Polarisation Coherence Imaging of Electric and Magnetic Fields in Plasmas
Creators: name: Thorman, Alex
Publisher: name: The Australian National University
Year: 2018
Subjects: Motional Stark Effect
Polarisation spectroscopy
Balmer-alpha
Stark-Zeeman effect
Coherence imaging
RF wave synchronous imaging

description: Polarisation coherence imaging is a powerful spectroscopic diagnostic for high spatial resolution measurements of strong electric and/or magnetic fields inside high temperature fusion plasmas. The motional Stark effect (MSE) is the principal application for the technique, involving measurement of the Balmer-α polarised emission from high velocity neutral beam atoms subjected to a strong vxB electric field. The research in this thesis examines all aspects of polarisation coherence imaging including: the atomic physics of the Stark-Zeeman split light emission; the optical physics and measurement principles involved in the technique; and experimental measurements on the DIII-D tokamak and H-1 heliac. The polarisation properties of single electron atoms in crossed electric and magnetic fields are revealed to be more complex than previously recognised due to the remaining degeneracy in the Stark-Zeeman energy levels and the absence of a well-defined magnetic quantum number. A linear perturbation theory analysis finds distinct polarisation structures for the σ emission that apply when the fine-structure of the atom and microscopic electric fields are considered. Only the σ±1 polarisation orientation is sensitive to upper-state populations, which are known to be non-statistical for MSE beam-into-gas calibrations, however with appropriate viewing geometries and neutral beam injection directions the effect can be made negligible. Similarly beam-into-gas calibrations of the σ±1:π±3 line intensity ratio are found to be sensitive to upper-state populations and are therefore potentially invalid. Equations for the linear and circular polarisation of each Balmer-α transition are formulated, correct to second order in the Zeeman:Stark splitting ratio, for straightforward interpretation of the Stokes parameters. Calculations reveal the net circular polarisation fraction for the π±3 and π±4 emission is ~20% for typical beam energies. Various imaging polarimeter designs exist and their different advantages and calibration challenges are examined. Some imaging MSE strategies encode the Stark-Zeeman circular polarisation at the same spatial frequency as the linear polarisation, however fortunately it is established that the interferometric delay can be tuned to mitigate the circular polarisation signal without severely reducing the linear polarisation signal. A newly developed non-axial ray model is capable of predicting and characterising additional spatial carriers generated by a sequence of displacer waveplates. An imaging MSE diagnostic was benchmarked against existing conventional MSE polarimeters on DIII-D and delivered new capabilities for measuring the magnetic pitch angle from 2 neutral beams and on the high field side of DIII-D. The imaging measurements from each neutral beam were self-consistent and good agreement was demonstrated with conventional MSE measurements for shots with magnetic field and plasma current in the standard direction, however the agreement is lost for shots with either reversed field or current direction. An analysis of the magnetic axis position independently measured with the conventional MSE, imaging MSE, electron cyclotron emission and magnetics is presented to elucidate differences between the MSE measurements. The ferroelectric liquid crystal waveplate used in the imaging polarimeter was discovered to have spatially non-uniform retardance, hence it is imperative for the illumination of the calibration source to replicate the ray paths of the neutral beam emission through the optical system. A systematic distortion is apparent in the images above and below the midplane, possibly due to remaining uncertainties from the Faraday rotation calibration or the illumination source dependence. Phase resolved imaging on the H-1 heliac revealed a 7MHz temporal oscillation in the light intensity that has the structure of a propagating wave. Using multiple viewing geometries and a magnetic field strength scan it was revealed that the wave characteristics are consistent with electron density perturbations produced by an electromagnetic ion cyclotron wave propagating near the last closed flux surface. The parallel velocity of the observed RF wave is comparable to the electron thermal speed suggesting that Landau damping of the wave energy to electrons drives the edge electron heating on H-1.