Speaker
Description
The information about the electron population emanating from a helicon source plasma is important in order to understand the formation of the current-free double layer (CFDL) between the source and the downstream region of a helicon plasma. The electrons need an energy higher than the potential drop across the CFDL to escape downstream from the source, and at these energies the signal of a standard Langmuir probe is less accurate.
We present measurements of the high-energy tail of the electrons by an inverted RFEA. To reach the probe, these electrons must have energies above Vp, which can vary over the region of the measurement. By constructing a full distribution from the electron temperature Te obtained from the electron IV curve and the Vp obtained from the ion IV-curve from a standard RFEA setup, we obtained a density measure of the hot distribution independent of Vp. We investigate variation of the high-energy tail of the EEDF in the radial as well as axial directions, in the two different cases of a purely expanding magnetic field nozzle, and a more constricted one by applying current in a third, downstream coil. The derived densities and temperatures of electrons from the source are then compared to an analytic model of the downstream development of the electron density from the source.
The method also allows for directional measurement of the electron current to the probe. This property is used in order to distinguish between the densities from the downstream and upstream direction.
