Possible excitation of solitary electron holes in a laboratory plasma

Abstract

Plasma response to a fast rising high positive voltage pulse is experimentally studied in a uniform and unmagnetized plasma. The pulse is applied to a metallic disk electrode immersed in a low pressure argon plasma (np∼109 cm−3 and Te∼0.5–2 eV) with the pulse magnitude U0⪢kTe/e, where Te is the electron temperature. Experiments have been carried out for various applied pulse widths τp ranging from less than 3fi−1 to greater than 3fi−1, where fi is the ion plasma frequency. For pulse widths less than 3fi−1, potential disturbances are observed to propagate in two opposite directions from a location different from the actual exciter (metal disk electrode), indicating the presence of a virtual source. For pulse widths equal or greater than 3fi−1, there is no indication of such virtual source. These disturbances propagate with two phase speeds, i.e., vp/ve=1.36±0.11 and 0.4±0.15, where ve is the electron thermal speed. It is also observed that by increasing plasma density, the speed of these disturbances increases, whereas the speed is independent of pulse magnitude. Analysis of these disturbances indicates the excitation of solitary electron holes.