Helicon wave excitation with bifilar antennas

Summary form only given, as follows. A bifilar, helical antenna has been used to produce an RF field that rotates in time as well as space. A special match box was constructed to tune the two helices with adjustable phase and amplitude. In this way, one hoped to excite the left-hand polarized (m=-1) mode of the helicon wave, which is usually not seen. We find that the strongest coupling is achieved when both the spatial and temporal rotations match those of the m=+1 mode, with the temporal rotation being more important. When both the time and space variations are set to excite the m=-1 mode, it can indeed be seen, but only close to the antenna. It damps rapidly, and downstream from the antenna only the weakly excited m=+1 mode remains. These observations are in agreement with computations by D. Arnush, which show that the antenna coupling to the m=-1 mode is much weaker than to the m=+1 mode.