Modeling of microplasmas from GHz to THz

Abstract

We present a study of atmospheric-pressure microdischarges sustained over a wide range of continuous excitation frequencies. A fluid model is used to describe the spatial and temporal evolution of the plasma properties within a 200μm discharge gap. At 0.5GHz, the behavior is similar to a typical rf collisional discharge. As frequency increases at constant power density, we observe a decrease in the discharge voltage from greater than 100 V to less than 10 V. A minimum of the voltage amplitude is attained when electron temporal inertia delays the discharge current to be in phase with the applied voltage. Above this frequency, the plasma develops resonant regions where the excitation frequency equals the local plasma frequency. In these volumes, the instantaneous quasi-neutrality is perturbed and intense internal currents emerge ensuring a low voltage operation range. This enhanced plasma heating mechanism vanishes when the excitation frequency is larger than the local plasma frequency everywhere in the plasma volume. For a typical peak electron density of 5 × 10 20 m-3, this condition corresponds to ∼ 0.2 THz. Beyond the plasma frequency, the discharge performs like a low loss dielectric and an increasingly large voltage is necessary to preserve a constant absorbed power.