Here, we present a modular constructed metal-grid micro cavity plasma array as a flexible, robust, and simple alternative to micro-structured devices based on silicon. They show great potential for applications requiring large-area treatment, catalytic conversion or decomposition of volatile organic compounds. The metal-grid array is an easily assembled layered structure consisting of a metal grid, a dielectric foil and a magnet. The grid contains between hundreds and thousands of uniformly arranged cavities with a diameter of 150 μm. The whole system is kept together by magnetic force. This also allows disassembling and exchange of the components independently. Typically, the arrays are operated close to atmospheric pressure with an alternating voltage of up to 1.4 kV peak-to-peak in the kHz range. For a first comparison with silicon-based configurations, the metal-grid array is examined from two different perspectives using phase-resolved imaging. The individual cavities show the same asymmetric discharge behaviour as in the silicon-based arrays. In addition, the expansion width of the discharge from the cavities could be measured. The same interaction between the cavities with the propagation of an ionization wave with velocities in the km/s range is observed as for the silicon-based devices. Thus, with respect to the most basic discharge properties, both configurations show the same behaviour, although they are different in structure and composition.

Microhollow cathode discharges have been produced on silicon platforms using processes usually used for MEMS fabrication. Microreactors consist of 100 or 150 μm-diameter cavities made from Ni and SiO 2 film layers deposited on a silicon substrate. They were studied in the direct current operating mode in two different geometries: planar and cavity configuration. Currents in the order of 1 mA could be injected in microdischarges operating in different gases such as argon and helium at a working pressure between 130 and 1000 mbar. When silicon was used as a cathode, the microdischarge operation was very unstable in both geometry configurations. Strong current spikes were produced and the microreactor lifetime was quite short. We evidenced the fast formation of blisters at the silicon surface which are responsible for the production of these high current pulses. EDX analysis showed that these blisters are filled with argon and indicate that an implantation mechanism is at the origin of this surface modification. Reversing the polarity of the microdischarge makes the discharge operate stably without current spikes, but the discharge appearance is quite different from the one obtained in direct polarity with the silicon cathode. By coating the silicon cathode with a 500 nm-thick nickel layer, the microdischarge becomes very stable with a much longer lifetime. No current spikes are observed and the cathode surface remains quite smooth compared to the one obtained without coating. Finally, arrays of 76 and 576 microdischarges were successfully ignited and studied in argon. At a working pressure of 130 mbar, all microdischarges are simultaneously ignited whereas they ignite one by one at higher pressure.

The neutral gas temperature (Tg) of a single micro-hollow cathode discharge (MHCD) elaborated on silicon wafer is investigated. The MHCD is continuously powered in DC yielding a plasma in He, Ar and N 2 gases close to atmospheric pressure. Values of Tg determined by means of optical emission spectroscopy inside and in the vicinity of a single cavity of 100 µm diameter. In this work, the use of the resonant broadening to infer Tg is thoroughly reconsidered taking into account the reevaluation of an empirical coefficient as well as including the weaker contribution of the Van der Waals interaction in the line profile analysis. Tg is found to be around 360 K in He while in Ar it ranges from 460 K to 860 K depending on the current. A temperature gradient is observed between the cavity and the surrounding gas, which is correlated with the design of the MHCD reactor.A second approach involves the study of the relative rotational population distributions of several N 2 (C-B) vibrational bands to infer the rotational temperature (Trot). While the latter is commonly assumed to approximated Tg, its validity will be discussed based on experimental results. Discrepancies between Trot from different vibrational bands as well as with values found with the resonant broadening are investigated using different operating regimes of the MHCD. The validity of both approaches will be discussed supported by a comprehensive evaluation of the Tg and Trot uncertainty estimations. The analysis of resonant atomic line profile appears to be a reliable and accurate method to measure Tg in absolute room temperature atmospheric pressure plasma sources used in industrial processes as well as in environmental and biomedical applications.