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Photo ionization plays a critical role in the formation and propagation of atmospheric pressure plasma jet plumes.But in experiments,it is very difficult to observe the photo ionization due to its relative lower density of photo electrons.In the present study,we develop a portable cold air plasma jet device and observe the ionization wave in adc spark air plasma jet.The discharge images acquired by an ICCD camera show that the ionization wave front performs as a quickly moving bright ball.Breakdown could take place at another side of the quartz plate or pork tissue layer (6 mm thick),which suggests that the ionization should be attributed to photo ionization.The laser schlieren images indicate there is propagation of a shock wave along with the plasma bullet.Based on the photo ionization theory and the photo-electric measurement,the direct photo ionization and multistage photo ionization are the main factors in charge of generating the cold air plasma jet.In addition,the plasma jet outside of the cathode nozzle is colder than 320 K and can be touched safely by a human.In view of the plasma jet including a large amount of active particles,such as NO,O,OH,emitted photons,etc,the proposed portable cold air plasma jet device could be qualified for plasma bio-medicine applications.