Characterizing spatially varying optical emissions in a steady-state dipole plasma: inversion based experiments and modeling

This study characterizes spatially varying optical emissions in a compact dipole plasma device driven at steady-state by continuous mode microwaves. The study is motivated by visual observations, which indicate a distinct pattern of alternate bright and less bright regions (bearing structural resemblance to the two particle radiation belts found in the Earth's magnetosphere). The investigation is performed in two experimental systems of cylindrical and spherical geometries, and boundary effects in the optical emissivity are observed in the smaller cylindrical system. Two optical diagnostic techniques are employed, namely, a simplistic linear inversion method, and the standard Abel inversion method, to invert the measured intensities and determine the local (spatially varying) emissivities in the equatorial plane of the dipole plasma. The study involves the design and development of the two optical probes, specifically, a telescopic probe capable of motion along a radial line (for linear inversion), and a mechanical gear-operated probe capable of bidirectional motion to obtain chord integrated intensities (for Abel inversion). Finally, the transition specific photon emission rates are determined by the application of a modified corona model, and the emission rates are compared with the experimental results. The existence of two bright belts separated by a darker band in the dipole plasma is confirmed by both the experimental and modeling results. (paper)