A corona pin encased in a plastic tube was placed annularly through a 1/16 inch brass Swagelok union tee (Swagelok), with the top nut removed and the opening sealed with superglue. The tee was connected to a gas line providing 0.5 bar of argon. The top of the corona pin was connected to a 50 MΩ resistor, which in turn was connected to a PS350/5000V-255W high voltage power supply (Stanford Research Systems Inc, Sunnyvale, USA) providing +5 kV direct current at around 5 mA. The return electrode was connected to indium-tin oxide slides (Merck). The distance between the tip of the corona pin and the ITO slide was 12 mm.