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.

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Plasma{material interactions in current tokamaks and their implications for next step fusion reactors

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.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Non-thermal atmospheric pressure plasma jets/plumes are playing an increasingly important role in various plasma processing applications. This is because of their practical capability to provide plasmas that are not spatially bound or confined by electrodes. This capability is very desirable in many situations such as in biomedical applications. Various types of ‘cold’ plasma jets have, therefore, been developed to better suit specific uses. In this paper a review of the different cold plasma jets developed to date is presented. The jets are classified according to their power sources, which cover a wide frequency spectrum from DC to microwaves. Each jet is characterized by providing its operational parameters such as its electrodes system, plasma temperature, jet/plume geometrical size (length, radius), power consumption, and gas mixtures used. Applications of each jet are also briefly covered.

Arc-Free Atmospheric Pressure Cold Plasma Jets: A Review

http://doc.rero.ch/record/9704/files/Shah_A.V._-_Material_and_solar_cell_research_20080822.pdf

Material and solar cell research in microcrystalline silicon

Macromolecular plasma-chemistry: an emerging field of polymer science

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.

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Frequency effects in silane plasmas for plasma enhanced chemical vapor deposition

The time‐resolved fluxes of negative polysilicon hydride ions from a power‐modulated rf silane plasma have been measured by quadrupole mass spectrometry and modeled using a simple polymerization scheme. Experiments were performed with plasma parameters suitable for high‐quality amorphous silicon deposition. Polysilicon hydride anions diffuse from the plasma with low energy (approximately 0.5 eV) during the afterglow after the electron density has decayed and the sheath fields have collapsed. The mass dependence of the temporal behavior of the anion loss flux demonstrates that the plasma composition is influenced by the modulation frequency. The negative species attain much higher masses than the positive or neutral species and anions containing as many as sixteen silicon atoms have been observed, corresponding to the 500 amu limit of the mass spectrometer. This suggests that negative ions could be the precursors to particle formation. Ion–molecule and ion–ion reactions are discussed and a simple negative ion polymerization scheme is proposed which qualitatively reproduces the experimental results. The model shows that the densities of high mass negative ions in the plasma are strongly reduced by modulation frequencies near 1 kHz. Each plasma period is then too short for the polymerization chain to propagate to high masses before the elementary anions are lost in each subsequent afterglow period. This explains why modulation of the rf power can reduce particle contamination. We conclude that for the case of silane rf plasmas, the initiation steps which ultimately lead to particle contamination proceed by negative ion polymerization.

Time-resolved measurements of highly polymerized negative ions in radio frequency silane plasma deposition experiments

Negative ions have been clearly identified in silane rf plasmas used for the deposition of amorphous silicon. Mass spectra were measured for monosilicon up to pentasilicon negative ion radical groups in power‐modulated plasmas by means of a mass spectrometer mounted just outside the glow region. Negative ions were only observed over a limited range of power modulation frequency which corresponds to particle‐free plasma conditions. The importance of negative ions regarding particulate formation is demonstrated and commented upon.

Negative-Ion Mass-Spectra and Particulate Formation in Radio-Frequency Silane Plasma Deposition Experiments

Diagnostics of particle genesis and growth in RF silane plasmas by ion mass spectrometry and light scattering

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Stable negative ions containing up to sixteen silicon atoms have been measured by mass spectrometry in RF power-modulated silane plasmas for amorphous silicon deposition. These hydrogenated silicon cluster ions reach much higher masses than the positive ions, which have no more then six silicon atoms. This supports the view that negative ions are the precursors to particulate formation in silane plasmas. The time-dependent fluxes of positive and negative ions from the plasma are shown with a 5 mu s time resolution. Possible cluster reaction sequences are discussed and the effect of visible light on the negative ion signal is commented upon.

/pdf/negative-hydrogenated-silicon-ion-clusters-as-particle-12gw8y3wh4.pdf

J.-L. Dorier

Papers

Frequency effects in silane plasmas for plasma enhanced chemical vapor deposition

Time-resolved measurements of highly polymerized negative ions in radio frequency silane plasma deposition experiments

Negative-Ion Mass-Spectra and Particulate Formation in Radio-Frequency Silane Plasma Deposition Experiments

Diagnostics of particle genesis and growth in RF silane plasmas by ion mass spectrometry and light scattering

Negative hydrogenated silicon ion clusters as particle precursors in RF silane plasma deposition experiments