Effects of Frequency on the Electrical Characteristic of Pulse Discharges in Atmospheric-Pressure Pure Helium

The effects of the frequency on the electrical characteristics in a dielectric barrier discharge in pure helium excited by repetitive voltage pulses with the frequency f from 5 to 500 kHz are systematically investigated by means of a 1-D fluid model under different operating conditions, including gap width d(g), secondary electron emission coefficient gamma, dielectric thickness d(s), and dielectric constant epsilon(r). The important characteristic quantities of describing the pulse discharge, i.e., discharge current density J(g), averaged electron density N-ave, averaged dissipated power density P-ave, and the axial distributions of both electron density N-e and electron temperature T-e, are calculated and analyzed in detail. This paper gives the following significant results. N-ave increases with increasing f and P-ave is almost proportional to f. Especially, there is a characteristic frequency of similar to 50 kHz, namely, when f is lower than similar to 50 kHz, J(m1) (peak value of Jg in the first discharge) decreases evidently with increasing f, and then changes very slightly for f s larger than similar to 50 kHz. In addition, with the increase of f, J(m2) (peak value of J(g) in the second discharge) increases very slightly for small d(s) and the reverse is true for large d(s). The frequency dependence of J(m2) for different epsilon(r)s is similar to that by changing ds. For a fixed frequency, the change of J(m1) with gamma is very small, which differs from that at low frequencies. In addition, J(m1) and J(m2) decrease with increasing ds and increase with increasing epsilon(r), but there are almost constant Delta J(m1) and Delta J(m2) for high frequencies. When the frequency is larger than similar to 50 kHz, there will be the second peak of N-e outside the cathode sheath in the first discharge, and both large ds and small er can result in the formation of the evident peak of N-e nearby the momentary anode. It should be noted that the analysis in this paper pertains only to the incompressible motion of the plasma.