Spatiotemporal analysis of a short pulse argon plasma submerged in water

Abstract

Atmospheric pressure plasmas create a unique chemical and electrical environment that is well-suited for the treatment of liquids. These plasmas can operate in a nonthermal regime when driven by pulsed power of sufficiently short duration to prevent significant heating of the surrounding gas, thus providing unique opportunities in liquid decontamination and remediation. This study investigates how varying the pulse width affects the properties of an argon plasma submerged in water. Plasmas produced using high-voltage (11 kV) pulses ranging from 50 to 350 ns at a frequency of 1 kHz were studied using electrical measurements and spatio-temporal optical emission spectroscopy. Results show that the plasma fills the gap between electrodes within the first 15 ns of the pulse. Once the gap closes, the voltage collapses and the current rises to a value determined by the power supply. The plasma remains in this current-limited state for the remainder of the pulse. Optical emission spectroscopy revealed that argon dominates the emission immediately after plasma formation, but over time, hydrogen emission becomes more prominent as the amount of dissociated water vapor in the plasma increases. Spatial emission profiles show uniform hydrogen emission across the reactor, whereas argon emission weakens near the positive electrode. Stark broadening analysis of hydrogen lines indicate a substantial electron density persists for several microseconds after the pulse, likely due to a small residual voltage on the electrodes associated with the power supply architecture. This post-pulse duration scales with pulse width and is found to last nearly 10 µs for the longest tested pulse width of 350 ns, highlighting not only the influence of pulse width on plasma dynamics but also showing the importance of other system parameters on determining the lifetime of the plasma.