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Experimental study on the growth and transport of nanoparticles in plasmas of high density

par Elodie PICO - publié le

Thesis advisor : Pr Cécile ARNAS

The production of nano-structured material is at the heart of modern sciences. Gaseous, liquid, solid and colloidal systems are used as precursor medium for various nanoscale assemblies.
Ionized gases or “plasmas” have the property to generate nanoparticles (NPs) either from sputtered material by ion fluxes or from discharges in reactive gases. Because of the superior properties of NPs with respect to bulk materials, their field of potential applications cover a wide range of domains : electronics, optics, chemistry, material, medicine... On the other hand, in plasmas, when their formation is not expected, they produce harmful effects : plasma contamination, modification of the ion and electron fluxes and various instabilities. In particular, in tokamaks(1) where hydrogen (deuterium) plasmas are produced, “dust” particles can be generated by plasma-wall
interaction (wall sputtering, instabilities). In this context, in the future tokamak ITER
(International Thermonuclear Experimental Reactor), the dust production rate by plasma-wall interaction must be limited, in order to achieve hydrogen fusion reactions as well as for safety reasons. The topic of the proposed experimental thesis is in this context.
The PhD student will study in laboratory plasmas of high density that will mimic tokamak edge plasmas (near the tokamak wall) the following topics :
- the formation of NPs from metallic sputtered atoms (cathode sputtering by ion flux)
- the transport of growing NPs in the plasma and their deposition in structured targets. The chosen structure will look like the one which exists on a tokamak first wall.
The nanoparticle synthesis follows 2 steps. The first one is link to the formation of ionic clusters. They grow from muliple (but specific) collisions betwwen the sputtered atoms which are injected continuously in the plasma and clusters already formed, up to the nucleation (appearance of NPs). The second step is the growth of NPs in the plasma : they agglomerate to form bigger particles and they grow by atom/cluster deposition on their surface. It is this second step that the applicant will study as well as their transport/deposition.
For the above mentioned studies, argon and deuterium plasmas of high density will be
produced between two plane-parallel électrodes : a magnetron cathode, negatively biased and a grounded anode. The cathode in tungsten (W) or aluminum (Al), will be equiped with permanent magnets to get an efficient ionization process by trapping electrons in magnetic field lines. The concomitent cathode bombardement by ions (sputtering) will introduce continuously in the plasma, Al or W sputtered atoms, in order to produce NPs. The location of their apperance/growth and their transport will be followed by laser scattering and laser extinction. In a first step, they will be collected on substrates at the end of the plasma. Their morphologies and size distributions will be studied by scanning/transmission electron microscopy (SEM, TEM) as a function of the plasma duration and for various discharge currents.
In a second part, substrates will be replaced by “targets” of structured pattern. The same in-situ (laser) and ex-situ diagnostics (SEM, TEM) will be used to study the transport towards the structures and deposition on the structures.
The dynamics of dust transport being linked to the plasma properties, the latter will be
characterized by various basic diagnostics of plasma physics. During the phases of formation and transport, the plasma will be characterized through its light emission by optical spectroscopy. The ion and electron densities, temperatures, fluxes will be established by a set of electrical diagnostics such as Langmuir probes, emissive probes, flux probes and using the optical diagnostic of laser induced fluorescence.
Results will be discussed with specialists of tokamak plasmas during meetings that will be organized at the CEA (Center of atomic studies) of Cadarache. This Center is at 80 km from the PIIM laboratory where the proposed experiments will be carried out. The CEA of Cadarache possesses a tokamak called WEST (tungstene (W) Environment Steady-state Tokamak). The plasma facing components in tungsten of WEST have the same design as those planed for the futur ITER.
(1)Tokamak : Device that uses magnetic fields to confine plasmas in the shape of torus. The tokamak is one of several types of magnetic confinement devices dedicated to the study of controlled energy production by hydrogen atoms fusion.

References :
- C. Arnas, A. Michau, G. Lombardi, L. Couëdel and Kishor Kumar K., Effects of the
growth and the charge of carbon nanoparticles on DC discharges, Phys. of Plasmas 20, 013705 (2013)
- L. Couëdel, Kishor Kumar K. and C. Arnas, Detrapping of tungsten nanoparticles in a
direct-current argon glow discharge, Phys. of Plasmas 21, 123703 (2014)
- C. Arnas, C. Pardanaud, C. Martin, P. Roubin, G. De Temmerman and G. Counsell, Analyses of dust samples collected in the MAST tokamak , J. Nucl. Mater. 401, 130 (2010)