LABORATOIRE PLASMA ET CONVERSION D’ENERGIE - UMR5213

PARTICIPANTS

Post-Docs K. Makasheva, E. Muonz-Serrano (Oct 2005 to Jan 2007)

Students B. Eismann

Permanent L. Pitchford, T. Callegari, G. Hagelaar, J.-P. Boeuf

BACKGROUND

We are a partner in the project ANR MICROPLASRé, which is a joint modelling/experimental project aimed at using the non-equilibrium atmospheric pressure plasmas created in microdischarges for producing large fluxes of radicals and metastables. In non-equilibrium plasmas, the chemistry is cntrolled primarily by the energetic electrons. “Microdischarges” or “microplasmas” are electric discharges or plasmas created in very small geometries – 100s of micron-sized – operating at neutral gas pressures up to an atmosphere. The recent and considerable interest in microdischarges is due to the fact that these discharge configurations are stable at atmospheric pressure and for high discharge power densities ( 100 kW/cm3). This stability is remarkable because it is otherwise quite difficult to maintain a non-equilibrium high pressure plasma for any length of time ; small fluctuations tend to be unstable and trigger a rapid rise in the gas temperature (ie transition to a thermal plasma arc). In contrast to dielectric barrier discharges where the transition to an arc is impeded by a capacitive effect, microdischarges provide a means for generating stable high pressure plasmas with relative low voltage (several 100 V) and thus it is possible to control the deposition of high power densities into atmospheric pressure gases and to produce large quantities of reactive species.

OBJECTIVES

The applications targeted in this work are production of O2 metastables (at 0.98 eV) in the context of high power infra-red lasers (O2/I2 systems) and production of oxidative radicals (O, OH, O3), which are interesting for atmospheric pollution control. Our objectives, however, are more general than these applications ; we propose to explore the extent to which we can control the stability and adjust the properties of the plasmas created in microdischarges by suitable design of the geometry and voltage pulses and thereby to identify operating conditions leading to enhanced radical fluxes.

DISCHARGE CONFIGURATION

The specific discharge configuration we are presently studying is based on that proposed by Schoenbach and colleagues as shown in fig. 1. It consists of a MicroHollow Cathode Discharge – a metal-dielctric-metal sandwich some 100’s of microns in thickness through which a central hole, also some 100’s of micron in diameter, has been drilled. A larger volume plasma region can be generated by using the MHCD as a plasma cathode, placing a 3rd positively biased electrode some 5 to 10 mm away. The discharge between the MHCD and the third electrode (A2) is called a MicroCathode sustained Discharge – MCSD.

	Fig. 1. Schematic of 3-electrode geometry from Stark and Schoenbach. The MHCD is used as an electron source for the Microhollow Cathode Sustained (MCS) glow.

APPROACH

Models developed in the group GREPHE and collaborative experiments in microdischarge configurations are underway at LPTP (Orsay), LPTP (Palaiseau) and LSP (Grenoble) as well as in our group using a "macrocell" - same pd, pressure x distance, product but 50 times larger. The electrical properties of the discharge and the properties of the plasmas generated in the MHCD and MCSD are measured and calculated.

HIGHLIGHTS

Our work has helped to eludicate reasons for the enhanced plasma stability and to quantify the extent to which plasma properties can be controlled in microdischarges.

Quantitatively good agreement has been obtained for the plasma properties in a MHCD in argon, as shown in fig. 2.

Fig. 2. Comparisons of calculated and measured peak, on-axis electron density in MHCDs in argon at 75 (blue), 100 (black) and 200 (red) torr. The measurments were obtained by N. Sadeghi, LSP, Grenoble.

The initiation of a plasma in the MCSD depends on the cathode current, the voltage on the third electrode as well as on pd. Model calculations reproduce well the experimental trends. The equipotential contours in the MCS region are shown in fig. 3 before and after initiaion of a plasma in this zone. The reduced electric field strength on-axis in the plasma is about 3 Td and typical of a positive column.

Fig. 3. Calculated equipotential contours in the MCS region before (left) and after (right) initiation of a discharge in this zone. The conditions are argon, 60 torr, 1 mA, with the electrode A2 (right) is placed 4 mm from the MHCD (left).

The conditions in the MCSD plasmas in oxygen containing mixtures are ideal for the production of the metastable oxygen molecule, O2(1D), which is interesting for a number of applications and experiments at LPGP aim to optimize these sources. Excimer production should also be efficient in this zone.

COLLABORATIONS

National : Vincent Puech (LPGP) Antoine Rousseau (LPTP) Nader Sadeghi (LSP) Michel Touzeau (LTM)

International : Klaus Frank (Erlangen) Leonid Simonchik (Minsk)

PUBLICATIONS

J.P. Boeuf, L.C. Pitchford and KH Schoenbach, "Predicted properties of microhollow cathode discharges in xenon", Appl. Phys. Letts. 86 071501 (2005)..

E. Muñoz−Serrano, G. Hagelaar, Th. Callegari, J.P. Boeuf and L.C. Pitchford, "Properties of plasmas generated in microdischarges", Plasma Phys. Control. Fusion 48 B391–B397 (2006).

G Bauville, B Lacour, L Magne, V Puech, JP Boeuf, E Munoz-Serrano and LC Pitchford, "Singlet oxygen production in a microcathode sustained discharge", Appl Phys Lett 90 031501 (2007).

K. Makasheva, E. Munoz-Serrano, G Hagelaar, JP Bœuf and LC Pitchford, "A better understanding of microcathode sustained discharges", Plasma Phys. Conrol. Fusion 49 B233 (2007).