Plasma bullets in atmospheric pressure plasma jets have drawn much scholarly attention in the past decade, especially its shape. This Letter reports the formation and evolution mechanism of a solid-in-hollow structured bullet. At high pulse repetition frequency (≥20 kHz) and long rising time (≥200 ns), the traditional hollow ring bullet turns into the solid-in-hollow structure, and the plasma plume correspondingly appears as a purple external plume with an extra internal filamentary red core under bare eyes. An image process method based on color recognition and decoupling is designed to separate the images of the external plume and the internal filamentary core and is then used to analyze the discharge mechanism together with the intensified charge-coupled device imaging and spectrum measurement. Analyses reveal that the dominant ionization sources for external ring and internal core are N2 and He, respectively, and the internal discharge can be enhanced by increasing the pulse repetition frequency, which might be explained by the electron oscillating heating in high frequency. Finally, the evolution mechanism of the internal filamentary core is studied, and the internal discharge is initially a hollow ring as well when it just exits the tube end, making the bullet a hollow-in-hollow structure, but quickly contracts into a solid core due to the Coulomb repulsion between the two plasma rings.

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