Measurements of electric field in an atmospheric pressure helium plasma jet by the E-FISH method

Temporal and spatial distributions of the electric field in an atmospheric pressure, ns pulse, positive and negative polarity helium plasma jets are measured by ps electric field induced second harmonic generation. The measurements have been done in a quasi-two-dimensional plasma jet impinging on liquid water, using a laser sheet and a focused laser beam positioned at different heights above the water surface. Absolute calibration of the electric field is obtained by measuring a known Laplacian electric field distribution for the same geometry and at the same flow conditions. The vertical component of the electric field is determined by isolating the second harmonic signal with the vertical polarization. The measured electric field is averaged over the span of the plasma jet, in the direction of the laser sheet or the focused laser beam. The spatial resolution of the laser sheet measurements is approximately 15 μm across the sheet, with the temporal resolution of 10 ns. The spatial resolution of the focused laser beam measurements is approximately 180 μm across the beam, with the temporal resolution of 2.5 ns. The results show non-monotonous electric field distribution across the jet, with two maxima produced by the surface ionization waves propagating over water. Considerable electric field enhancement is detected near the surface. Residual charge accumulation on the water surface is detected only in the negative polarity pulse discharge. The results provide new insight into the charge species kinetics and transport in atmospheric pressure plasma jets, and produce data for detailed validation of high-fidelity kinetic models.