Excitation and Propagation of Alfvén Waves in a Helicon Discharge

An experimental study of shear Alfvén waves in a linearly magnetized plasma is presented. Shear Alfvén waves are electromagnetic waves propagating parallel to the background magnetic field without compression of the plasma at a frequency well below the ion cyclotron frequency and a wavelength inversely proportional to the square root of the plasma density. A basic condition on laboratory investigations is that the Alfvén wavelength must be significantly smaller than the device dimension. This makes Alfvén waves difficult to investigate in laboratory experiments and most studies are performed in space, where typical Alfvén wavelengths of several kilometers are observed. The results of these studies are often ambiguous due to difficulties concerning the measurements of plasma parameters and the magnetic field geometry. The primary motivation for the present paper is the investigation of Alfvén wave propagation in a well defined laboratory situation. The experiments are conducted in the linear VINETA device. The necessary operational regime is achieved by the large axial device length of 4.5m and the use of a helicon plasma source providing high density plasmas with ionization degrees of up to 100%. The Argon plasma is magnetized by a set of 36 magnetic field coils, which produce a maximum magnetic field of 0.1T on the device axis. With this configuration a plasma-β of ≥ 10^–4 is achieved, which exceeds the electron to ion mass ration, and the ion cyclotron frequency is [approximate] 250kHz. Langmuir probes provide detailed informations on the time-averaged plasma profiles. Magnetic field perturbations for the excitation of Alfvén waves are generated by a current loop, which is introduced into the plasma. The surface normal of the current loop is directed perpendicular to the magnetic field. The waves's dispersion relation in dependence of plasma parameters is determined by spatially resolved B-dot probe measurements