Plasma power balance: methodology and investigations of microwave capillary discharges

Abstract

Nowadays, where strong energy constrains are set by the environmental and economical context, the real power coupled in plasmas becomes a critical aspect particularly for applications where plasma technologies are facing competition. This issue is enhanced for atmospheric pressures micro-plasmas, where the local power density can be very high (e.g. 10^5 Wcm-3) which implies fast heat exchanges. The precise knowledge of the power coupled to plasma is also a key for fundamental understanding of discharge properties. This is often used as an input parameter for plasma modeling and its inaccuracy can mitigate the predictive quality of plasma simulation tools. In the present paper the macroscopic power balance was established for continuous microwave (MW) plasmas generated in capillaries in argon gas flows. The macroscopic power budgets were performed based on measurements of microwave leak fluxes, heat fluxes (thermal radiation, convection, conduction), optical radiation and chemical powers. It was proven that the traditional power measurements at the MW generator would largely overestimate the real power coupled to plasmas, namely by at least a factor two. An important power fraction is found in the heat transfer through the MW launcher, the impedance mismatching being here better assessed. The proposed methodology, which is based on the physics of transport phenomena, can be employed for characterization of other (micro-) plasma sources.

Nitric oxide generation by microwave capillary discharges under thermal control at atmospheric pressure

“…The density of NO was measured at tens of cm from the discharge zone and at room temperature conditions, therefore in a frozen chemical regime. Moreover, as shown in the recent paper of Coquery et al [22] the actual coupled power in microwave capillary plasmas could be up to an order of magnitude less than the value given by the usual balance of incident and reflected powers. This method was retained here and is commonly employed in the literature.…”

“…The corresponding conducto-convective resistance R CC ext can be significantly modified when the convection regime changes. As shown in [22], the capillary can be considered here as an opaque body for the thermal radiation domain. Also note that the thermal radiation flux at inner capillary surface is negligible.…”

“…The analogous resistance for the capillary external cooling by natural convection, is estimated as dxR CC ext ∈ (6 ÷ 11) Km/W, for the external capillary surface between 700 K and ambient air. The Nusselt number is determined here for a horizontal cylinder of diameter D, following the procedure from [22] with the correlation,…”

“…Recently, mw surface discharges within capillary diameters in the range of 1 mm-100 μm were efficiently generated by surfatron or stripline launchers. These capillary discharge sources are particularly attractive because they can produce high power density plasmas (up to 0.1 MW cm -3 ) for low power inputs (tens of W) in gas mixtures ranging from mbar to atmospheric pressures [19][20][21][22]. A noteworthy feature is the slight power required to sustain plasmas that can be as low as 1 W. Studies in dry air at intermediate pressures have shown large radical productions (e.g., ~3% NO molar fraction, ~70% dissociation of O 2 ) and high vibrational excitations (e.g., T vib ~10,000 K), opening up prospects for applications also at atmospheric pressure [21,23].…”