Electric field measurements in nanosecond pulse discharges in air over liquid water surface

Electric field in nanosecond pulse discharges in ambient air is measured by picosecond four-wave mixing, with absolute calibration by a known electrostatic field. The measurements are done in two geometries, (a) the discharge between two parallel cylinder electrodes placed inside quartz tubes, and (b) the discharge between a razor edge electrode and distilled water surface. In the first case, breakdown field exceeds DC breakdown threshold by approximately a factor of four, 140 ±10 kV cm^-1. In the second case, electric field is measured for both positive and negative pulse polarities, with pulse durations of ∼10 ns and ∼100 ns, respectively. In the short duration, positive polarity pulse, breakdown occurs at 85 kV cm^-1, after which the electric field decreases over several ns due to charge separation in the plasma, with no field reversal detected when the applied voltage is reduced. In a long duration, negative polarity pulse, breakdown occurs at a lower electric field, 30 kV cm^-1, after which the field decays over several tens of ns and reverses direction when the applied voltage is reduced at the end of the pulse. For both pulse polarities, electric field after the pulse decays on a microsecond time scale, due to residual surface charge neutralization by transport of opposite polarity charges from the plasma. Measurements 1 mm away from the discharge center plane, ∼100 μm from the water surface, show that during the voltage rise, horizontal field component (E_x ) lags in time behind the vertical component (E_y ). After breakdown, E_y is reduced to near zero and reverses direction. Further away from the water surface (≈0.9 mm), E_x is much higher compared to E_y during the entire voltage pulse. The results provide insight into air plasma kinetics and charge transport processes near plasma-liquid interface, over a wide range of time scales.