Nanosecond pulse discharges in electronegative gases

Abstract

Experiments have been conducted using C^Fn-Ar and SFg-N^ mixtures as f i l l i ng gases in a uniform-field spark gap subjected to highly overvolted nanosecond pulses. The electrode separation is 1 cm. U.V. radiation is applied prior to the arrival of the pulse to the gap. In C2Fg-Ar mixtures the pressure ranges from 500 Torr to 960 Torr and the overvoltage ranges from 50% to 300%. After the i n i t i a l rate of current rise at the load a phase of constant current is observed the length of which changes with the concentration of ^3^8* ^^^^ phase has large resistance and the energy deposited into the discharge ranges from 6% to 20% of the energy in the applied pulse as the concentration of C^Fg increased from 30% to 100%. The physical appearance is a multichannel discharge. In light of the above observation, the use of C^Fg-Ar mixtures in self and non-self sustained discharges is discussed. Another experiment was conducted in a 30-70% C^Fg-Ar mixture at 1480 Torr to observe the difference in current development for single and multiple electron initiated discharges. When the initial number of electrons is large the observational time lag decreased with an increase in the initial rate of current rise. The stability of the observational time lag in the formation of discharges in SFg-Np mixtures was also investigated with parameters: pressure (up to 1360 Torr), percent SFg (0 to 100%), type of U.V. (pulsed, continuous) and separation in time, T, between U.V. irradiation and the arrival of the voltage pulse to the gap. The results indicate that: 1) In C-Fg-Ar mixtures significant energy loss may occur in "turning-on" the discharge because of the low conductance of the first state of equilibrium through which the discharge passes; 2) For self-sustaining application, a trade off exists between the energy loss in the discharge and the desired recovery properties of the gas; 3) Applications which utilize self-sustaining discharges cannot take advantage of the properties of C^Fg-AR mixtures because the equilibrium state is in a region where ionization and attachment are high, i.e., requiring high E/N to maintain this balance; 4) There is a significant change in the shape of the current growth in C^Fg-Ar mixtures as the initial number of electrons vary, indicating a difference in processes for single and multiple electron initiation; 5) In SFg-Np mixtures the stability of the observational delay was observed to increase for decreasing T and low percentages of SFg. The best stability was obtained when the U.V. radiation was coincidental or shortly before the arrival of the voltage pulse to the gap, and the percentage of SFg was 10%.