The effect the power supply frequency and the dielectric barrier material have on the radiation spectrum from surface discharge plasma

Abstract

The results from experimental studies of irradiance in the UV range and the spectral composition of radiation from the surface discharge plasma as a function of the dielectric barrier material and power supply frequency are presented. The experiments were carried out using axially symmetrical electrode systems containing a disk-shaped electrode by applying a sine-wave power supply voltage with a frequency from 0.6 to 35 kHz. Alumina ceramics of different composition, ceramics from pure aluminum nitride, ultrahigh molecular weight polyethylene, and synthetic ruby were used as the barrier material. The thickness of all barriers used in the study was equal to 1 mm. It is shown that the interaction of discharge plasma with the dielectric barrier material can change the discharge's optical spectrum. For example, the occurrence of a continuous spectrum for aluminum nitride and separate spectral lines in case of adding a small quantity of chromium oxide in the composition of alumina ceramics were observed. The presence of a continuous spectrum of discharge plasma radiation over the aluminum nitride surface facilitates the growth of the UV-A radiation component. The most intense radiation among the considered barrier material versions was observed in the UV-A band. The irradiance formed by surface discharge plasma is three orders of magnitude lower than the values used as doses in subjecting seeds to preplant stimulation. With the exposure time equal to 5 min, the discharge plasma produces the radiation dose equal to 45 J/m2 necessary to stimulate seeds (the stimulating dose is in the range from 4 to 60 kJ/m2). On the other hand, the plasma radiation cannot provoke even the minimal damage to the DNA of the irradiated object. The radiation dose in the UV-C band with a 15-min exposure does not exceed the minimum DNA damage inducing dose, namely, the formation of thymine dimer in recalculation for a 254 nm wavelength. By using the discharge energy versus the power supply frequency curve, it is possible to determine the range of frequencies at which the transition to a more high-energy form of discharge takes place. This transition is associated with an increase of the UV-C component in the radiation.