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Effect of the Plasma Inhomogeneity on the Wave Excitation and the rf Power Deposition in Helicon Discharges

The rf power deposition in helicon discharges generated through helical antennas depends sensitively on the radial density profile. The results obtained on two distinct devices give evidence that the density gradient is decisive for the formation of helicon discharges which are predominantly sustained by the helicon modes with azimuthal mode numbers m > 0. The width of the density profile determines the degree of axial asymmetry of the discharge caused by the different propagation behaviours of m > 0 and m < 0 helicon modes. Furthermore, the total rf power absorption is significantly affected by the edge density. Our computational results show, in particular, that the Trivelpiece–Gould mode carries a considerable fraction of the rf power which is most likely rapidly lost at the plasma edge. With respect to plasma applications, the rf power coupling via m = +2 helical antennas may be very attractive because large-volume plasmas can be generated.