Multipacting Simulations Research Articles

On the parallelization of multipacting simulation codes for the design of particle accelerator components

Particle trajectory and collision simulation is a critical step of the design and construction of novel particle accelerator components. However it requires a huge computational effort which can slow down the design process. We started from a sequential simulation program which is used to study an event called Multipacting. Our work explains the physical problem that is simulated and the implications it can have on the behavior of the components. Then we analyze the original program’s operation to find the best options for parallelization. We first developed a parallel version of the Multipacting simulation and were able to accelerate the execution up to ∼35×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 35\ imes $$\\end{document} with 48 or 56 cores. In the best cases, parallelization efficiency was maintained up to 16 cores (∼95\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 95$$\\end{document}%) and the speed-up plateaus at around 40–48 cores. When this first parallelization effort was tried for multi-power simulations, we found that parallelism was severely limited with a maximum of 20×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$20\ imes $$\\end{document} speed-up. For this reason, we introduced a new method to improve the parallelization efficiency for this second use case. This method uses a shared processor pool for all simulations of electrons (OnePool). OnePool improved scalability by pushing the speed-up to over 32×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$32\ imes $$\\end{document}.

Design and optimization of a novel bent-vane type radio frequency quadrupole

A novel radio frequency quadrupole (RFQ) resonant structure with bent vanes is proposed at the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). Compared with the traditional four-vane type RFQs, the bent-vane type structure is more compact at low frequency with significant reduction in the cross section size, and is capable of using a simple cooling system due to its rotationally symmetric structure. An 81.25 MHz proof-of-principle cavity is now under development at IMP. The beam dynamics design, electromagnetic optimizations with multipacting simulations, multiphysics analyses, and mechanical design of this novel bent-vane type RFQ (BV-RFQ) will be presented in this paper.

Design studies of a compact superconducting rf crab cavity for future colliders using Nb/Cu technology

The design of a compact crab cavity using Nb/Cu technology is presented. The cavity shape is based on a ridged waveguide resonator with wide-open apertures to provide access to the inner surface of the cavity and facilitate coating. It also provides natural damping for higher-order modes (HOMs) and comparatively low longitudinal and transverse impedances. A first prototype is being fabricated, originally within the HL-LHC framework and now for the Future Circular Collider study. The optimized shape is characterized and compared with respect to peak surface field balance, multipolar momentum components, and longitudinal and transverse impedances. Further investigations include the identification of multipacting barriers, the frequency sensitivity to pressure fluctuations in the helium bath, the radio frequency (rf) design of a fundamental mode coupler, and considerations of HOM damping. Along these topics, several methods have been established serving a wider range of applications, in particular, for the rf loss evaluation, the postprocessing of multipacting simulations, and a detailed error analysis of the pressure sensitivity simulations.

Design and development of radio frequency output window for circular electron–positron collider klystron**Project supported by Yifang Wangʼs Science Studio of the Ten Thousand Talents Project, China.

This paper presents the first phase of design, analysis, and simulation for the klystron coaxial radio frequency (RF) output window. This study is motivated by 800 kW continuous wave (CW), 650 MHz klystrons for the future plan of circular electron–positron collider (CEPC) project. The RF window which is used in the klystron output section has a function to separate the klystron from the inner vacuum side to the outside, and high RF power propagates through the window with small power dissipation. Therefore, the window is a key component for the high power klystron. However, it is vulnerable to the high thermal stress and multipacting, so this paper presents the window design and analysis for these problems. The microwave design has been performed by using the computer simulation technology (CST) microwave studio and the return loss of the window has been established to be less than −90 dB. The multipacting simulation of the window has been carried out using MultiPac and CST particles studio. Through the multipacting analysis, it is shown that with thin coating of TiN, the multipacting effect has been suppressed effectively on the ceramic surface. The thermal analysis is carried out using ANSYS code and the temperature of alumina ceramic is lower than 310 K with water cooling. The design result successfully meets the requirement of the CEPC 650 MHz klystron. The manufacturing and high power test plan are also described in this paper.

Electromagnetic simulations, manufacturing and low power measurements of a 352.21 MHz RF power coupler for the SANAEM RFQ project

Radio frequency quadrupole (RFQ) linear accelerators occupy an important place among modern accelerators since the days it was conceived and constructed because of its ability to focus, bunch and accelerate the low energy beams at high efficiency. In Turkey, recently a 352.21 MHz RFQ accelerator cavity is designed and manufactured within a proton beamline located at the Turkish Atomic Energy Authority (TAEK), Sarayköy Nuclear Research and Training Center (SANAEM). The SANAEM RFQ cavity, operating in pulsed mode needs an overall RF power of 120 kW to accelerate the proton beam up to a final beam energy of 1.3 MeV . A new prototype RF power coupler, considering multipacting effect is designed, developed and manufactured to be used to deliver the required power for the RFQ. At the beginning of this study, design and 3D EM simulations of a coaxial power coupler considered for the SANAEM RFQ accelerator is presented. The RF properties of the coupler such as external quality factor, power loss on the loop, coupling coefficient and scattering parameters are evaluated and the geometrical parameters are optimized with the CST Microwave Studio. Also, various dielectric window materials to be used in the coupler are discussed. Moreover, multipacting simulations for the coupler are carried out using the CST Particle Studio and RF power levels at which the multipacting effect occurs are obtained. Finally, the manufacturing of a prototype coupler, low power measurements and discussions are presented in the present article.

Simulation of Multipacting with Space Charge Effect

The electron multiplication on surfaces exposed to an oscillating electromagnetic field causes the phenomenon of multipacting, which can degrade significantly the performance of vacuum RF devices, especially accelerating cavities. It is a serious obstacle to be avoided for normal operation of particle accelerator and their RF components. Many types of room temperature and superconducting accelerating cavities are designed and produced at Fermilab for different projects. The extensive simulations of multipacting in the cavities with updated material properties and comparison of the simulation results with experimental data are routinely performed during electromagnetic design of the cavities. The new advanced computing capabilities made it possible to take the space charge effect into account in the multipacting simulations. The basic new features of multipacting process that appear due to the space charge effect are shown for the classic case of the parallel plates and discussed. As the first practical application of the multipacting simulations with space charge effect the study of multipacting in the low-beta and high-beta 650 MHz elliptical superconducting cavities is also presented.

Preliminary design and multipacting simulation of a SRF quarter wave electron gun at Peking University

Peking University is designing a new SRF gun that is composed of a quarter wave resonator (QWR) and an elliptical cavity. Compared to the elliptical cavity, the QWR is sufficiently compact at the same frequency and its electric field is quasi-DC. The RF parameters are determined by optimization of QWR cavity structure and the possible multipacting locations are analyzed by 2D MP simulation. The simulation results show that multipacting is not a critical issue for our optimized cavity structure.

RF thermal and new cold part design studies on a TTF-III input coupler for Project-X

An RF power coupler is one of the key components in a superconducting (SC) linac. It provides RF power to the SC cavity and interconnects different temperature layers (1.8 K, 4.2 K, 70 K and 300 K). The TTF-III coupler is one of the most promising candidates for the High Energy (HE) linac of Project X, but it cannot meet the average power requirements because of the relatively high temperature rise on the warm inner conductor, so some design modifications will be required. In this paper, we describe our simulation studies on the copper coating thickness on the warm inner conductor with RRR values of 10 and 100. Our purpose is to rebalance the dynamic and static loads, and finally lower the temperature rise along the warm inner conductor. In addition, to get stronger coupling, better power handling and less multipacting probability, one new cold part design was proposed using a 60 mm coaxial line; the corresponding multipacting simulation studies have also been investigated.