Laser Induced Fluorescence Measurements of Spheromak Plasmas

Abstract

This thesis reports the results of investigations intended to further the understanding of the formation and evolution of spheromak plasmas via the use of laser induced fluorescence (LIF) measurements of laboratory plasmas. LIF is a spectroscopic technique in which laser radiation induces atomic energy level transitions in a target species within a specified volume. LIF experiments have been performed on Ar II plasmas produced in a spheromak confinement configuration. The term spheromak refers to a class of plasmas whose internal magnetic fields satisfy a particular topology, the details of which are presented as they relate to the formation and evolution of plasmas generated in this work. LIF measurements made on these spheromak plasmas suggest that 20 eV Ar II ions have been produced at densities of 10²¹ m⁻³. LIF experiments studying plasmas generated by a second spheromak device are discussed. The planar electrodes in this new device produce spheromaks with a distinct central column of plasma whose evolution is related to [alpha], an important parameter in the theory of force-free Taylor states, where the internal magnetic field B of the plasma satisfies ∇xB=αB. Experiments have been conducted with the probing laser oriented both parallel to and perpendicular to the axis of symmetry of the spheromak. Ion parameter estimates calculated from LIF measurements are found to agree with those obtained from other diagnostics, including passive spectroscopy and high speed photography. Details are presented concerning the design and operation of a portable device capable of generating plasma discharges. The motivation for the construction of this device is to provide a convenient plasma source that may be used to calibrate the laser and photodetection systems used in LIF experiments. The Ar II ion temperature and density values reported in this work are believed to be among the first such measurements performed on plasmas produced in a spheromak confinement configuration. Suggestions are offered for several modifications that could be made to the experiment that might serve to increase the amount of information that can be gained during each plasma discharge and thus augment the future value of the experiment.