Modes of oscillation in dc driven high pressure microplasma discharges

Atmospheric pressure microplasma devices have been the subject of considerable research during the last decade [1]. Most of the operation regime of the discharges studied fall in the `abnormal´, `normal´ and `corona´ modes - increasing and a `flat´ voltage current characteristics. However, the negative differential resistance (NDR) regime at atmospheric and high pressures has been less studied and possesses unique characteristics that can be employed for novel applications. The NDR regime has been studied for low pressure systems and has been characterized to be associated to relaxation oscillation only [2-4]. In this work we report a detailed study on the different modes of self oscillation in high pressure micro plasma discharges. Detailed multidimensional numerical simulations have been conducted with a validated model. The different self-pulsing modes of oscillation have been identified as, relaxation oscillation having medium to low frequency oscillation at low discharge current and high frequency free running oscillation at comparatively high discharge current condition. In the relaxation oscillation, the discharge switches between a dark and glow like discharge whereas in the free running mode the transition is observed to occur within glow like modes. Predictions from the model are in favorable agreement with experimental measurements. These two modes of oscillation are found to be more prevalent at higher pressure. Depending on pressure, the frequency of relaxation and free running oscillations are in the kHz - MHz and MHz - GHz range respectively. External parameters influencing these self oscillations are also studied, conditions suitable for suppression of these oscillation is discussed.