Diagnostics of a nanosecond atmospheric plasma jet. Ionization waves, plasma density and electric field dynamics

Atmospheric repetitive He discharge with 10 ns current peak width and $3× 10 11$ V/s voltage front rise working in jet geometry is studied. This part deals with the ionization waves, electron density, and electric field dynamics. The electron density (n $e$⁠) is measured by Stark broadening of the H Balmer  $β$ (H $β$⁠) and He emission lines, the electric field is analyzed using Stark polarization spectroscopy, and the ionization waves are studied by fast imaging. We found that the ionization fronts propagate in the quartz tube with a velocity of about $5× 10 5$ m/s; this velocity slowly decreases along the tube but may jump in the open air at some conditions. In the space between electrodes, n $e$ increases rapidly at the beginning, reaching about $7× 10 15$ cm $− 3$⁠, which corresponds to electron avalanche defining the discharge current peak. In the tube, the electrons are concentrated in the ionization wavefronts having low density (⁠ $< 10 14$ cm $− 3$⁠). Before the avalanche, a macroscopic (electrode-induced) electric field dominates between the electrodes peaking at about 8 kV/cm as deduced from H $β$ peak splitting, whereas during the avalanche, H $β$ reveals a double-Lorentzian polarization-insensitive profile imposed by two electron populations. In the low-density electron group, n $e$ does not exceed 10 $14$ cm $− 3$⁠, whereas the high-density group is responsible for the observed electron density peak formation. After a rapid decay of the electrode-induced field, the microscopic electric field (induced by space-charge) dominates, peaking at about 25 kV/cm after the electron density peak. Certain electric field anisotropy is also detected in the quartz tube, confirming the wavefront propagation.