Surface charge dynamics and OH and H number density distributions in near-surface nanosecond pulse discharges at a liquid / vapor interface

摘要

The present experiments provide new insight into surface charge dynamics in repetitive nanosecond pulse, surface ionization wave discharges sustained over quartz and distilled water surfaces. At low pulse repetition rates (～100 Hz), residual surface charge density on quartz surface and distilled water surface does not exceed ～10% and ～25-30%, respectively, of the peak value during the discharge pulse, with the exception of data taken near the maximum extent of the surface ionization wave from the high voltage electrode, where wave amplitude becomes fairly low. This corresponds to ～1% and ～ 5-10% fraction of energy stored on the surface, compared to energy coupled to the plasma. In the discharge over distilled water surface, surface charge deposition is more pronounced, residual surface charge density is higher, and its decay is considerably slower. However, net discharge energy fraction stored on the surface remains quite low, and surface charge is almost completely neutralized several μs after the pulse. At high pulse repetition rates (~ 10 kHz), ionization wave discharge sustained by an alternating polarity pulse train results in neutralization of surface charge deposited by a previous pulse by the next, opposite polarity pulse. This effect is observed in discharges over both quartz surface and distilled water surface. In the discharge over quartz surface, sustained by the same polarity pulse trains, no significant residual surface charge accumulation over multiple pulses is detected. In the discharge over distilled water surface, sustained by same polarity pulse trains, significant residual surface charge accumulation over multiple pulses is detected for negative polarity pulses, while the effect for positive polarity pulses is nearly absent. Laser Induced Fluorescence (LIF) and Two-Photon Absorption LIF (TALIF) line imaging are used for in situ measurements of two-dimensional spatial distributions of absolute OH and H atom number density distributions in near-surface repetitive nanosecond pulse discharge plasma over distilled water surface in argon buffer. Measurement results after a burst of 20 discharge pulses at a pulse repetition rate of 1 kHz demonstrated that peak H number density, [H] = 2・10~(16) cm~(-3), is much higher compared to peak OH number density, [OH] = 3・10~(14) cm~(-3). Highest OH number density was measured in the region where plasma emission peaks, while H atoms convect downstream with the flow and diffuse further away from the surface, compared to OH.