Discharge equilibrium of a helicon plasma

The axial dependence of the plasma density, electron temperature, plasma potentials, and the 488 nm argon ion emission intensity have been measured in argon helicon discharges excited by both right helical and Nagoya III antennas for various magnitudes and directions of the magnetic field B. The plasma parameters were monitored with RF-compensated probes, while the emission line was detected with an optical emission spectrometer that incorporates an optical fibre and a miniature lens. The right helical antennas were designed to excite the m = +1 azimuthal mode when B is parallel () to the propagation vector k, and the m = -1 mode when B is antiparallel (#) to k. The plasma is found to be more dense in the former case (, m = +1), and the density peaks several antenna lengths downstream in the k direction. Nagoya III antennas are symmetric antennas that should excite the same azimuthal mode content in either magnetic field direction; indeed, the light profile was found to be independent of field direction. In the near field, under the antennas, the density is approximately the same for both antenna geometries and magnetic field directions. These results indicate that the m = +1 mode is preferentially excited regardless of the antenna helicity.

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