20 – Plasma and fusion technology

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					2.0 – Plasma and fusion technology



2.1 Surface treatment of materials with plasma

        a) - A plant for SiOx film deposition on different materials is functioning. In
particular films with the followings characteristics have been produced:
* Gas barrier films on glass and PET.
* Hydrophobic films on plastics, metals, glass, ceramics, papers and textiles.
As an example, the modification of the wetting characteristics of application on a
textile material is reported. The plasma is used to increase or decrease the wetting
(measured by the decrease or the increase of the contact angle on the surface of
different liquids including water). Therefore, a surface can be transformed from
hydrophobic to hydrophilic and vice versa. The treatment time changes from some
seconds to minutes in function of the material.




         Fig. 2.1.i - Application of plasma processing on textile substrates. The surface textile
has been treated by plasma polymerisation. The contact angle for water is around 135° after
silicon-like polymerisation. The textile surface after treatment is totally water-repellent

These activities, fully supported by contracts with national industries, have
produced the industrial patent "Plasma treatment of textile fibers (natural,
synthetic and composite) and of animal hides for the conferring to the surface of
liquid repulsion properties (hydro and oil repulsion)".

        b) - Another recent research activity at the Istituto di Fisica del Plasma
refers to the removal or reduction of microbial contamination and paper
consolidation by means of plasma treatment. Deterioration of paper-based
materials is mainly due to degradation of cellulose that is caused by many factors
such as chemical attack due to acidic hydrolysis, oxidative agent, light, air pollution
and biological attack due to presence of micro-organism like bacteria and fungi. Our
activity aims to develop appropriate treatments for restoration and conservation
techniques. The results of this research have produced a CNR Patent (n°
Mi2004A000068, 21/01/2004).




2001/2004 IFP Activity Report                                                              23
        c) - Silicon-like thin films can be deposited on glass and silicon substrates
by Plasma Enhanced Chemical Vapour Deposition (PECVD). The investigation of the
optical properties of polymer films is particularly interesting because of their use in
optoelectronic devices. These films can be realized with different kinds of monomer
and different feed gases. Inorganic SiO2-like films as well as polymer-like films can
be realized, depending on the mixture of monomer and feed gases and on the
different plasma process conditions (pressure, applied power, deposition time,
geometrical configuration of the reaction chamber). A research activity at the
Istituto di Fisica del Plasma is the study of structural and optical properties of
Si:Ox:Cy:Hz films, deposited by PECVD method using hexamethyldisiloxane
(HMDSO) as precursor and Ar as feed gas. Optical properties, such as absorption
coefficient, energy gap and refractive index, have been investigated. Particularly we
attribute the width of energy band gap to the degree of disorder present in the
material and the value of refractive index to the degree of porosity and
compactness of the film.




2.2 RF ion species separation in plasma

        A plasma device with a cusp magnetic configuration has been completed. It
works at a steady state regime thanks to water-cooled solenoids; the maximum
magnetic field strengths are 0.25 T at the line cusp and 0.40 T at the point cusp,
respectively. A photo of the plasma device is shown in Fig. 2.2.i. The plasma source
is located at one point cusp and it is fed with gas through a mass flow controller
that maintains a discharge pressure from 10-3 to 10-2 mbar. The plasma source is a
rf capacitatively coupled type: it consists of an inner electrode rf powered at a fixed
frequency (13.6 MHz) while the vacuum tube is the outer (grounded) electrode.




     Fig. 2.2.i - Photographic view of the cusp plasma device.


      The produced plasma has a disc shape at the line cusp (see Fig. 2.2.ii for an
argon plasma). Plasma parameters are obtained from a Langmuir probe located at
the line cusp, the plasma density is, typically, 108 ~ 109 cm-3 and the electron
temperatures are about 10 eV. An additional diagnostics tools is furnished by
optical spectroscopy. The main experimental activity concerns the plugging and the
separation of the ion species comprising the plasma. This is achieved by coupling rf


24                                                        2001-2004 IFP Activity Report
power to the plasma. Radiofrequency field is applied to two ring shaped-electrodes
which enclose the thin plasma located at the line cusp.




     Fig. 2.2.ii - Photograph of the argon plasma confined at the line cusp.
        The electrodes generate an rf electric field almost perpendicular to the
magnetic field direction at the line cusp; according to the rf frequency applied to
the electrodes, a repulsive ponderomotive force (directed from the line cusp to the
plasma centre) acts on the plasma species according to their mass to charge ratio.
In this way when the rf frequency approaches to the cyclotron frequency of a
particular ion species it is possible to plug (reduce the plasma outflow) of these ions
and consequently obtaining a separation between of the ion species of the plasma.
Additional experimental activities have been carried out:
    - Hydrogen formation by methane cracking. In this experiment methane or an
        argon/methane mixture is fed to the plasma device, the produced plasma
        presents a high reactivity that improves the dissociation of the methane
        molecules. The reaction products have been analysed by a mass
        spectrometer located at the line cusp. An increase of the hydrogen
        concentration (up to 75%) has been achieved at high values of the applied
        magnetic field.
    - Production of hydrocarbon (a-H:C) film due to the methane cracking.
        Analyses by scanning electron microscopy and transmission electron
        microscopy have indicated that the film has a prevalently amorphous
        structure with a significant presence of crystalline clusters.



2.3 Destruction of organic toxic-dangerous substances

        The plasma plant for waste destruction has been used for the disposal of
Halons. Tests have been performed on different species of Halons using the plasma
torch operated in argon with a nominal power of 15kW. The flow of the injected
substance was up to 2 slm at 1 bar. Particular care was dedicated to the design of
the injector integrated in the plasma source. Tests with Halon 1301 (CF3Br) have
been carried out. The complete destruction of the Halon molecules is demonstrated
by a large increase of the mass spectrometer signal of the cracking products and by
optical emission spectroscopy.




2001/2004 IFP Activity Report                                                     25
2.4 Material surface analysis for ITER

        The data analyses of the material deposition relevant to the graphite tiles of
the RFX first wall were continued. The imperfect axial symmetry and the horizontal
shift of the plasma column, with respect to the first wall, could influence the erosion
and deposition rates for the alloy metals. This, along with the differential sputtering
rates during the profiling, strongly modified the original concentrations. The boron
distribution is not homogeneous along the poloidal and toroidal coordinates and will
be subject of a future investigation.
        The interim report for the EFDA–ITER task TW4-TPP-ERCAR has been
delivered: the objective of the task is to characterize the properties of carbon based
materials after exposure to the large fluences expected in ITER and the chemical
sputtering of carbon under impact of Hydrogen and noble gases (He, Ne, Ar) in the
energy range close to those expected at the ITER divertor target. A graphite tile of
the toroidal belt limiter of TEXTOR was provided and was analyzed by Scanning
Electron Microscopy and EDS microanalysis.




26                                                    2001-2004 IFP Activity Report

				
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