Polarisation Coherence Imaging of Electric and Magnetic Fields in Plasmas
Abstract
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.