Whistler Modes in Highly Nonuniform Magnetic Fields

点击翻译按钮获取中文摘要

Fundamental properties of certain electromagneticwaves in plasmas are investigated. These waves, specifically known as whistler modes, are very important in space plasmas and in plasma processing and propulsion. They have not yet been studied in plasmas immersed in highly nonuniform magnetic fields that change within one wavelength or less. The waves are excited and measured in a large laboratory plasma with local magnetic fields produced by strong permanent magnets and current-carrying coils. Wave propagation, wave refraction and reflection, standing waves and nonlinear effects are obtained from detailed measurements of three vector components of the wave magnetic field in three dimensional space and time. The findings are expected to produce a new understanding of wave effects in space plasmas and to improve industrial plasma sources for plasma processing.Specifically, whistler wave propagation will be studied near magnetic X- and O-type null points, magnetic neutral sheets and dipole fields with high mirror ratios. Such geometries are often found in space and laboratory plasmas, but there is little knowledge about the wave properties in a regime where the spatial changes of the magnetic field are comparable to the wavelength. Under those circumstances, different group and phase velocities are possible and complicate the wave propagation. The experiments produce complete data sets that provide valuable insight for the understanding of whistler-mediated reconnection, whistler eigenmodes on closed field lines, and whistler wave reflection at the lower hybrid frequency layer as well as whistler modes near lunar crustal magnetic fields, whistlers in small toroidal plasmas, and in helicon and magnetron discharges. Thus, the research will open up unexplored areas in plasma physics with broad relevance.