Measurements of electric field in high-pressure plasmas by ps and ns e-fish generation

Temporal and spatial distributions of the electric field in an atmospheric pressure, quasi-two-dimensional, ns pulse helium plasma jet impinging on liquid water are measured by ps Electric Field Induced Second Harmonic (EFISH) generation. The measurements have been done using a laser sheet and a focused laser beam. Absolute calibration is obtained by measuring a known Laplacian electric field distribution for the same geometry and at the same flow conditions. 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 insight into the charge species kinetics and transport in atmospheric pressure plasma jets, and produce data for detailed validation of high-fidelity kinetic models. EFISH generation using a ns pulse duration laser is employed for time-resolved measurements of the axial and transverse electric field in a pin-to-pin ns pulse discharge in ambient air, at atmospheric pressure and at 2 bar. The results demonstrate that the time-varying electric field can be measured accurately over the duration of the laser pulse, 15-20 ns long, with the temporal resolution limited by the response time of the detector. Each data set provides the time-accurate electric field over a period of up to 10 ns. Time-resolved electric fields over longer time periods are obtained by overlapping the individual data sets. Absolute calibration is obtained by measuring the Laplacian field distribution in the discharge gap before breakdown, taking into account the residual charge accumulation on the surface of the dielectric encapsulating the grounded electrode. The results demonstrate the feasibility of ns EFISH measurements in high-pressure transient plasmas, using widely available ns pulse duration Nd:YAG lasers.