Experimental Observation of Slow Ion Acoustic Double Layers

Abstract

Laboratory results are presented to show the existence of a new type of double layer which has monotonic potential variations but can be triggered with an electron drift velocity less than the electron thermal velocity. These double layers are always found to evolve from virtual-cathode potential wells at the electron injection boundary, independent of the electron drift velocity. However, the amplitudes of these double layers are determined by the initial drift velocity of the injected electrons. Double layers with potential drops less than the ambient electron temperature are found to be unstable to ion-ion streaming instabilities and decay into ion acoustic double layers.