Oscillations of low-current electrical discharges between parallel-plane electrodes. I. dc discharges

The oscillatory behavior of dc discharges between parallel-plane electrodes (1 cm spacing) at low pressures (0.5–3 Torr) is measured and compared with models. For hydrogen these discharges are unstable for wide ranges of discharge current below 0.2 mA/${\mathrm{cm}}^2$ and external circuit resistances above ≊4000 Ω. Lateral constrictions of the discharge occur over a much more limited range of currents and pressures than do oscillations. Laser-induced photoelectron pulses produce damped oscillations for discharge currents below those at which self-sustained oscillations are observed. The frequency of the oscillations varies approximately as the square root of the discharge current and the damping of the oscillations increases with the discharge current. These results agree with simple models developed in an accompanying paper [this issue, Phelps, Petrović, and Jelenković, Phys. Rev. E 47, 2825 (1993)]. Some data are presented for argon and nitrogen.