The electron number density measurement from needle-to-cylinder gas discharge source in helium at atmospheric pressure

Abstract

The optical emission spectroscopic methods are applied for the characterization of a newly constructed needle-to-cylinder atmospheric pressure gas discharge source operating in helium. The discharge source comprises stainless steel cathode and graphite anode. The focus of this study is on electron number density Ne measurement employing a dedicated fitting procedure on asymmetric He I 447.2 nm and He I 492.2 nm lines emitted during pulsed and DC voltage regime. In the pulsed regime, Ne = (0.55–0.75) × 1015 cm−3, while during DC voltage excitation, Ne = (1.3–1.7) × 1015 cm−3 was measured. The reliable gas temperature Tg measurement was performed using a Boltzmann plot technique on N2(C3Πu–B3Πg) R2 branch lines. The Tg jump from 780 K to 1720 K is observed when switching from DC to pulsed regime. In both regimes, the increase in current caused the increase in Tg. The gas discharge interaction with water drops located on the anode surface was investigated as well. During the water drop treatment, intensive Fe I originating from cathode material were detected. The tentative explanation is that iron-oxide layers are formed on the cathode surface during the plasma–water interaction with subsequent decomposition in plasma. This feature opens the possibility for analytical application on conductive, liquid samples.