Experimental Investigations of Wave Motion and Electric Resistance in Collisionfree Plasmas

by Wendt, Martin

The work presented in this thesis concerns processes in collisionfree laboratory plasma when high voltage drops give particle acceleration. The three processes investigated are electron beam-plasma interaction in the presence of density gradients, the electrical resistance of an inhomogeneous plasma after the application of a step voltage, and non-linear current oscillations caused by electric double layers. An experimental investigation of the electron beam-plasma interaction in an inhomogeneous plasma diode is presented. Even though the spread of the observed slow space charge waves follows linear growth and damping, experimental evidence and particle-in-cell simulations show that the oscillation frequencies are determined by standing waves trapped in the low density region at the diode boundary. The standing waves are identi ed as eigenmodes of the inhomogeneous plasma diode with the help of a uid model. Particle-in-cell simulations have shown that the application of a step voltage to a plasma with a density minimum may cause high potential drops which extend over the region where the electrons move towards decreasing density. The transient current pulses and the current limiting potential minimum are investigated experimentally. Their levels show good agreement with a one-dimensional model assuming xed ions and steady electron motion under quasi-neutral conditions. The decrease of the current after its maximum is shown to be partly caused by radial ion losses due to the transient radial electric eld. However, strong electric eld uctuations may also contribute to the resistance during the current decrease. A steady double layer has formed after an ion transit time. Its negative di erential resistance can give rise to nonlinear, large amplitude oscillations of potential and current. The circuit equation which has the form of a modi ed van der Pol equation, is shown to give excellent agreement with experiments and to explain the hysteresis and the jumps of the oscillation amplitude observed experimentally.