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Ch. Hollenstein

Bio: Ch. Hollenstein is an academic researcher from École Polytechnique Fédérale de Lausanne. The author has contributed to research in topics: Plasma & Silane. The author has an hindex of 34, co-authored 136 publications receiving 3990 citations.


Papers
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Journal ArticleDOI
TL;DR: An overview of the most recent experimental and modelling efforts on powder formation in reactive plasmas is given in this paper, where particle charging and charge fluctuations regarding the particle agglomeration is emphasised.
Abstract: An overview of the most recent experimental and modelling efforts on powder formation in reactive plasmas is given. The physics and chemistry of these dusty plasmas and their fundamental mechanisms leading to the production of nanometre-sized particles and their successive agglomeration leading to micrometre-sized particles are reviewed. The central role of particle charging and of charge fluctuations regarding the particle agglomeration is emphasised. Finally, the influence of the dust particles on the plasma parameters is described and an outlook on the most eminent problems towards the understanding of the reactive, dusty plasmas is given.

205 citations

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TL;DR: In this paper, contact angle measurements, FTIR-ATR, AFM, and XPS were used to study the surface structure and dynamics of PET surfaces, showing that the surface acquired oxygen containing polar functional groups such as -C=O, -OH, and -OOH.
Abstract: Plasma treatment of PET films was carried out under argon, followed by exposure to an oxygen atmosphere. The films underwent considerable changes in surface composition and morphology, as demonstrated by contact angle measurements, FTIR-ATR, AFM, and XPS. It was found that the surface acquired oxygen containing polar functional groups such as -C=O, -OH, and -OOH, which increased in number as the plasma treatment time increased. During storage, the treated films underwent significant surface reorganization, and both the time and temperature contributed to the increase in the contact angle. As revealed by AFM measurements, these changes were accompanied by an increase in roughness in the form of ridges. The ridges were observed to grow in height with increasing treatment time, although their spacing showed little evolution. A correlation among the observations obtained from various techniques was established, giving a comprehensive picture of the structure and dynamics of plasma-treated PET surfaces. (C) 2000 John Wiley & Sons, Inc.

201 citations

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TL;DR: In this paper, a combination of IR absorption spectroscopy and mass spectrometry has been applied to dusty radiofrequency (RF) plasmas in methane, acetylene and ethylene.
Abstract: Infrared (IR) absorption spectroscopy and mass spectrometry have been simultaneously applied to dusty radiofrequency (RF) plasmas in methane, acetylene and ethylene. The combination of IR absorption spectroscopy and mass spectrometry allows the chemical composition and structure of the most relevant plasma-produced neutral species, the ionic plasma composition and the chemical composition of the nanometer-sized particles to be precisely identified. The production of acetylenic compounds (C2Hx) seems to be a key mechanism for the powder formation in all the investigated hydrocarbon plasmas. Electron attachment to acetylenic compounds and the following ion-neutral reactions might lead to the high-mass carbon anions, which are trapped in the plasma and finally end in powder formation. The hydrogenation of the monomer strongly influences the composition of the ions. Finally the composition of the plasma-produced particles is mainly sp3 bonded carbon and the infrared spectra show similarities to that of polyethylene.

201 citations

Journal ArticleDOI
TL;DR: In this paper, a shape control algorithm based on finite element reconstruction of the plasma current in real time was implemented, which achieved a stability margin f=1.15 in the first year of operation of the TCV tokamak.
Abstract: During the first year of operation, the TCV tokamak has produced a large variety of plasma shapes and magnetic configurations, with 1.0

178 citations

Journal ArticleDOI
TL;DR: In this paper, the authors compared the excitation frequency in radio-frequency (RF) plasmas with the 13.56 MHz industrial frequency in the same reactor and presented a comparative study of key discharge parameters such as deposition rates, plasma uniformity, ion impact energy, power transfer efficiency, and powder formation.
Abstract: It is now generally recognized that the excitation frequency is an important parameter in radio‐frequency (rf) plasma‐assisted deposition. Very‐high‐frequency (VHF) silane plasmas (50–100 MHz) have been shown to produce high quality amorphous silicon films up to 20 A/s [H. Curtins, N. Wyrsch, M. Favre, and A. V. Shah, Plasma Chem. Plasma Processing 7, 267 (1987)], and therefore the aim of this work is to compare the VHF range with the 13.56 MHz industrial frequency in the same reactor. The principal diagnostics used are electrical measurements and a charge coupled device camera for spatially resolved plasma‐induced emission with Abel inversion of the plasma image. We present a comparative study of key discharge parameters such as deposition rates, plasma uniformity, ion impact energy, power transfer efficiency, and powder formation for the rf range 13–70 MHz.

165 citations


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TL;DR: In this article, the authors review the underlying physical processes and the existing experimental database of plasma-material interactions both in tokamaks and laboratory simulation facilities for conditions of direct relevance to next-step fusion reactors.
Abstract: The major increase in discharge duration and plasma energy in a next step DT fusion reactor will give rise to important plasma-material effects that will critically influence its operation, safety and performance. Erosion will increase to a scale of several centimetres from being barely measurable at a micron scale in today's tokamaks. Tritium co-deposited with carbon will strongly affect the operation of machines with carbon plasma facing components. Controlling plasma-wall interactions is critical to achieving high performance in present day tokamaks, and this is likely to continue to be the case in the approach to practical fusion reactors. Recognition of the important consequences of these phenomena stimulated an internationally co-ordinated effort in the field of plasma-surface interactions supporting the Engineering Design Activities of the International Thermonuclear Experimental Reactor project (ITER), and significant progress has been made in better understanding these issues. The paper reviews the underlying physical processes and the existing experimental database of plasma-material interactions both in tokamaks and laboratory simulation facilities for conditions of direct relevance to next step fusion reactors. Two main topical groups of interaction are considered: (i) erosion/redeposition from plasma sputtering and disruptions, including dust and flake generation and (ii) tritium retention and removal. The use of modelling tools to interpret the experimental results and make projections for conditions expected in future devices is explained. Outstanding technical issues and specific recommendations on potential R&D avenues for their resolution are presented.

1,187 citations

01 Jan 1993
TL;DR: In this article, particle-in-cell (PIC) combined with Monte Carlo collision (MCC) calculations are used for simulation of partially ionized gases, with many of the features met in low-temperature collision plasmas.
Abstract: Many-particle charged-particle plasma simulations using spatial meshes for the electromagnetic field solutions, particle-in-cell (PIC) merged with Monte Carlo collision (MCC) calculations, are coming into wide use for application to partially ionized gases. The author emphasizes the development of PIC computer experiments since the 1950s starting with one-dimensional (1-D) charged-sheet models, the addition of the mesh, and fast direct Poisson equation solvers for 2-D and 3-D. Details are provided for adding the collisions between the charged particles and neutral atoms. The result is many-particle simulations with many of the features met in low-temperature collision plasmas; for example, with applications to plasma-assisted materials processing, but also related to warmer plasmas at the edges of magnetized fusion plasmas. >

1,022 citations

Journal ArticleDOI
TL;DR: An overview of gas discharge plasmas can be found in this paper, where the most important applications of these and related plasmmas are discussed, as well as their working principles.

928 citations

Journal Article
TL;DR: In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and the phonon broadening of these lines is considered.
Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

788 citations

Journal ArticleDOI
TL;DR: A review of measurements of microscopic fluctuations and theories of turbulence and anomalous transport for tokamaks is given in this article, and some comparisons between theory and experiment are presented.
Abstract: A review of measurements of microscopic fluctuations and theories of turbulence and anomalous transport for tokamaks is given, and some comparisons between theory and experiment are presented. The results of the measurements indicate that all tokamaks have rather similar, broadband, incoherent microscopic fluctuations. Such fluctuations have been measured in the density, potential, electric field, and magnetic field. In the edge regions of three tokamaks, the particle transport caused by the turbulent electric field fluctuations has been measured directly. Although tokamak microturbulence has been studied extensively, neither its source nor its role in anomalous energy transport is yet understood. The incoherent, turbulent nature of the fluctuations has made it difficult to understand them theoretically. Recently, however, significant theoretical progress has been made in several areas including non-linear models of drift wave turbulence and transport, models of anomalous electron thermal conduction by stochastic magnetic field fluctuations, and non-linear models of localized resistive-MHD instabilities.

726 citations