomposite, underwent thorough structural characterisation to understand their nature. Characteristic bands of C-N/C=N bonds and a content of 8.03 at% N were identified, with a high N:O ratio of 1.31, indicating efficient doping of the conductive matrix. The aerogel NrGO phase, with a quality of just 5 layers, and the Fe3O4 phase, with domains around 5 nm, were confirmed, as well as the superposition of the signals in the Fe3O4@NrGO pattern. Electron microscopy images and elemental map also demonstrated the efficient distribution of Fe3O4 in the matrix and its porous appearance. Thus, leveraging the advantages of aerogel, extremely light ultra-thin pellets measuring 0.1 mm and weighing 10 mg were produced. The high nitrogen doping of the graphene network, the presence of magnetic nanoparticles, as well as the high surface area and porosity achieved in the production of the nanocomposite, were responsible for high specific shielding effectiveness of 2730 dB · cm−1 at 10.7 GHz and an average in the X-band of 1590 dB · cm−1. Thus, the improved and synergistic mechanisms highlighted the promising advantages of using these highly porous doped materials, demonstrating that the Fe3O4@NrGO hybrid presents high value for investigations in the application of EMI SE. 

Binder-free Ultrathin Pellets of Nanocomposites based on Fe3O4@Nitrogen-doped Reduced Graphene Oxide Aerogel for Electromagnetic Interference Shielding

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.

/pdf/analytical-rf-pulse-heating-analysis-for-high-gradient-33p84ncpt5.pdf

Analytical RF Pulse Heating Analysis for High Gradient Accelerating Structures

The ultrafast electron beam diffraction technology, which involves an electron beam at the level of several megaelectron volts, is crucial for studying ultrafast dynamic processes at the atomic level. To discern these processes at a temporal resolution of a few femtoseconds, the demand to focus on electron beams with femtosecond or even subfemtosecond width and time jitter is increasing. In this paper, we investigate the dynamics of electron beams as they interact with lasers, present an explanation of the mechanism that results in time jitter, and propose a method to mitigate or possibly eliminate the time jitter by inducing a specific energy chirp in the electron beam through the use of $X$-band photocathode rf gun. The feasibility of this method is confirmed by implementing bunch compression with two radially polarized laser pulses. Simulation results demonstrate that the time jitter can be reduced to less than 1 fs when the amplitude jitter of the rf gun is within $\ifmmode\pm\else\textpm\fi{}0.05%$ and the phase jitter is within $\ifmmode\pm\else\textpm\fi{}0.5\ifmmode^\circ\else\textdegree\fi{}$. Based on our findings, we anticipate that this approach will substantially advance the field of ultrafast science and technology. 

Ultrashort electron bunches with subfemtosecond jitter from an 
X
-band photocathode rf gun

An S-band High-Gradient (HG) Radio Frequency (RF) laboratory is under construction and commissioning at IFIC. The purpose of the laboratory is to perform investigations of high-gradient phenomena and to develop normal-conducting RF technology, with special focus on RF systems for hadron-therapy. The layout of the facility is derived from the scheme of the Xbox-3 test facility at CERN [1] and uses medium peak-power (7.5 MW) and high repetition rate (400 Hz) klystrons, whose RF output is combined to drive two testing slots to the required power. The design and construction of the various components of the system started in 2016 and has been completed. The installation and commissioning of the laboratory is progressing, with first results expected before mid-2018. The technical characteristics of the different elements of the system and the commissioning status together with preliminary results are described.

/pdf/construction-and-commissioning-of-the-s-band-high-gradient-44ao7tcu0q.pdf

Construction and Commissioning of the S-Band High-Gradient RF Laboratory at IFIC

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X-band RF photoinjector design for the CompactLight project

Construction and Commissioning of the S-Band High-Gradient RF Laboratory at IFIC

Analytical RF Pulse Heating Analysis for High Gradient Accelerating Structures