DC-excited discharges in vapour bubbles in capillaries

DC-excited discharges in vapour bubbles in capillaries are studied. A bubble is generated in a capillary filled with a NaCl solution due to Joule heating. The fluid columns on either side of the bubble serve as electrodes for the electrical discharges inside the bubble. The electrical breakdown (corresponding to a corona-to-spark transition) of quasi-static vapour bubbles is discussed. The breakdown electrical field decreases with bubble length. For larger bubbles, the reduced electrical field is smaller than the electrical field where electron attachment equals ionization, indicating that the discharge is a surface discharge. Linear translation of bubbles in the cathode direction, coinciding with intense discharges inside the bubbles, is observed and can be explained by asymmetric heating due to the plasma. The optical emission spectrum of a vapour bubble discharge consists of excited hydroxyl, hydrogen and sodium emission. A delay in the range of 0.1 s is observed between the emission of hydroxyl and sodium. The sodium emission is most intense on the anode side of the bubble where orange anode spots are visible.