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Ionization channel formation is observed in an atmospheric-pressure plasma jet. A radial electric field is applied between coaxial electrodes. Helium flows in the axial direction through an inner metal tube electrode. In this device configuration, time-resolved images show that a plasma bullet can propagate only when the inner electrode is an anode. This phenomenon is experimental evidence that the bullet head is a positive space charge region. Moreover, it supports the hypothesis that the mechanism of bullet propagation is similar to that of a positive streamer. A comet-tail-like feature appears during the bullet propagation. This interesting shape occurs probably because of the Penning ionization near the helium-air boundary.