Influence of ion-induced secondary electron emission parameters in PICMC plasma simulations with experimental validation in DC cylindrical diode and magnetron discharges

Niobium thin lms are used at CERN (European Organization for Nuclear Research) for coatings of superconducting radio-frequency (SRF) accelerating cavities. Numerical simulations can help to better understand the physical processes involved in such coatings and provide predictions of thin lm properties. In this article, Particle-in-Cell Monte Carlo (PICMC) 3D plasma simulations are validated against experimental data in a coaxial cylindrical system allowing both DC diode and DC magnetron operation. A proper choice of ion induced secondary electron emission (IISEE) parameters enables to match experimental and simulated discharge currents and voltages, with argon as the process gas and niobium as the target element. Langmuir probe measurements are presented to further support simulation results. The choice of argon gas with a niobium target is driven by CERN applications, but the methodology described in this paper is applicable to other discharge gases and target elements. Validation of plasma simulations is the rst step towards developing an accurate methodology for predicting thin lm coatings characteristics in complex objects such as SRF cavities.

Experiments and modelling for glow discharge plasmas applied to niobium sputter deposition in superconducting radiofrequency cavities

Thibaut François Hugo Richard

Thin film coatings are ubiquitous in modern daily lives, from the semi-conductor industry to aesthetic applications, and rely on the modification of a given material to enhance its surface properties. At CERN (European Organization for Nuclear Research), thin film coatings are used for several applications related to the accelerator technology field, such as electron cloud mitigation, continuous pumping or radio-frequency (RF) superconductivity. This PhD thesis aims at studying the use of a Particle-in-Cell Monte Carlo/ Direct Simulation Monte Carlo (PICMC/DSMC) numerical code applied to the modelling of deposition processes. The present study focuses on niobium thin film deposition on copper RF cavities of peculiar geometries and large sizes. Numerical simulations can help to understand and improve process parameters in existing systems, and guide the design of future coating systems such that thin ï¬lm characteristics can be predicted and expensive times of Research and Development can be reduced. In the first part of this thesis, critical parameters are identified to provide an accurate modelling of glow discharge plasmas commonly used in coating applications with the PICMC module of the numerical code. A dedicated experimental system is designed to enable both DC diode and DC magnetron operation with a coaxial cylindrical plasma source. Suitable ion-induced secondary electron emission yield and energy distribution are numerically assessed by matching simulated discharge voltages and currents with experimental ones. Simulated local plasma parameters, such as electron density and energy, are compared with experimental measurements obtained with a Langmuir probe. In the second part, transport simulations of sputtered neutral niobium atoms performed with the DSMC method are validated by comparing simulated deposition rates on a substrate with experimental ones with a compact hollow cathode magnetron sputtering source. At last, the validated ab initio methodology for coating process modelling, from plasma simulation and extraction of sputtering profiles to sputtered niobium atoms transport, is applied to typical coating systems used at CERN. The scalability of low discharge power simulation results to realistic powers is first demonstrated for a planar magnetron source in terms of cathode erosion and deposited thin film thickness profiles. Then, an elliptical RF cavity is used as a case study to apply the full methodology in a real substrate of complex shape. PICMC and DSMC simulation results are further extended towards thin film growth modelling, and simulated thin film morphology is compared with experimental Focused Ion Beam/Scanning Electron Microscopy imaging of the niobium layer.

EPFL2019

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.

EPFL2017

Object-oriented approach for dynamic system modelling and simulation

Adel Besrour

Software engineering always cares to provide solutions for building applications as close as possible to what they should be, according to the requirements and the final users needs. Systems behavior simulation is a very common application to virtually reproduce and often predict the real-world behavior. Simulation is one of the most operational research tool in a large variety of engineering and scientific domains: Transport, telecommunication, medicine, chemical processes, physics, etc. The complexity of such application is relative to the increasing complexity of the systems. In this context, it is relevant to bring together different tools and formalisms such as markovian chain, Petri nets, etc., to improve the existent approaches and so to answer the simulations performances needs. The principle objective of this thesis is to bring together techniques from software engineering and safety engineering in order to improve the state of the art of modeling and simulation of dynamic systems in the industrial context. In addressing this objective, this work initially involves defining the essential limitations of the used formalisms, methods and tools regarding from one hand the software engineering modeling and simulation techniques and from the other hand the existent risk analysis methodologies. This work is conducted with respect to the problem of danger identification, considering the context of the complex systems behavior and their interaction with the human operator. In software engineering, it is well known that Petri/high-level nets have attractive characteristics to be used in systems simulation and behavior prediction such as the natural graphical representation, and their well-defined semantic. They are well-suited for the description of complex situations with concurrency (interleaving and true concurrency depending on the underlying semantics), conflict and confusion. However, the absence of structuring capabilities has been one of the main criticisms raised against Petri nets/high-level nets. Thus, there have been many attempts to introduce structuring principles in nets of this kind [BCM88] [Kie89] [JR91]. The attractive characteristics of Petri/high-level nets have prompted researchers to enrich these formalisms with object-oriented features. CO-OPN (Concurrent Object-Oriented Petri Net) approach, brings together the power of both Petri/high-level nets and object-orientation techniques, it has been devised so as to offer an adequate framework for the specification and design of large scale concurrent system [BG91]. CO-OPN, as a powerful modeling tool, has been used in a limited way to simulate systems. This work aims to provide a CO-OPN extension to allow a more realistic systems' simulation. Actually, its simulator semantic uses to be a suitable approach for modeling near closed systems and software components, because they need to loose coupling with external world. But, when we model more realistic problems like industrial processes, where human interaction is a relevant event, this approach is not sufficient to catch all system activity attributes. Moreover, the CO-OPN interpretation process does not allow interaction with the object internal states. This work provides a new solution to overcome CO-OPN simulation limitations and a set of prototypes to assist dynamic systems simulations. Furthermore, this work has been conducted in a Risk Analysis (RA) context, a domain where computer-based simulations research are of utmost interest. Actually, classical approaches used to address complex workplace hazard in a limited way, using checklists or sequence models. Moreover, the use of single oriented methods, such as AEA (man-oriented), FMEA (machine oriented) or HAZOP (process oriented), is not satisfactory to overcome the increasing sophistication of industrial processes. The automation of a part of the analysis process as well as the multiple-oriented approach allowed by dynamic modeling may indeed enhance significantly the analysis completeness and reduce the time analyzing time. This work, based on Object Oriented Petri net formalism (CO-OPN), propose an alternative multi-oriented approach where existent methods limitations have been criticized to develop a dynamic model, MORM (Man-machine Occupational Risk Modeling). A real industrial system (metal wire making process) has been specified to implement the different approach steps (system identification, model application, system simulation, system analysis).

EPFL2005