Electromagnetic Simulation of CERN Accelerator Components and Experimental Applications

Wakes and impedances of single accelerator elements can be obtained by means of theoretical calculation, electromagnetic (EM) simulations or bench measurements. Since theoretical calculations apply only to simple structures and bench measurements have some intrinsic limitations, EM simulations can be used as a reliable tool to determine wakes and impedances. This thesis will focus on the use of time domain 3D CST Particle Studio EM simulations to calculate wakes and/or impedances. First, the results of the EM simulations are compared with the known analytical solutions and other codes. In this exercise, the driving and the detuning terms of the wakes/impedances, in the transverse plane, are disentangled for both symmetric and asymmetric geometries. The sensitivity of the simulations results to the numerical parameters is discussed, as well as the limits of validity of the wake formalism and its extension to the nonlinear regime. Using the CST Wakefield Solver, the SPS kicker impedance contribution is then estimated. The simulation model was improved step by step, and successfully benchmarked with existing and new theoretical models, giving confidence in the numerical results and allowing a better understanding of the EM problems. In the case of the resistive wall impedance of simple chamber geometries a handy theoretical model has been proposed. In order to calculate the resistive wall impedance of a round chamber, a theoretical approach based on the transmission line (TL) theory, is demonstrated to be valid and practical to use. By means of appropriate form factors the method is then extended to rectangular or elliptical chambers. Moreover the method was successfully benchmarked with the most recent codes based on the field matching technique developed at CERN and was used to construct the SPS wall impedance model. For more complicated geometries (asymmetries, small inserts, holes etc.), a theoretical estimation without involving EM simulation becomes unworkable. An example of interest is the LHC beam-screen, for which CST 3D simulations were used to estimate the impedance. In order to allow the simulator to cover the whole frequency range of interest (few KHz to several tens of MHz) a novel scaling technique was developed and applied. Where possible, the EM models developed throughout this thesis were also successfully benchmarked with bench measurements (wire methods) and observations with beam. On the specific subject of bench measurements, a numerical investigation of coaxial wire measurements has been also presented. Finally, in order to verify the adopted EM models of the materials both in theoretical calculations and 3D simulations, an experimental setup for measuring EM properties (permittivity and permeability) of materials has been presented. The method is based on fitting the measured reflection transmission coefficients through a coaxial line filled with the material to be probed, with the outcome of EM simulations or theoretical models. It was successfully applied for measuring NiZn ferrites and dielectric material (e.g. SiC)

Coaxial Wire Measurements of Ferrite Kicker Magnets

Fast kicker magnets are used to inject beam into and eject beam out of the CERN accelerator rings. These kickers are generally transmission line type magnets with a rectangular shaped aperture through which the beam passes. Unless special precautions are taken the impedance of the yoke can provoke significant beam induced heating, especially for high intensities. In addition the impedance may contribute to beam instabilities. The results of longitudinal and transverse impedance measurements, for various kicker magnets, are presented and compared with analytical calculations: in addition predictions from a numerical analysis are discussed

EM SIMULATIONS IN BEAM COUPLING IMPEDANCE STUDIES: SOME EXAMPLES OF APPLICATION

In the frame of the SPS upgrade an accurate impedance model is needed in order to predict the instability threshold and if necessary to start a campaign of impedance reduction. Analytical models, 3-D simulations and bench measurements are used to estimate the impedance contribution of the different devices along the machine. Special attention is devoted to the estimation of the impedance contribution of the kicker magnets that are suspected to be the most important impedance source in SPS. In particular a numerical study is carried out to analyze the effect of the serigraphy in the SPS extraction kicker. An important part of the devices simulations are the ferrite model. For this reason a numerical based method to measure the electromagnetic properties of the material has been developed to measure the ferrite properties. A simulation technique, in order to account for external cable is developed. The simulation results were benchmarked with analytical models and observations with beam. A numerical study was also performed to investigate the limits of the wire method for beam coupling impedance measurements

QUADRUPOLAR TRANSVERSE IMPEDANCE OF SIMPLE MODELS OF KICKERS

The SPS kickers are major contributors to the SPS transverse beam coupling impedance. The current ”flat chamber” impedance model for a kicker is obtained by applying form factors to the theoretical impedance of an axisymmetric ferrite beam pipe. This model was believed to be acceptable for the vertical dipolar impedance, as twowire measurements on SPS kickers revealed a satisfactory agreement. However, one-wire measurements on PS kickers suggested that this model underestimates the kickers’ transverse quadrupolar (detuning) impedance. The longitudinal and transverse dipolar impedances of another kicker model that accounts for the metallic plates on each side of the ferrite were derived in the past by H. Tsutsui. The same formalism is used in this paper to derive the quadrupolar impedance. These formulae were then successfully benchmarked to electromagnetic (EM) simulations. Finally, simulating the interaction of an SPS bunch with the improved kickers’ model results in a positive horizontal tune shift, which is very close to the tune shift measured with the SPS beam

Update of the SPS Impedance Model

The beam coupling impedance of the CERN SPS is expected to be one of the limitations to an intensity upgrade of the LHC complex. In order to be able to reduce the SPS impedance, its main contributors need to be identified. An impedance model for the SPS has been gathered from theoretical calculations, electromagnetic simulations and bench measurements of single SPS elements. The current model accounts for the longitudinal and transverse impedance of the kickers, the horizontal and vertical electrostatic beam position monitors, the RF cavities and the 6.7 km beam pipe. In order to assess the validity of this model, macroparticle simulations of a bunch interacting with this updated SPS impedance model are compared to measurements performed with the SPS beam

ELECTROMAGNETIC SIMULATIONS OF SIMPLE MODELS OF FERRITE LOADED KICKERS

The kickers are major contributors to the CERN SPS beam coupling impedance. As such, they may represent a limitation to increasing the SPS bunch current in the frame of an intensity upgrade of the LHC. In this paper, CST Particle Studio time domain electromagnetic simulations are performed to obtain the longitudinal and transverse impedances/wake potentials of simplified models of ferrite loaded kickers. The simulation results have been successfully compared with some existing analytical expressions. In the transverse plane, the dipolar and quadrupolar contributions to the wake potentials have been estimated from the results of these simulations. For some cases, simulations have also been benchmarked against measurements on PS kickers. It turns out that the large simulated quadrupolar contributions of these kickers could explain both the negative total (dipolar + quadrupolar) horizontal impedance observed in bench measurements and the positive horizontal tune shift measured with the SPS beam

Unveiling the morphogenetic code: A new path at the intersection of physical energies and chemical signaling

In this editorial, we discuss the remarkable role of physical energies in the control of cell signaling networks and in the specification of the architectural plan of both somatic and stem cells. In particular, we focus on the biological relevance of bioelectricity in the pattern control that orchestrates both developmental and regenerative pathways. To this end, the narrative starts from the dawn of the first studies on animal electricity, reconsidering the pioneer work of Harold Saxton Burr in the light of the current achievements. We finally discuss the most recent evidence showing that bioelectric signaling is an essential component of the informational processes that control pattern specification during embryogenesis, regeneration, or even malignant transformation. We conclude that there is now mounting evidence for the existence of a Morphogenetic Code, and that deciphering this code may lead to unprecedented opportunities for the development of novel paradigms of cure in regenerative and precision medicine

Effects of an Asymmetric Chamber on the Beam Coupling Impedance

The wake function of an accelerator device appears to have a constant term if the geometry of the device is asymmetric or when the beam passes off axis in a symmetric geometry. Its contribution can be significant and has to be taken into account. In this paper a generalized definition of the impedance/wake is presented to take into account also this constant term. An example of a device where the constant term appears is analyzed. Moreover, the impact of a constant wake on the beam dynamics is discussed and illustrated by a HEADTAIL simulation

ELECTROMAGNETIC CHARACTERIZATION OF MATERIALS FOR THE CLIC DAMPING RINGS

The performance of the Compact LinearCollider (CLIC) damping rings (DR) is likely to be limited by collective effects due to the unprecedented brilliance of the beams. Coating will be used in both electron (EDR) and positron damping rings (PDR) to suppress effects like electron cloud formation or ion instabilities. The impedance modeling of the chambers, necessary for the instabilities studies which will ensure safe operation under nominal conditions, must include the contribution from the coating materials applied for electron cloud mitigation and/or ultra-low vacuumpres- sure. This advocates for a correct characterization of this impedance in a high frequency range, which is still widely unexplored. The electrical conductivity of the materials in the frequency range of few GHz is determined with the waveguide method, based on a combination of experimen- tal measurements of the complex transmission coefficient S21 and CST 3D electromagnetic (EM) simulations