Excitation of terahertz radiation by an electron beam in a dielectric lined waveguide with rippled dielectric surface Phys. Plasmas 19, 093105 (2012) Inductance and near fields of a loop antenna in a cold magnetoplasma in the whistler frequency band Phys. Plasmas 19, 093301 (2012) Radiofrequency antenna for suppression of parasitic discharges in a helicon plasma thruster experiment Rev. Sci. Instrum. 83, 083508 (2012) First results from EBW emission diagnostics on COMPASS Rev. Sci. Instrum. 83, 10E327 (2012) Additional information on Appl. Phys. Lett.

By using phased bifilar antennas, helicon waves have been excited by applying fields which rotate either in space or in time, or both simultaneously. The direction of rotation is made to favor either m = +1 or m = -1 waves, where m is the azimuthal mode number, and m is measured directly. Rotation in time is found to be more effective than rotation in space and makes possible a direct comparison of m = ±1 excitation. An m = -1 structure was seen only in the antenna near-field, while m = +1 modes propagate far downstream. Up to 2 kW of RF power, density profiles and antenna loading measurements show that m < 0 (left-hand) waves are poorly coupled, and m = +1 (right-hand) waves are necessary for good plasma production. Loading results indicate that antennas also couple to absorption mechanisms unrelated to helicon waves.

Abstract. To produce the highest possible density in a helicon discharge, two helical antennas were used to launch m = +1 waves colliding at the midplane. With 2 kW of RF power on each antenna, a maximum plasma density of 8 × 10 13 cm −3 was achieved, not much higher than with a single antenna. This density limit is explained by gas depletion, and a method to avoid this is suggested.

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Effects of elecmn mass on A l f h waves in a cylindrical, inhomogeneous plasma are examined. in the cold plasma approximation, and in the frequency range from well below to several times the ion cyclotron frequency. oei. Both propagaling (k: > 0) and axially evanescent (k: c 0) modes are considered. It is shown how the cold plasma modes are coupled by a density gradient and how these modes are connected across the four domains bounded by kf = 0 and o = oci. Low-frequency surface-wave modes, predicted for a density step profile, lose their identity in a diffuse profile and merge with a discrete set of shear wave modes. High frequency surface modes survive in a diffuse profile. These, and other high frequency fast wave modes are axially evanescent at frequencies below ule k, = 0 cutdff fr&quency, and are Alfv6n-resonant if the cut& frequency is above the ion cyclotron frequency.