Breakdown, stability and similarity laws of microplasmas
Zusammenfassung der Projektergebnisse
The fundamentals of parallel plate direct current (dc) micro discharges were investigated in the working gas argon. The discharge phenomena were investigated by means of voltagecurrent measurements and synchronised phase resolved optical emission spectroscopy (PROES) with a fast ICCD camera and tunable diode laser absorption spectroscopy (TDLAS). Absolute metastable densities as well as gas temperatures were measured spatially and phase resolved. The voltage-current characteristics show the typical shape with distinguished low current Townsend discharge, voltage-current oscillations region, (constricted) normal glow and high current abnormal glow, previously reported in low-pressure standard size discharges (pressure around 100 Pa, electrode spacing a few centimetres). The emission profile of the Townsend discharge shows the typical diffuse, zero order Bessel function like profile. Since collisions with electrons are the main excitation and ionisation sources in this type of discharge, the emission is exponentially increasing from the cathode towards the anode, as expected from the breakdown condition. In the normal glow, the discharge is highly constricted for the lower current values. As the discharge current increases, the area of the effective discharge conducting channel growth until the discharge covers the full electrode diameter and the transition to the abnormal glow occurs. The different scaling laws, namely the Paschen’s law (scaling of the breakdown voltage with pd), the scaling of the voltage-current characteristic (scaling of the discharge voltage with j∼p2, where j is the current density) and the scaling of the cathode fall width (scaling of the size of the cathode fall times the pressure with j∼p2) were validated with the experiments. Furthermore, the measured position of the Paschen minimum (pd ∼ 130 Pa cm) and the value of the breakdown voltage (Vb ∼ 215 V) at the Paschen minimum are in good agreement with the other experiments. TDLAS investigations on the Ar metastable transition reveal that the broadening of the spectral line profiles is in the order of 1 GHz for all investigated discharge conditions. An analysis of the spectral profiles shows, that in the low current Townsend discharge the gas temperature stays at around room temperature and the density of the metastable state reaches maximum values from 1 ... 8 × 10^11cm^−3 depending on the discharge mode. The largest metastable density production rate is found at the position of the current peak maximum, which is reasonable since electron impact excitation is the main excitation source. Different types of voltage-current oscillations (relaxation, damped and free running oscillations), which occur between the Townsend discharge and the normal glow, were investigated. The relaxation oscillation frequency is increasing as the discharge working point is shifted away from the steady state Townsend region towards the normal glow. The oscillations are caused by an interplay of the discharge and the external circuit parameters and resistances. However, in the micro discharge, it was not possible to completely suppress the oscillations, regardless of choice of the external circuit resistances, as it is the case in the low-pressure standard size discharges.
Projektbezogene Publikationen (Auswahl)
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Axial light emission and Ar metastable densities in a parallel plate dc micro discharge in steady state and transient regime, Plasma Sources Sci. and Technol. 20, 065001 (2011)
T. Kuschel, B. Niermann, I. Stefanović, M. Böke, N. Škoro, D. Marić, Z. Lj. Petrović and J. Winter
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Oscillation modes of dc microdischarges with parallel-plate geometry, J. Appl. Phys. 110, 083310 (2011)
I. Stefanović, T. Kuschel, N. Škoro, D. Marić, G. Malović, Z. Lj. Petrović, and J. Winter
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Spatiotemporal profile of emission from oscillating micro discharges, IEEE Transactions on Plasma Sci. 39, 2692 (2011)
T. Kuschel, N. Škoro, D. Marić, I. Stefanović, G. Malović, J. Winter, and Z. Lj. Petrović
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Argon metastable dynamics in a filamentary jet micro-discharge at atmospheric pressure, Plasma Sources Sci. and Technol. 21, 034002 (2012)
B. Niermann, R. Reuter, T. Kuschel, J. Benedikt, M. Böke, and J. Winter
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Ionization coefficients for argon in a micro-discharge, Plasma Sources Sci. and Technol. 22, 045001 (2013)
T. Kuschel, I. Stefanović, G. Malović, D. Marić, and Z. Petrović
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Argon metastable dynamics and lifetimes in a direct current microdischarge, J. Appl. Phys 116, 113302 (2014)
I. Stefanović, T. Kuschel, S. Schröter, and M. Böke
