Investigation of the Influence of Different Boundary Conditions on Helicon Discharges

In this paper we present investigations on the influence of boundary conditions on helicon discharges. This is done using two approaches: Firstly, the influence of the plasma size is studied by excitation of whistler waves with different wavelengths (frequency range 100 – 1000 MHz). For wavelengths much smaller than the plasma dimensions (4.5 m in length and up to 40 cm in diameter), the unbounded whistler dispersion relation turns out to be appropriate. For increasing wavelengths, the measured dispersion deviates more and more from the predictions of unbounded plasma theory and the helicon wave dispersion relation must be used. Secondly, the boundary condition between the antenna and the plasma is investigated. Standard right-helical antennae are used to operate an rf helicon plasma at a typical frequency of 13.56 MHz. In the linear magnetised plasma experiment VINETA, two different setups for the RF plasma source are available: a standard helicon setup with a glass cylinder attached to one end of the chamber, and a helicon antenna inserted into the stainless steel chamber. The spatial distribution of magnetic fluctuations as well as the plasma parameters are measured with high resolution in a plane perpendicular to the magnetic field as well as in a parallel plane. The influence of the different electric boundaries in the two setups is studied extensively under different operational conditions. This is done by studying the transitions between the capacitive, the inductive and the helicon mode operation