Optical Properties and Parameters of Plasma of Nanosecond Diffuse and Spark Discharges in Air at Various Humidity

The results of an experimental study of a nanosecond discharge in a 1.75 mm long tip-to-tip gap in dry and humid air at voltage pulse amplitudes across a matched load of 10 and 25 kV are presented. The tip electrodes have been made of carbon. The discharge has been initiated by trapezoidal voltage pulses with a duration of 13 ns and a rise time of 2.2 ns. A coaxial electrical circuit for connecting the gas-discharge chamber, ensuring a single breakdown mode, is used. A set of diagnostic methods is employed: high-frequency voltage and current measurements, a four-channel ICCD camera, high-resolution time-integral photography, and time-integral and time-resolved optical emission spectroscopy. It is established that the breakdown occurs in 0.1–0.2 ns via counter-propagating streamers, after the collision of which a diffuse discharge is formed lasting ≈2–5 ns. Regardless of the pulse amplitude, approximately 1/3 of its total energy is deposited into the plasma (5 and 32 mJ, respectively). The discharge channel has a non-uniform filamentary structure. In the plasma emission spectrum, a broadband continuum with a maximum near 400 nm, the intensity of which increases in humid air at high energy inputs, bands of the second positive system of the nitrogen molecule, as well as N I, O I, N II, C I, Na I, K I, and H I lines are identified. It is shown that immediately after the breakdown (the diffuse stage of the discharge), the electron temperature reaches ≈10 eV, and the reduced electric field is ≈450–500 Td. The electron density is determined from the Stark broadening of the Hα line.