Optical and electrical diagnostics of microdischarges at moderate to high pressure in argon

459 Brazilian Journal of Physics, vol. 40, no. 4, December, 2010 Optical and electrical diagnostics of microdischarges at moderate to high pressure in argon B.N. Sismanoglu∗ and C.L.A. Cunha Departamento de Fı́sica, Instituto Tecnológico de Aeronáutica, Comando-Geral de Tecnologia Aeroespacial, 12228-900, São José dos Campos, SP, Brasil M.P. Gomes and R. Caetano Laboratório de Óptica e Espectroscopia, Departamento de Fı́sica, Instituto Tecnolgico de Aeronáutica, Comando-Geral de Tecnologia Aeroespacial, 12228-900, São José dos Campos, SP, Brasil K.G. Grigorov Institute of Electronics, Bulgarian Academy of Science, 72 Tzarigradsko Chaussee, Sofia 1784, Bulgária (Received on 21 October, 2010) Microdischarges at moderate to high pressure in argon were investigated. A hole opening diameter of 500 µm direct current (dc) microhollow cathode discharges (MHCD) were characterized by electrical measurements and optical emission spectroscopy (OES) for pressures ranging between 90 and 800 Torr and current from 5 to 20 mA. Current-voltage characteristic curves were obtained as a function of the pressure for this hole diameter. MHCD enables stable dc discharges for molybdenum electrodes material at constant Ar + 2%H2 flow of 0.03 `/min. Optical emission spectroscopy and analysis of the spectral line broadening of plasma line emissions were performed in order to measure gas discharge parameters. Electron number densities were obtained from Hβ Balmer line (∼ 1014 cm−3 ). For the above mentioned discharge conditions, gas temperature was estimated to be 550 – 850 K from OH rotational bands. Excitation temperature was measured based on two lines method (from atomic Mo lines) and from 4p - 4s and 5p – 4s Ar radiative transitions. Hydrogen atom temperature was measured for 800 Torr (∼ 12000 K). Keywords: Microdischarges, microhollow cathode discharges; optical emission spectroscopy. 1. INTRODUCTION Following Schoenbach group studies about microdischarges - notably of microhollow cathode discharges (MHCD) at moderate to atmospheric pressure - the operation and applications of these kinds of microplasmas has attracted the attention of researchers worldwide due to the convenience and easiness of operation, besides the low cost [110]. These non-thermal microplasmas, where the electron temperature is much higher than the gas temperature (here, electron temperature is associated with the mean electron energy), allied to small size and low power consumption, is an attractive to the plasma applications possibilities in industry, like surface treatment, generation of UV and VUV radiation, reduction of pollutants, gas lasers, biological decontamination, thin film deposition, mainly in a high pressure operation [1,2]. These microdischarges consist of a cathode with a borehole and an arbitrarily shaped anode, separated by an insulator, where a hole is drilled through a cathode-mica-anode sandwich structure. Both current-voltage characteristics and Paschen’s curves have been studied previously [6-8] for different pressures and hole diameters. They showed three distinct modes of operation: abnormal (at low current), selfpulsed and normal. Stable atmospheric pressure operation is easily obtained in small holes or by employing a certain constant gas flow (dynamic mode). As an example, for a flow of 0.7 `/min it was possible to ignite a discharge in a 200 µm diameter hole using rare gas [11]. For atmospheric pressure operation, the hole diameter should be in the order of 100 µm ∗ Electronic address: in a static mode of operation (without gas flowing) [3]. The understanding of physical and chemical processes occurring in this kind of plasma is fundamental for the optimization of the some industrial applications. Reliable MHCD operating at low voltage seems to be useful and important both for industrial and research applications. It is important to notice that at high-pressure, the cathode heating and sputtering is a problem in these devices mainly in static mode of operation, diminishing the lifetime. Therefore, low gas temperature at higher current operation is desirable to avoid these issues. The investigation of plasma parameters, like electron number density (ne ), gas temperature (Tg ), electron temperature (Te ) and excitation temperature (Texc ) generated in MHCD is important to understand the mechanisms that govern these new high-pressure discharges and at the moment, there are no systematic experimental results of plasma parameters (ne , Tg , Te and Texc ) in the literature. Recent works have shown results on plasma parameters and applications for high pressure (p) quasi-static MHCD, or in a low argon flux, and now they will be presented. For diagnostics and application of an analytical plasma, Miclea et al [12] have measured ne = 9 × 1015 cm−3 , Tg = 2000 K, Te = 1.2 eV for discharge current Id = 3 mA in dc MHCD at atmospheric pressure. Limitation in these applications is the low current and reduced lifetime of the microstructures due to overheating, or sputtering of the micro-hole, at high pressure. Investigating excimer emission from MHCD, Moselhy et al [13] found ne = 1 × 1015 cm−3 in atmospheric pressure, for flowing argon at a rate of 0.04 `/min in the gas chamber (gas temperature was not measured). These authors estimated Te = 1.2 eV for dc operation and 2.25 eV in the pulsed mode. Naming micro-structured-electrode arrays, Penache et al [14] have investigated static dc MHCD in pure 460 monochromator (THR1000). The radiation collimated at thebyexit slit where a photomultiplier borehole was focused onto the optical fiberwas 1mm i.d. aperture means of convergent lens. This tube (PMT) converts photons into an electric signal. The current generated by the PMT was sent to radiation was send onto the entrance slit (width of 100 µm) of 1 m Czerny-Turner (Jobin-Yvon) B.N. Sismanoglu et al. was data acquisition electronics, where the signal was processed by software. The apparatus function monochromator (THR1000). The radiation was collimated at the exit slit where a photomultiplier obtained from Ar low-pressure lamp and found to be 0.0463 nm. tube (PMT) converts photons into an electric signal. The current generated by the PMT was sent to data acquisition electronics, where the signal was processed by software. The apparatus function was Ar for pressure ranging from 50 to 400 mbar. From specobtained from Ar low-pressure lamp and found to be 0.0463 nm. troscopic measurements of the absorption line profiles they found Tg = 1100 K and ne = 5 × 1015 cm−3 at p = 400 mbar. In a device for water treatment, Yamatake et al [15] implemented dc-driven MHCD in argon flow rate 1`/min for 200 µm hole diameter and they have only measured the currentvoltage characteristics, showing abnormal mode of operation. Otherwise, we showed that a dynamic mode MHCD can be operated with intense gas flow in a low gas temperaFIG. 1: Experimental set-up showing MHCD and light acquisition; inset: end-on pho (...truncated)