Laboratory investigations of Alfven waves in a high power helicon plasma with density gradients.

摘要

Alfven wave dissipation is the primary physical process that underlies a leading theoretical model of coronal ion heating in the Sun. Data from Hinode, the recently launched high-resolution solar imaging mission [Erdelyi and Fedun, Science, Dec. 2007], has provided strong evidence for the presence of Alfven waves in the corona and in coronal loops. These Hinode observations have the potential to explain the million-degree difference between ion temperatures in the corona and at the top of the photosphere.;Laboratory investigations of Alfven wave propagation and damping typically require long plasma sources, needed to accommodate the long wavelength waves. Shorter experimental systems can be employed if the plasma density in the source is orders of magnitude larger. Such high densities can be achieved in helicon sources. Their high plasma density (∼1013 cm -3) yields Alfven wavelengths on the order of a few meters. However, helicon sources usually have a steep radial density gradient. The steep gradient introduces two regimes of Alfven wave propagation: kinetic (beta > me/mi) and inertial (beta me/mi), where beta is the ratio of total pressure to total magnetic field pressure. At the boundary between the two regimes, magnetic energy can accumulate due to a decrease in perpendicular group velocity of the waves.;This dissertation reports the first experimental observation of propagating kinetic Alfven waves in a helicon source. The kinetic model fit to the dispersion measurements includes the effects of ion-neutral damping, the magnitude of which is determined through in-situ neutral density measurements. The measurements are also consistent with the pileup of wave energy at the kinetic and inertial Alfven boundary.