Ultrafast optical diagnostics of evolving nanosecond plasmas in liquid water and signatures of discharge mechanisms.

The dynamics of transient in-liquid discharges is very complex and highly dependent on experimental conditions such as electrode geometry, high voltage (HV) polarity, rise time and duration of the high voltage pulse, impedance matching, liquid conductivity, and the presence of vapor bubbles [1]. Our results obtained from different phases of the nanosecond discharge event show that discharges produced in liquid deionised water by short-rise time (a few ns) high-voltage pulses evolve through a sequence of at least two (dark and light) phases, each characterized by a specific morphology [1-3]. In this contribution, we will discuss our recent results obtained by combining several techniques with enhanced spatiotemporal resolution. We will describe approaches based on shadow/interferometric imaging with a resolution of several tens of picoseconds and optical emission with a resolution from hundreds of picoseconds to few nanoseconds [4].

Under investigated conditions, the images reveal the morphology and dynamics of expanding discharge, while ICCD spectra obtained as a function of distance from the anode tip provide time-dependent emission spectra. The discharge emits characteristic UV-vis-NIR continua along the discharge developed from the anode surface, which we have recently resolved spatially (tens of micrometers) along the anode symmetry axis with (sub)nanosecond temporal resolution. The very initial emission spectra show a broadband continuum followed by atomic lines during the later stages. Line profiles can be extracted using the modeled continuum to obtain a spectral line shape at a specific spatial and temporal coordinate. We show that electron number densities at later times (tens of nanoseconds) can be estimated from the broadening and shifts of atomic hydrogen and oxygen lines. The temporal and spatial electron number densities evolve significantly with time (10²⁰ – 10¹⁸ cm^-3 from 30 ns to 450 ns) and with only small changes in the electron density with distance from the anode tip. These results are consistent with images taken with high temporal resolution (ns) using either an ICCD spectrometer (0th diffraction order) or a 4-channel ICCD imaging device.