Measurement of the energy distribution of trapped and free electrons in a current-free double layer

In the high potential plasma, upstream of the double layer, the measured electron energy distribution function EEDF shows a very clear change in slope at energies break corresponding to the double layer potential drop. Electrons with lower energy are Maxwellian with a temperature of 8 eV, whereas those with higher energy have a temperature of 5 eV. The EEDF in the downstream plasma has a temperature of 5 eV. Over the range of pressures wherein the double layer and accelerated ion beam are detected by analysis of a retarding field energy analyzer, the strength of the double layer corresponds to the energy where the slope changes in the EEDF break. We deduce that the downstream electrons come from upstream electrons that have sufficient energy to overcome the potential of the double layer, and that only a single upstream plasma source is required to maintain this phenomenon. © 2007 American Institute of Physics. DOI: 10.1063/1.2803763 High energy charged particles in space are thought to be accelerated by mechanisms such as proposed by Fermi, by waves, and more recently, by electric double layers DLs.1-3 Although some experimental data obtained from probes on satellites are available, they are rarely sufficient to fully de- velop self-consistent models of such space DLs and assump- tions on the form of the accelerated and trapped particle distribution functions have to be made. Perkins and Sun pre- dicted the existence of current-free double layer solutions,4 and soon after their prediction, current-driven laboratory double layers were set up in a current-free "mode,"5-7 con- firming the prediction. More recently, a new class of current- free double layers were experimentally found in expanding plasmas.8-10