The coupling modes and efficiency in terms of transmitted and absorbed powers were studied using an open-ended microwave applicator (MWA) provided with a magnet. This configuration allowed the acquirement and investigation of plasmas over four decades in pressure, from 10 −4 to a few torrs, for powers up to 200 W. Plasma impedance measurements revealed different coupling modes and transitions from the capacitive to the inductive mode. These also permitted the unraveling of the conditions for which maximum power and heating efficiencies can be obtained, which were found to be primarily linked with the resistive coupling. Moreover, the MWA was seen to be effective in extending the domains of pressures (1-50 mTorr) and powers (higher than 60 W), maintaining at least 80% of the transmitted/absorbed power within the working range. However, by knowing the values of impedance for different pressures, the MWA design can be adjusted for other domains as well.

The objective of this exploratory study is to demonstrate that sulfur can offer an advantageous alternative to mercury for the production of UV photons in fluorescent discharges not only in terms of environment and health impacts, but also in terms of Stokes shift energy losses for the conversion of UV photons by luminophors into visible light. The experiments are performed in surface-wave plasma columns sustained at the output end of a coaxial applicator by microwaves at 2.45 GHz frequency and 10 W applied microwave power. After introducing 10 mg sulfur in argon at 1 Torr, it can be observed that most of the emission spectrum due to sulfur is maximum in the UV range between 300 and 400 nm, well above the 253.7 nm resonant peak line met with mercury in low-pressure discharges. Therefore, by comparison with mercury, the Stokes shift is considerably reduced in the case of sulfur. By surrounding the silica discharge tube with a segment of commercial fluorescent tube coated inside with luminophors, the UV spectrum of sulfur appears quite efficiently transferred into visible light.

In order to improve significantly the performance of fluorescent discharges in lighting applications, the concept of hybrid positive columns, which combines a direct current DC positive-column and a microwave plasma source at the cathode surface is introduced. The feasibility of such hybrid discharges, which can operate in the 10 W power range or below, is substantiated experimentally in pure argon. A first analysis of performance in terms of energy efficiency and energy savings is reported for comparison purposes with current fluorescent lamps.