External axial magnetic field enhances discharge and water treatment of cold plasma jets

In this Letter, the effect of an external axial magnetic field on the propagation characteristics of a helium plasma jet and plasma-activated water was investigated. Stacked magnets surrounding the quartz tube significantly enhance the brightness and length of the ionization wave. The discharge enhancement is more pronounced at a position farther along the tube axis than near the jet nozzle. Under the action of the external magnetic field, the concentrations of aqueous reactive oxygen species, including H2O2, OH, and O2−, significantly increase with the increasing electron density, whereas the vitality of melanoma cells effectively drops. A magnetic fluid model suggests that the discharge enhancement arises mainly from the confinement of the radial motion of electrons imposed by the axial magnetic field and the radial magnetic pressure, suggesting that the external magnetic field may be beneficial in improving the effects of plasma jet applications.