Spectral characteristics of a radiofrequency nitrogen plasma jet continuously passing from low to atmospheric pressure

We report and discuss the characteristics of the optical emission of a radiofrequency (13.56 MHz) expanding plasma generated in nitrogen in a double chamber configuration, when the discharge passes continuously from the low pressure (10(-1) mbar) to the high pressure region (760 mbar), at constant gas mass flow. The pressure increase leads to a sudden change in plasma aspect, occurring at intermediate pressure of 200-400 torr. The plasma temperature estimation from spectral measurements indicates that the change in the aspect is accompanied by a sudden decrease of the plasma temperature.