A large-diameter helium atmospheric pressure plasma jet (APPJ) system was developed using two floating electrodes and an insulating layer beneath the grounded electrode. This configuration enabled relatively stable and uniform plasma plume generation, making it suitable for biomedical applications. To investigate the underlying mechanisms, we conducted optical emission spectroscopy (OES), ICCD imaging, and numerical simulations. Results show that the addition of a floating electrode between the high-voltage and ground electrodes reduces the local electric field during the positive half-cycle, suppressing electron avalanche formation. Furthermore, insulating tape between the tube and ground electrode lowers the field strength and potential drop, enabling more electrons to reach the nozzle while limiting space charge accumulation. The floating electrode near the nozzle increases the local potential, helping confine electrons within the tube. These effects contribute to a more uniform and extended plume. Our findings highlight the importance of radial electron dynamics in large-diameter APPJs and offer insights for improving their design and control.
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