Investigation of a Magnetically Enhanced Inductively Coupled Negative Ion Plasma Source

Abstract

Experiments and numerical models were used to investigate an inductively coupled plasma source (ICPS) operating with a magnetic filter field. The work shows that applying magnetic filters transversely to the plasma offers several new control parameters to help enhance the properties of a plasma source. The application of these new results using magnetic enhancement is discussed with respect to both industrial plasma fabrication processes and neutral beam injection for fusion power. Experimental measurements of the power transfer efficiency of the ICPS were undertaken comparing the effect of the magnetic field for both hydrogen and argon plasmas. The location and strength of the magnetic field was varied while measurements of the plasma resistance and power transfer efficiency were performed. The changes in forward power transfer were correlated with plasma density measurements and a numerical model of the electrical plasma circuit was used to guide the optimal choice for the power system components. The results demonstrate that the magnetic field increases the total efficiency of the plasma source and that the gains are strongly dependant on the choice of location for the magnetic field. Plasma properties were then investigated across the plasma source 1 cm intervals. Experimental measurements comparing the effect of the magnetic filter on the plasma properties include: electron densities using a hairpin probe, electron energy probability functions using a compensated Langmuir probe, negative ion densities by laser photo detachment and rotational gas temperatures by optical emission spectroscopy. These measurements revealed interesting new properties of the plasma when a magnetic filter is applied including: the formation of a high density cold particle trap, changes in particle transport and drift motions, increased gas temperatures, and a peak in negative ion density under the magnetic filter center. Pulsing the plasma can greatly affect the plasma dynamics, leading to electron cooling in the afterglow and increased negative ion production. A combination of a pulsed plasma with a magnetic filter was then investigated. Measurements of the negative ion and electron populations were performed in the plasma afterglow with the magnetic filter applied. The results reveal a complex and dynamic afterglow process including strong spatial dependencies measured for diffusive transport, ambipolar breakdown and ion-ion plasma formation. The applications for this work include offering new avenues for control over processing plasma chemistry as well as initial results toward the future viability of a caesium-free pulsed negative ion neutral beam source.