Time- and spatially resolved emission spectroscopy of the dielectric barrier discharge for soft ionization sustained by a quasi-sinusoidal high voltage

A helium capillary dielectric barrier discharge was investigated by means of time-resolved optical emission spectroscopy with the aim of elucidating the process of the formation of the plasma jet. The helium emission line at 706 nm was utilized to monitor spatial and temporal propagation of the excitation of helium atoms. The discharge was sustained with quasi-sinusoidal high voltage, and the temporal evolution of the helium atomic emission was measured simultaneously with the discharge current. The spatial development of the plasma was investigated along the discharge axis in the whole region, which covers the positions in the capillary between the electrodes as well as the plasma jet outside the capillary. The high voltage electrode was placed 2 mm from the capillary orifice, and the distance between the ground and high voltage electrode was 10 mm. The complete spatiotemporal grid of the development of the helium excitation has shown that during the positive half-period of the applied voltage, two independent plasmas, separated in time, are formed. First, the early plasma that constitutes the plasma jet is formed, while the discharge in the capillary follows subsequently. In the early plasma, the helium atom excitation propagation starts in the vicinity of the high voltage electrode and departs from the capillary towards the ground electrode as well as several millimeters outside of the capillary in the form of the plasma jet. After relatively slow propagation of the early plasma in the capillary and the jet, the second plasma starts between the electrodes. During the negative voltage period, only the plasma in the capillary between the electrodes occurs.