Plasma wall interaction - ITER
The main topic concerns the plasma wall interaction, and especially the dust generation, in the frame program of thermonuclear fusion (tokamak). There is a strong coupling of lab studies to develop diagnostic tools and methods with experiments on large facilities TORE SUPRA (Cadarache, France) and ASDEX (Garching, Germany). We particularly implement high speed video recording (> 650 000 fps) and develop the TRACE software for image processing which allows us to detect and follow various events (dusts, hot spots, ...) These tools have also been implemented within other plasma reactors in the group to study short time plasma events (micro-arcs in liquid) or to investigate arc discharges (coll. LAEPT, Univ. Clermont Ferrand and LIEN, Univ. de Lorraine).
Lab studies
Two plasma reactors are presently developped to study the etching of the wall and the resulting formation and transport of dust:
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Dusty plasmas are generated in a capacitive coupled reactor using a C2H2/Ar gas mixture. Time and space resolved diagnostic tools have been developed and implemented, especially 1D or 2D laser light diffusion to detect the dust formation and study their behavior and transport across the plasma chamber. Two simulation tools are coupled to analyze the results from diffusion measurements:
a first model is used to discretize the continuous population balance equations that characterize the dust agglomeration process
The second model is based on a Monte Carlo ray-tracing code coupled to a Mie theory calculation of dust absorption and scattering parameters.
Combining experimental and numerical results allows us to perform time and space investigation of the dust formation and transport. For instance we showed that dust formation occurs according a periodic behavior under certain conditions. This results from the time variation of the dust size distribution which evolves from small to large size due to dust agglomeration. Indeed large size dusts are formed to the detriment of small ones. As the particle size exceeds a critical value – and therefore a critical mass – and fall down in the reactor. Then a new cycle starts. This periodic behavior was confirmed by electrical measurements (bias, voltage and current) that showed the strong influence of the variation of the size distribution of dusts on the flux of ions and electrons, the latter being consumed by the dust particles.
Meanwhile OES, FTIR spectroscopy and fast video recording diagnostics were used to investigate the dynamic of dust particles.
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- Since 2008 a new reactor with a helicon configuration (Boswell's antena) has been operated to study chemical etching phenomena of various carbonaceous material by hydrogen atoms. Plasma investigations have been carried out within different working conditions of pressure, magnetic field in the source and diffusion chambers, Ar to H2 ratio in the gas mixture, etc. It was shown that the coupling regime – capacitive, inductive, , Trivelpiece Gould and resonant Trivelpiece Gould – are very sensitive to the plamsa parameters.
LIF diagnostics coupled with Langmuir probe measurements pointed out that there exists a threshold value of the electron density above which Ar and Ar+metastable states are excited towards higher energy levels by electron collisions (ne>5 1010 for Ar et ne>2 1011 cm-3 for Ar+). This results is of prime importance since metastable states play a fundamental role in such rather dense low pressure plasmas (1010<ne<1012 cm-3).
Finally, TALIF is going to be implemented in order to determine the influence of the plasma parameters on the H-atom ground state density while experiment with carbonaceous surfaces are on the way (biasing and heating substrate holder, CFC tiles from Tore Supra, etc.).
Studies on tokamak machines
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We have been developing since 2007 important work on the application of fast video imaging to plasma studies, including movies recording, image processing and data interpretation and exploitation. We have particularly developed TRACE (TRAcking and Classifying pin-point Events), a tool that allows us to automatically detect and follow dusts and hot-spots in fusion plasma machines. This tool was validated on our diode reactor and is now implemented at IPP Garching for the daily analysis of fast video movies recorded on teh ASDEX Upgrade tokamak. It is thus expected to establish relationships between dust formation and discharge conditions. First results showed that the dust formation rate decreases as the shot campain goes on.
On 2010 we carried out stereoscopic measurements in collaboration with IPP Garching in order to get the 3D trajectories of dust particles in the ASDEX tokamak.