Physical properties of plasma streamers produced on water surface

Abstract

Since high voltage plasma discharges over water surface in the air have been studied in recent years for their potential applicability in several biomedical and industrial domains. This paper reports on experimental investigations of basic physical characteristics of these discharges. The streamers were produced in a tungsten needle-to-water surface geometry, and were forced to propagate in one direction by presence of a glass streamer guide. The needle electrode was driven by 44 kV/3.5 μs voltage pulses. Experimental arrangements limited total length of the streamers to about 320 mm. The experiments were performed with different gases: air, N2, and O2, every at 1 atmosphere unit pressure. Time and spatially resolved spectroscopic measurements have been done. The emission spectra were significantly changing during life span of the produced streamers. The early spectra of the discharges in the air and nitrogen atmosphere were dominated by N2 2nd positive system. After about 150 ns the N2 radiation disappeared, and emissions from atomic hydrogen took place instead. Spectra of the discharges in O2 atmosphere provided possibility of calculation of the excitation temperature from both atomic hydrogen and oxygen emissions. The excitation temperatures were measured in different parts of the discharge channel; excitation temperature in the inner part of the discharge was 3×104 K (hydrogen emissions), excitation temperature in the outer part was 1.6×104 K (oxygen emissions). Stark broadening of the hydrogen H line (486.1 nm) was used for determination of the electron densities. It reached its maximum value of 1018 cm−3 just after the discharge initiation in every case, and then it monotonically decreased. The electron densities depended only slightly on the distance from the needle electrode. Direct observation of the streamers by a high speed camera has also been done. It showed alternation of active and passive phases, where the active phase was an ionization avalanche. Whereas the average propagation speed of the streamer head was 450 km⋅s−1, the maximum propagation speed of the ionization avalanche exceeded 760 km⋅s−1.