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.

Electromagnetic Simulation of CERN Accelerator Components and Experimental Applications

Carlo Zannini

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).

EPFL2013

Magneto-thermal finite element modeling of 2nd generation HTS for FCL design purposes

Bertrand Dutoit, Francesco Grilli, François Roy, Frédéric Sirois

Coated conductors are very promising for the design of novel and e±cient Fault Current Limiter (FCL). However, before considering using them in a power grid, their thermal and electromagnetic behaviors in the presence of over-critical currents need to be investigated in details. In this context, we performed ¯nite element magneto-thermal modeling of coated conductors under over-critical current on several geometries. Accordingly, we have investigated the substrate electrical connectivity and thermal properties on the HTS-FCL behavior. All simulations were performed using in COMSOL Multiphysics, a commercial finite element package, which has a built-in coupling between the thermal and electrical equations, allowing us to compute both quantities simultaneously during the solving process. Our simulations allowed us to formulate thresholds for the current density usable in coated HTS as well as limitation capability of a device made of these new conductors.

Iop Publishing Ltd, Dirac House, Temple Back, Bristol Bs1 6Be, England2008

H- ion source for CERN's Linac4 accelerator

Stefano Mattei

Linac4 is the new negative hydrogen ion (H-) linear accelerator of the European Organization for Nuclear Research (CERN). Its ion source operates on the principle of Radio-Frequency Inductively Coupled Plasma (RF-ICP) and it is required to provide 50 mA of H- beam in pulses of 600 us with a repetition rate up to 2 Hz and within an RMS emittance of 0.25 pi mm mrad in order to fullfil the requirements of the accelerator. This thesis is dedicated to the characterization of the hydrogen plasma in the Linac4 H- ion source. We have developed a Particle-In-Cell Monte Carlo Collision (PIC-MCC) code to simulate the RF-ICP heating mechanism and performed measurements to benchmark the fraction of the simulation outputs that can be experimentally accessed. The code solves self-consistently the interaction between the electromagnetic field generated by the RF coil and the resulting plasma response, including a kinetic description of charged and neutral species. A fully-implicit implementation allowed to simulate the high density regime of the Linac4 H- ion source, ensuring the energy conservation while maintaining the computational resources tractable. We studied the capacitive to inductive transition characteristic of the initial phase of the pulsed discharge. The simulation results were confirmed by time-resolved photometry measurements and allowed quantifying the effect of the hydrogen pressure and of the external magnetic cusp field on the transition dynamics. This provided insights into possible modifications to the magnetic cusp field configuration to maximize the power deposited to the plasma. The optimal ion source configuration maximizes the density of volume produced H-, the flux of H0 atoms onto the cesiated molybdenum plasma electrode surface at the origin of H- emission, and minimizes the electron density and energy in the beam formation region. We simulated the high-density regime (10^19 m-3) representative of the nominal operation of the Linac4 ion source during beam extraction. We performed a parametric study of the RF current, hydrogen pressure and magnetic configuration (cusp and filter) to assess their impact on the plasma parameters. The simulation results allowed assessment of these parameters and provided guidelines for the optimization of the ion source operational and design parameters. The simulation results of electron density, electron energy and hydrogen dissociation degree showed excellent agreement with optical emission spectroscopy measurements both as a function of RF coil current and magnetic configuration. The outputs of these simulations provide crucial inputs to beam formation and extraction physics models. Dedicated PIC software packages are being developed that will eventually shed insight into essential beam parameters such as the intensity and emittance of the H° beam and the intensity of the co-extracted electrons.