How dielectric, metallic and liquid targets influence the evolution of electron properties in a pulsed He jet measured by Thomson and Raman scattering

Thomson scattering using a Bragg grating notch filter is used to determine the electron properties of a pulsed, kHz-driven, non-thermal atmospheric pressure plasma jet in helium expanding in air. The plasma jet is allowed to freely expand or interact with targets with different electrical properties, i.e. glass, copper and water. With the same setup, Raman scattering is used to determine spatially- and time-resolved the densities and rotational temperatures of oxygen and nitrogen molecules entrained into the jet. Fast imaging is used to determine the development of the discharge in the plasma jet as well as its behavior in the plasma-target interaction zone. As the discharge approaches the target, the rise of electron density was followed by the fall of electron temperature. The discharge is influenced only over a few millimeters before it hits the target. The electron density and temperature during the spreading of the discharge on the low-permittivity target are measured to be resp. 2 × 1019 m−3 and ≈1 eV. During the return stroke on the high-permittivity and the metallic target the densities rise with a factor 1.5 resp. 2.2, and the temperature with a factor 2.5 for both cases. The discharges on the high- and low-permittivity targets extinguished soon after the initial impact of the ionization front, while the diffuse discharge on the metallic target extinguished only after the end of the voltage pulse (with a duration of 1 μs). In the diffuse discharge the electron temperature reaches 3.4 eV, the gas temperature increases by approximately 100 K and the electron density increases by approximately a factor three with respect to before its formation.