Possible excitation of solitary electron holes in a laboratory plasma

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 (n(p) similar to 10(9) cm(-3) and T-e similar to 0.5-2 eV) with the pulse magnitude U-0 >> kT(e)/e, where Te is the electron temperature. Experiments have been carried out for various applied pulse widths tau(p) ranging from less than 3f(i)(-1) to greater than 3f(i)(-1), where f(i) is the ion plasma frequency. For pulse widths less than 3f(i)(-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 3f(i)(-1), there is no indication of such virtual source. These disturbances propagate with two phase speeds, i.e., v(p)/v(e)=1.36 + 0.11 and 0.4 +/- 0.15, where v(e) 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. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3501994]