Transverse electric field and density gradient induced instabilities in a cylindrical magnetron discharge

Abstract

Recent one-dimensional particle-in-cell simulations of a cylindrical postcathode direct current magnetron discharge have shown a transition from the usual positive space charge mode, dominated by a strong cathode fall, to a higher impedance negative space charge mode, characterized by a broad anode fall, as either the pressure is decreased or the magnetic field is increased. Such behavior is consistent with a steady decrease in the cross-field electron transport coefficients as calculated from classical fluid theory. However, experimental measurements of the distribution of electric potential between the electrodes do not exhibit any systematic variation with changes in magnetic field or pressure. This suggests that anomalous transport mechanisms increase the electron transport above the level predicted by classical transport theory, so maintaining the positive space charge mode at low pressures. It is shown using a fluid model that electrostatic oscillations with ω≳νi, where νi is the ion-neutral collision frequency, propagating in the E×B direction are unstable with growth rates comparable to the charged particle transit times. Such oscillations, which cannot be modeled in a one-dimensional particle-in-cell model, may contribute to the anomalous transport observed in experiments.