Influence of a target on the electric field profile in a kHz atmospheric pressure plasma jet with the full calculation of the Stark shifts

The electric field in the head of the plasma bullet (ionization wave) in a cold atmospheric pressure plasma jet is measured using the Stark polarization spectroscopy technique, a noninvasive method. The jet is driven by 1 μ s long voltage pulses at 6 kV amplitude and 5 kHz frequency, and a helium gas flow of 1.5 slm. Two helium lines (447.1 nm and 492.2 nm) are studied, from which the peak-to-peak wavelength difference between the allowed and forbidden band of the spectral lines is determined. The full derivation to obtain the electric field from this peak-to-peak difference is included in this paper. The electric field is determined both inside and outside the capillary of the jet, up to about 2 cm in the effluent of the jet. Measurements are performed on the freely expanding jet, but especially the influence is studied when a target is placed in front of the plasma jet. Targets with different properties are used: insulating (polyvinyl chloride, PVC), conducting (copper), liquid (distilled water and saline), and organic (chicken breast). It is found that a target changes the electric field of the plasma jet and thus changes the plasma itself. This change depends on the dielectric constant or conductivity of the target: a higher dielectric constant or higher conductivity yields a higher electric field. For a low dielectric constant ( ϵ r ≈ 3), the change in the electric field is negligible. Decreasing the distance between the target and the capillary to below 2 cm yields an increase in the electric field.