Measurements and code comparison of wave dispersion and antenna radiation resistance for helicon waves in a high density cylindrical plasma source

Abstract

Helicon wave dispersion and radiation resistance measurements in a high density (ne≈1019−1020 m−3) and magnetic field (B<0.2 T) cylindrical plasma source are compared to the results of a recently developed numerical plasma wave code [I. V. Kamenski and G. G. Borg, Phys. Plasmas 3, 4396 (1996)]. Results are compared for plasmas formed by a double saddle coil antenna and a helical antenna. In both cases, measurements reveal a dominance of the m=+1 azimuthal mode to the exclusion of most other modes; in particular, no significant m=−1 mode was observed. The helical antenna, designed to launch m<0 and m>0 modes in opposite directions along the field, resulted in an axially asymmetric discharge with very little plasma on the m<0 side of the antenna. For both antennas, good agreement of the antenna radiation resistance and wave dispersion with the model was obtained. It is concluded that unshielded antennas formed from current loops with an important |m|=1 component for the conditions of our experiment, couple most of their power to the m=+1 helicon mode and thus have negligible parasitic, nonhelicon plasma loading. This result greatly simplifies calculations of power balance in these sources by identifying the helicon as the mode by which energy is transferred to the plasma.