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