Atmospheric pressure plasma jets operated by shielded and unshielded high voltage electrodes: Physicochemical characteristics and application to bacterial killing

This study investigates how the shielding of a high voltage (HV) electrode influences the physicochemical properties and the bacterial inactivation efficacy of an atmospheric pressure plasma jet. The plasma jet is comprised of a tungsten electrode inserted inside either a single closed end quartz capillary tube (termed as shielded) or double open end quartz capillary tube (termed as unshielded). A second double open end quartz tube surround the capillary tube to create an annulus area where gas flows. The entire assembly is held inside a plastic Swagelok Tee fitting. Plasma is generated by flowing helium gas through the annulus between the quartz tubes and applying a pulsed DC voltage of 8 kV, 1 µs pulse width, and 6 kHz between the HV tungsten electrode, and a copper ring placed near the end of the outer quartz tube that serves as the ground. Results from fast imaging reveal that the plasma bullets in an unshielded jet has a higher velocity by an order of magnitude, and propagates twice as far as the shielded jet. The higher propagation speed and longer plume length for the unshielded configuration are due to higher accumulation of surface charges which also induce higher electric field. These physical characteristics of the unshielded jet also result in the production of more than two times higher concentration of hydrogen peroxide in plasma activated water. Measurement of rotational temperature for both plasma jets showed that they operate at room temperature facilitating their use in real-world applications. The importance of the study for practical application has been demonstrated by treating Escherichia coli, a bacterial pathogen commonly found in the human body.