Microplasmas refer to electric discharges created in very small geometries
able to operate in DC mode at high pressure without glow to arc transition.
The recent and considerable interest in microplasmas is due to their unique
properties in term of discharge stability and power loading. A microplasma
configuration which has proven to be stable at atmospheric pressure and up
to a power density of some 100 kW/cm3 is the Micro Hollow Cathode
Discharge (MHCD) developed by Schoenbach and coworkers. MHCDs are created by
applying a voltage between two closely spaced hollow electrodes separated by
a dielectric layer. The thickness of the dielectric and the diameter of the
hole are both on the order of some 100's microns. These MHCDs can be used as
plasma cathodes for generating a diffuse discharge between the MHCD cathode
and a third positively-biased electrode placed some distance away. This is
the so-called Micro Cathode Sustained Discharge (MCSD) configuration, which
can be operated as a non self-sustained discharge. In that mode, the MCSD
appears as a unique tool for producing, at high pressure, large fluxes of
O2(a1Δg) metastable states which cannot be
efficiently produced in classical self-sustained discharges.
Based on experimental works performed by our group and on modeling studies
done by Pitchford and coworkers at Toulouse University, this paper
summarizes the properties of the plasmas generated by the MHCD and the MCSD,
with an emphasis on the new fascinating opportunities for the production of
O2(a1Δg) metastable states by electrical discharges
in high pressure rare gas-oxygen mixtures.