Plasma ignition in free standing gas bubbles

Summary form only given. Achieving efficient plasma production in water is limited by two main factors: (1) the high breakdown strength of water, which increases the required voltage or energy input to produce plasma and (2) the high permittivity of water, which limits the penetration of the applied electric field. A recent approach used to circumvent these issues is to ignite plasma in gas bubbles injected into the water. These bubbles provide the high reduced fields (E/N) needed to maintain the discharge. To date, such discharges have been restricted to bubbles in contact with the electrode. In this study we investigate the formation of plasma inside free standing gas bubbles that are completely isolated from the driving electrode. Observation of discharge mechanisms in this geometry has yielded a host of new, complex phenomena. Chief among them is a multi-phase streamer that originates in the liquid but later makes contact with the bubble, igniting a secondary discharge inside the gas. Accompanying this interaction is an intense transfer of energy that destabilizes the bubble's shape. We also report on the successful ignition of plasma that is confined solely within the isolated gas bubble. In our experimental approach, the gas bubble, air or helium, is isolated by trapping it in the node of an ultrasonic acoustic wave. A pair of electrodes are positioned around the bubble using a 3-D translation stage. The electrodes are pulsed at 13.6 kV with a 1 us pulse length. Images of the resulting plasma formation are captured using a PIMAX ICCD camera with gate times in the 50-500 ns range.