INstabilities and Dust in Ionized Gases: Outlook – INDIGO

The objective of this project is to go further in the understanding of closely linked aspects: mechanisms of dust particle formation, instabilities concerning both plasma and dust particles, void dynamics. One of the main originalities of this project will be to study these phenomena with different reactor geometries, using different methods of dust production (reactive gases and particularly containing hydrocarbons, sputtering). Thus, a global understanding of these dusty plasma aspects will be possible, by comparing, correlating and interpreting results. Dust particle formation will be explored to identify universal evolution schemes. Growth kinetics will be followed as a function of discharge conditions thanks to electrical measurements. An analysis of carbonaceous molecules involved in chemical reactions will be performed by mass spectrometry and optical emission spectroscopy. Void characterisation will be performed in term of gas nature dependence. The inner plasma will be analysed by Laser Induced Fluorescence, absorption and optical emission spectroscopy. The plasma/dust interface will be also finely examined. Several types of instabilities will be studied as a main and universal dusty plasma behaviour thanks to electrical measurements and high speed imaging. These unstable phenomena could be useful diagnostics for detecting dust presence and characterising their growth. These instabilities are usually strongly nonlinear. Classical dynamical system theories will be used to characterize them and deduce the set of equations governing their dynamics.

Thanks to the project, joint activities around dusty plasmas between Orléans and Bourges have started. A real synergy is emerging between the two sites.
The dust particle growth kinetics and its impact on the plasma characteristics were studied in Orléans (sputtering) and Bourges (reactive gas). The electrical characterization of the plasma modifications was used to follow the growth kinetics. A comparison was conducted with emission spectroscopy that showed the increase of the electron temperature during dust particle growth and the formation of successive dust generations.
The formation of dust particles is often accompanied by instabilities. The use of a new gas for the sputtering (Krypton) revealed a wide variety of new phenomena that have been characterized by electrical measurements and high-speed imaging. The existence and behavior of very bright plasma regions have been evidenced and are major results that we published in Phys. Rev. Lett.. The interactions between these regions (merging and splitting) emphasize the existence of physical phenomena that could be found in other types of plasmas.
The diagnostics of Laser Induced Fluorescence highlighted the variation of the excited neutral density during dust particle growth instabilities and the spatial variation of this density in the void.
Concerning the appearance of non-linear instabilities, a more detailed analysis was conducted on the experimental results. The shape and evolution of the peak amplitudes were analyzed and showed in particular that a new small oscillation has a higher amplitude than those already present.
Our good results on the dust particle formation allowed us to start new collaborations with two European laboratories that were not originally planned in the project.

Results from this project rise new questions particularly on the topics related to instabilities et the void. Non linear aspects are relatively complex and the interdisciplinary approaches have to be continued.

At mid-term, this work leads to 3 articles in international peer-reviewed journals including 1 paper in Physical Review Letters.
2 other papers are close to submission.
This project also allowed us to participate in international conferences with 2 oral contributions, 8 posters and 4 proceedings

Electron temperature evolution in a low pressure dusty RF nitrogen-rich methane plasma
V. Massereau-Guilbaud, I. Géraud-Grenier, J.-F. Lagrange, H. Tawidian and M. Mikikian
IEEE Trans. Plasma Sci. 41, 816 (2013)

Zoom into dusty plasma instabilities
H. Tawidian, T. Lecas, M. Mikikian
IEEE Trans. Plasma Sci., 41, 754 (2013)

Merging and Splitting of Plasma Spheroids in a Dusty Plasma
M. Mikikian, H. Tawidian, T. Lecas
Phys. Rev. Lett. 109, 245007 (2012)

Dusty (or complex) plasmas are ionised gases containing solid dust particles with sizes ranging from nanometre to millimetre. These media are of great interest for astrophysics (comet tails, planetary atmospheres) and for industry where dust particles are usually fatal for the processes in microelectronics, but useful to build small objects in nanotechnologies (single electron devices, solar cells). Thermonuclear fusion is also a topical field concerned by dust production due to wall erosion.
Dust particle formation in plasmas usually results from the presence of reactive gases or from the surface sputtering by plasma ions. Growth mechanisms involved complex chemical pathways and physical reactions still misunderstood. Dust particles are trapped in the plasma due to the negative charge they acquire by attaching free electrons. This charge loss can be drastic for the plasma equilibrium and instabilities can appear with characteristics related to dust particle ones. Charged dust particles experience different forces creating spatial inhomogeneity in the dust cloud like a localised dust-free region called "void".
The objective of this project is to go further in the understanding of closely linked aspects: mechanisms of dust particle formation, instabilities concerning both plasma and dust particles, void dynamics. Significant breakthroughs are expected on these little known aspects that are central for a good understanding of these media. For potential applications, these studies could improve methods for growing, detecting, controlling, eliminating or using dust particles.
One of the main originalities of this project will be to study these phenomena with different reactor geometries, using different methods of dust production (reactive gases and particularly containing hydrocarbons, sputtering). Thus, a global understanding of these dusty plasma aspects will be possible, by comparing, correlating and interpreting results.
Dust particle formation will be explored to identify universal evolution schemes. Growth kinetics will be followed as a function of discharge conditions and an analysis of carbonaceous molecules involved in chemical reactions will be performed.
Void characterisation will be performed in term of gas nature dependence. The inner plasma will be analysed by Laser Induced Fluorescence, absorption and optical emission spectroscopy. The plasma/dust interface will be also finely examined.
Several types of instabilities will be studied as a main and universal dusty plasma behaviour. These unstable phenomena could be useful diagnostics for detecting dust presence and characterising their growth. These instabilities are usually strongly nonlinear. Classical dynamical system theories will be used to characterise them and deduce the set of equations governing their dynamics. This aspect will create a new interdisciplinary topic in the laboratory.
This project will be conducted in the GREMI laboratory, internationally recognized for its skills about dusty plasmas. One particularity of the project will be to develop original approaches and to initiate interdisciplinary collaborations, especially on the nonlinear aspects. Since 2009 and the merge with LASEP in Bourges, the GREMI is on 2 places (Orléans, Bourges). Before 2009, both laboratories worked independently on dusty plasmas. Since the merge, the association of the respective skills from both places opens new perspectives. This project would allow to federate these works and to give birth to a common project on dusty plasmas.