Observation of increased upstream ionization resulting from spontaneous formation of a double layer in an expanding argon helicon plasma

Abstract

Summary form only given. We report observations that confirm the theoretical prediction1 that formation of a current-free double layer in a plasma expanding into a chamber of larger diameter is accompanied by an increase in ionization upstream of the double layer. The increased ionization is caused by electrons accelerated to the upstream side of the double layer. The theoretical model argues that the increased ionization is needed to balance the difference in diffusive losses upstream and downstream of the expansion region. We use both Laser Induced Fluorescence [LIF] and a Retarded Field Energy Analyzer [RFEA] to measure the ion velocity distribution functions ivdfj upstream and downstream of the double layer. In our expanding helicon source experiments, measurements of ion-beam energy and density indicate that a stable ion beam appears for rf frequencies above 11.5 MHz. The upstream plasma density increases sharply at the same threshold rf frequency. Below 11.5 MHz, large electrostatic instabilities are observed and appear to suppress formation of the double layer - simultaneously reducing the upstream ionization rate. Further studies of the electrostatic noise suggest a beam driven instability at ~ 18 kHz.