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Home > About us > Media > Archived news > 2012 > Student Award for Excellence to LPP PhD student Jérôme Bredin

Student Award for Excellence to LPP PhD student Jérôme Bredin

The GEC Student Award for Excellence was given to Jérôme Bredin at the 65th Annual Gaseous Electronics Conference (GEC), held October 22 to 26, 2012 in Austin, Texas.
Jérôme is a 3rd year PhD student at the LPP in the group plasmas froids under the supervision of Ane Aanesland and Pascal Chabert.

The title of his talk at the GEC was “Measurements of the Positive and Negative Ion Energy Distribution Function obtained from a Langmuir probe in an ion-ion plasma” (GEC abstract).

The work presented by Jérôme at the GEC is the first complete analysis of Langmuir probe data obtained in an ion-ion plasma where the electron density is orders of magnitude lower than the ion densities. Jérôme has persistently carried out experiments and developed a model showing that the measured I-V (Current-Voltage) characterisitics can provide the energy distribution functions of both the positively and the negatively charged species. From this analysis the negative and positive ion temperatures and densities can be found. His work provides strong insight into the physics of strongly electronegative plasmas where the key unknown is the value of the ion temperatures. The scientific level and originality of this work are very high and can be compared to the best work presented at the GEC in previous years. This can be reached in particular thanks to Jerôme deep understanding of plasma physics and to his utmost careful and precise manner to carry out experiments.
This work is funded by the EADS Astrium and the ANR EPIC project ANR-11-BS09-040.

The ion-ion plasmas are interesting for a variety of applications as space propulsion, etching, polymer treatment, neutral beam injection for ITER, etc. To improve these applications, our aim is to deeper understand how the plasma behaves when the negative charges (normally electrons) are dominated by negative ion. The electrons are light and therefore very mobile whereas the negative ions are heavy and have almost the same mobility as positive ions, due to their similar masses. The plasma physics will therefore change drastically between an electro-positive plasma and an ion-ion plasma. Experiments and simulations have been carried out to investigate ion-ion plasma (or strongly electronegative plasmas as they are also called), however one of the major challenges when studying these plamas has been to determine the ions temperatures. This quantity is crucial in both simulations (determining for example the collisions processes) and in experiments (needed for example to determine the ion densities from the ion saturation currents). Our study of electronegative plasmas has been realized with Langmuir probe measurements, where a new model to analyze the measured Current-Voltage (I-V) characterisitic was developed. The model describes the various electron and ion currents measured by the probe as a function of the applied voltage. A theoretical expression of the I-V characterisitic is found, and can be fitted to the experimental data. By fitting both the I-V characteristics and its second derivative we can determined the positive and negative ion densities as well as their temperatures.

Figure 1 shows a typical example of the fitted second derivative with the various electron and ion contributions. The blue line is the measured data and the red line is the total fitted data.

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Figure 1

Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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