Effect of permanent magnets on plasma confinement and ion beam generation in a double layer helicon plasma source

Abstract

The work described in this thesis was carried out to investigate how additional permanent magnets (PM) could affect the confinement and ion beam generation in a plasma which expands from a helicon source. PMs added to a plasma source systems has been shown to provide an increased plasma confinement, but is not widely used. This inexpensive and cheap method of using PMs which does not require any external power supply can be attached basically anywhere on any plasma source, and is especially effective when placed around a cylindrical source.

The effect of the added PMs was investigated experimentally by using a Retarding Field Energy Analyzer (RFEA) probe with and without the PMs. The ion current obtained from the plasma by biasing the RFEA at different potentials was analyzed as a function of the potential, and the ion distribution function was extracted. The different plasma characteristics was extracted from the distribution functions, and the results from the characteristics with and without the PMs were compared.

Highly mobile electrons and/or high-energetic ions which would be lost to the walls of the port can be reflected back towards the plasma interior by the magnetic cusp field from the PMs placed around the port. The PMs create a very strong magnetic field around the borders of the system of interest which decreases rapidly away from the magnets, providing little change to the original system but results in a better plasma confinement because of the reduced plasma loss to the port wall. By adding PMs around a the port wall of a cylindrical plasma source with an expanding axial magnetic field, results have shown that the plasma density can in some cases be doubled. Another interesting effect introduced by the PMs is that the generation of ion beams in a helicon plasma source provided ions with a slight velocity reduction compared to the beam without PMs, but because of the increased ion beam density the flux of the ion beam is increased by a factor of up to 1.5. This might be useful for ion thrusters and beam propulsion systems based on a similar principle for beam generation which are being developed for satellites and space probes. Placement of PMs around the source exhaust-ports could thus provide an increased thrust.