Experimental Investigation of Isolated Double Layer in a Current Carrying Plasma.

Experimental observation of a steady state, isolated, nonlaminar double layer is observed in the UC Davis STING, unmagnetized, current carrying device with v(,d) < v(,e). A variable current flows down an extension tube (diameter = 12 cm and length = 40 cm) using an anode at one end while a large plasma source (diameter = 60 and length = 50 cm) is connected to the other end. The plasma is approximately collisionless in the experimental regions. A double layer forms in the extension tube if current flows in the tube. The position of the double layer is determined by the current. As current is increased, ionization of neutral particles on the high potential side due to primary electrons and accelerated electrons occurs, thus generating a higher density pressure. The double layer is then pushed toward the lower potential side until a new steady state condition is reached. The double layer has typical values of the electric field of 7 V/cm, potential (e(DELTA)(phi)/kT(,e)) of 7, and length of 50(lamda)(,D). The plasma density on the low potential side increases slightly and drops sharply at the base of the double layer. It then increases and levels off in a similar manner to the potential. The localized potential structure accelerates charged particles to form beams on the high and low sides of the double layer. Large amplitude ((delta)n/n < 40%) ion turbulence and space potential ((delta)(phi)/(DELTA)(phi) < 13%) turbulence are observed in the double layer. On the high potential side, electron plasma turbulence driven by the electron beam and large amplitude ((delta)n/n < 10%) ion turbulence apparently due to the parametric decay of the electron plasma waves are also observed. Ion waves propagate in a direction oblique to the electron-beam flow. Electrons are mainly heated on the high potential side of the double layer. The density distribution of particles around the double layer agrees with a simple model which includes four species of particles: trapped ions, trapped electrons, free ions, and free electrons. The double layer in this experiment is three dimensional and has a convex shape so that electrons are focused on the high potential side. High electron drift (v(,d) (TURN) v(,e)) is not required to maintain a double layer. A peak value of v(,d)/v(,e) = 0.3 was observed in this experimental investigation.