Investigation of water dissociation by Nanosecond Repetitively Pulsed Discharges in superheated steam at atmospheric pressure

Nanosecond Repetitively Pulsed (NRP) discharges in atmospheric pressure water vapor at 450 K are studied with time-resolved optical emission spectroscopy (OES). A 20-ns high- voltage pulse is applied across two pin-shaped electrodes at a frequency of 10 kHz, with an energy of 2 mJ per pulse. Emission of OH(A-X) as well as atomic states of O and H are observed. The emission of these species increases during the 20-ns pulse, then decreases. Then, after about 150 ns, we observe again a strong increase of emission of these species. To determine the gas temperature, we add a small amount (1%) of molecular nitrogen to the flow of water vapor. The rotational temperature measured from N₂(C³II_u - B²II_g) second positive system of N₂ is measured and compared with the rotational temperature measure with the OH(A-X) transition. The electron density is obtained by the Stark broadening of the H_β emission line at 486 nm. The electron number density increases to about 6 × 10¹⁵cm^-3 during the pulse, then decays to 10¹⁴cm^-3 after 150 ns. But then, a surprising behavior occurs: the Full-Width at Half-Maximum (FWHM) of the H_β emission line increases again sharply, with no electric field applied, up to 5 nm, and then decays slowly to 1 nm over the next microsecond.