Multiple discharge dynamics of long-distance transferred atmospheric pressure plasma jets with floating electrode

The atmospheric pressure plasma jet with a floating electrode, operated in a long-distance transfer mode, plays a key role in tumor treatment within the human body and medical device sterilization. In this work, we primarily investigate the discharge inside the dielectric tube using a self-consistent fluid model that resolves the three-phase interaction among plasma, dielectric, and floating electrode and how they modulate the secondary discharge plume. The simulation agrees well with the floating electrode voltage measured using a differential voltage probe. After breakdown, the double-headed streamer forms at the center of the discharge gap and propagates bidirectionally to cause gap conduction. The negative streamer transforms to the radial expanding surface ionization wave when impinging on the dielectric, while the positive streamer charges the floating electrode, triggering a reverse restrike that neutralizes the gap. A second discharge is then initiated at the electrode tip, launching a plasma bullet. Gap conduction causes potential redistribution between the dielectric tube and the gap, leading to a drop in floating potential. Additionally, the nonlinear dependency on the relative permittivity of the dielectric tube on multiple discharge is discussed. The high relative permittivity results in more potential distributed on the gap, and subsequent sheath space charge will suppress the discharge development and limit the floating voltage increase. This study will provide a quantitative understanding of the discharge mechanisms in long-distance plasma jets and insight into the optimization of device operation.