Behaviour of a low frequency wave in a FRC plasma

Abstract

A low frequency torsional Alfven wave (angular frequency ω ∼ 0.3ωci0, ωci0; ion gyro frequency in vacuum) is induced in an extremely high beta (beta = plasma pressure/pressure of confining magnetic field) plasma with the field reversed configuration (FRC) by an external antenna. When the wave is introduced from outside towards the magnetic axis of the FRC plasma, not only the Alfven velocity vA but also the local ion gyro frequency ωci decreases as the density increases and the magnetic field decreases. The spatial structure of the wave is examined in the region between the separatrix and the location near the O-point. Near the former location, the plasma is tenuous and the magnetic field has nearly external value so that ω ∼ 0.3ωci0, and near the latter location, the beta value is large and ω > 0.7ωci. The observed amplitude of the toroidal component of the magnetic disturbance has a broad peak around the radial location of the resonance position obtained from the cold plasma theory, although the cold plasma theory yields a sharper spatial structure. The polarization of the magnetic disturbance component perpendicular to the static magnetic field is left-handed outside the theoretical resonance position and right-handed deep in the plasma. The measured perpendicular wave number kr is small except for the region near the resonance position where the kinetic Alfven wave is expected to have a large wave number. This behaviour will be interpreted as, with the help of the warm plasma theory, the torsional Alfven wave evolves into the kinetic Alfven wave and is then converted to the compressional wave with a smaller wave number when the wave propagates inside the plasma.