Stability of low-current needle-to-plane negative corona discharges in air

Negative DC corona discharges are known for their self-pulsing regime: the Trichel pulses. In some works, pulsed regimes have been observed immediately upon the inception of the discharge, while in other works the discharge was found to be ignited in a stationary (pulseless) mode after which the Trichel pulses develop. Recent theoretical and modelling work showed that the stationary negative corona between concentric cylinders in atmospheric-pressure air is stable immediately after the ignition. The pulseless mode was found also in the modelling of the needle-to-plane geometry, however in a quite narrow voltage range. This work studies conditions for a pulseless negative corona discharge in a needle-to-plane geometry to occur over a wide range of voltages, which will facilitate its unambiguous observation in the experiment. After the negative corona loses stability, the current evolution shows, after a small region of quasi-harmonic oscillations, pulses. These can be of small amplitude or regular Trichel pulses, which develop via standing-wave or ionization-wave mechanisms. Modelling results agree with available experimental data, both for the current-voltage characteristics and the stability limit of the pulseless discharge. An insight into stochastic Trichel pulses is given.