On the mechanism of the negative differential resistance of a Townsend discharge

The effects leading to the falling behavior of the current-voltage characteristic of a Townsend discharge from 'zero' current are considered. The conditions investigated correspond to the breakdown of argon at pd ∼ 1 Torr cm. Using a Monte Carlo method for modeling electron multiplication the probability of ion-induced electron emission is extracted from the experimental breakdown data and the initial negative slopes of the current-voltage characteristic are found. This paper shows that two mechanisms provide the existence of the negative differential resistance R_1D. One of them is connected to the decrease in the probability of an electron to return to the cathode due to backscattering when increasing the discharge current; this mechanism was discussed before. The other mechanism is a result of non-equilibrium electron motion in a discharge gap. The higher the current and consequently the electric field at the cathode, the earlier an emitted electron acquires the energy necessary for ionization. This leads to an increase in the electron multiplication. Although this non-equilibrium effect is known, it was not considered previously as the possible cause of the appearance of the negative differential resistance. This paper shows that the role of the second mechanism in the formation of the negative differential resistance turns out to be even higher than that of the first one. The calculated R_1D values are in good agreement with the experiment.