The present invention is related to a plasma jet apparatus for performing plasma processing of an article, comprising: - An elongated central electrode (2,15), - An elongated cylindrical outer electrode (1) or two outer electrodes (15,16) surrounding said central electrode and being coaxial with said central electrode, or two electrodes substantially parallel to the central electrode, - An electrical insulator (3) or insulators (18,19) disposed between said outer electrode(s) and said central electrode, wherein a discharge lumen having a distal end and a proximal end is defined between said central electrode and said electrical insulator(s), - A supply opening (6) disposed at said distal end of said discharge lumen for supplying a plasma producing gas to said discharge lumen, - A power source (9) for providing a voltage between said central electrode and said outer electrode characterised in that said electrical insulator has a radial or outward extension (40,20) at said proximal end beyond the outer surface of said outer electrode(s).