1. “Investigation of 50 Hz pulsed DC nitrogen plasma with active screen cage by trace rare gas optical emission spectroscopy”. Saeed A, Khan AW, Shafiq M, Jan F, Abrar M, Zaka-ul-Islam M, Zakaullah M, Plasma science & technology 16, 324 (2014).   http://doi.org/10.1088/1009-0630/16/4/05