Helicon waves in a plasma confined by a cylinder are treated. The undamped normal modes of the helicon (H) and Trivelpiece-Gould (TG) waves have distinctly different wave patterns at high magnetic fields but at low fields have similar patterns and therefore interact strongly. Damping of these modes, their excitation by antennas, and the RF plasma absorption efficiency are considered. Nonuniform plasmas are treated by solving a fourth order ordinary differential equation numerically. A significant difference between this and earlier codes which divide the plasma into uniform shells is made clear. Excitation of the weakly damped H wave, followed by conversion to the strongly damped TG wave which leads to high helicon discharge efficiency, is examined for realistic density profiles. A reason for the greater heating efficiency of the m = +1 versus the m = -1 mode for axially peaked profiles is provided.

The theory of helicon waves is extended to include finite electron mass. This introduces an additional branch to the dispersion relation that is essentially an electron cyclotron or Trivelpiece–Gould (TG) wave with a short radial wavelength. The effect of the TG wave is expected to be important only for low dc magnetic fields and long parallel wavelengths. The normal modes at low fields are mixtures of the TG wave and the usual helicon wave and depend on the nature of the boundaries. Computations show, however, that since the TG waves are damped near the surface of the plasma, the helicon wave at high fields is almost exactly the same as is found when the electron mass is neglected.

It is well known that the simple theory of helicon waves, in which the electron mass m e is neglected, is valid only if E z also vanishes, a condition which is not satisfied in experiment. Exact solutions of cold plasma theory with finite m e and E z predict the existence of additional highly damped Trivelpiece-Gould ͑TG͒ modes ͑H-TG theory͒, which can greatly modify the nature of helicon discharges. However, most experiments have been explained using only the simple theory for which the helicon waves are undamped. In that case, antenna-plasma-coupling calculations predict infinite resonances. To avoid this problem theorists have set E z ϭ0 and included finite m e effects ͑the TE-H theory͒. By comparing the TE-H theory with exact ͑i.e., closed form͒ solutions for uniform density, the role of TG modes has been clarified. To do so for nonuniform density, a new algorithm is developed to treat the case of high magnetic fields, when the wave equation becomes singular. The results show that, though the wave patterns are not greatly affected by TG modes except at low magnetic fields or near the radial boundary, the k z spectrum and radial profile of the energy deposition are greatly modified. In particular, the peaks in the TE-H-mode spectrum, which lead to predictions of erroneously high antenna loading, are suppressed and broadened by the TG modes which also produce high edge absorption. Both the TE-H and exact theories give maximum antenna loading for 1 2 m e (/k) 2 Ϸ10-100 eV, in contrast to several hundred electron volts predicted by the simple theory.

Traveling-and standing-wave characteristics of the wave fields have been measured in a helicon discharge using a five-turn, balanced magnetic probe movable along the discharge axis z. Helical and planepolarized antennas were used, and the magnitude and direction of the static magnetic field were varied, yielding three primary results. 1) As the density varies along z, the local wavelength agrees with the local dispersion relation. 2) Beats in the z variation of the wave intensity do not indicate standing waves but instead are caused by the simultaneous excitation of two radial eigenmodes. Quantitative agreement with theory is obtained.3)The damping rate of the helicon wave is consistent with theoretical predictions based on collisions alone.

In a plasma composed of negative (SF−6) ions, positive (Ar+) ions, and electrons (with ne/n+ reducible to less than 10−3) a fast ion mode (vph≈9Cs), due to the out-of-phase motion between positive and negative ions, is observed with reduced spatial attenuation. This fast ion mode exhibits spatial growth in the presence of a fast ion beam. Diffusion properties of the negative ion plasma are also presented.