The Application of Polarization as a Magnetic Field Diagnostic

Astrophysics, Plasma physics, Electromagnetics

Ameijenda, Victorino

University:University of Glasgow

关键词: Astrophysics, Plasma physics, Electromagnetics;   

Others  :  http://theses.gla.ac.uk/74695/1/10992317.pdf

来源: University of Glasgow

【 摘 要 】

The aim of this thesis is to explore the application and development of polarization diagnostics as a means of determining magnetic field structures in plasmas. It comprises two main themes: (1) an exploration of the center-to-limb Halpha polarization variation in the solar polar and equatorial regions and (2) an investigation of the Faraday rotation effect on the COMPASS-D tokamak. The former deals with the application of the Hanle effect in determing solar magnetic field structures while the latter is a means for diagnosing the magnetic field profile and hence the distribution of the current density in a tokamak plasma. Other solar features exhibiting polarization are also presented and the corresponding magnetic fields investigated. First an investigation of the center-to-limb linear polarization variation was conducted utilising an instrument capable of performing high precision polarimetry with a narrow band (FWHM Deltalambda = O.3A) Halpha filter tunable by +/- 0.33 A. A double beam imaging polarimeter was designed and constructed to measure the differential polarization between the center of the Sun and the limb at an angular resolution of 4 arcseconds per CCD pixel, with an expected polarimetric accuracy of +/- 0.02%. Other solar features exhibiting linear polarization such as prominences and areas of the quiet chromosphere are presented and the related magnetic fields determined. The radiation under investigation is engendered by coherent scattering processes associated with spectral lines and is modified by weak magnetic fields. This process is called the Hanle effect and involves a depolarization and a rotation of the plane of polarization. The second major part of the thesis relates to the results of the double diagnostic, namely an interferometer and a proposed polarimeter system operated at COMPASS-D. The interferometric part of the diagnostic serves as a tool to determine the electron density in the plasma. Once this is known, the polarimetric data may be analysed to give the cross-section distribution of the poloidal magnetic field, or, equivalently, the current density. The major limitation of the diagnostic is the line-integrated nature of the measurement. Since measurements are performed under one viewing angle only, assumptions related to plasma symmetry are essential for a proper interpretation of the data. This thesis focuses on the development and evaluation of the polarimetric aspects of the combined diagnostic. Plasma birefringence, resulting from the presence of a magnetic field component along the propagation direction of the beam, causes the linear or elliptical polarization of an incident probing wave to rotate. The rotation angle can be determined via a measurement of the polarization state of the emerging wave, hence the term polarimetry. A conventional but robust technique measures the relative intensity of two orthogonal linear polarization components separately after passing through or reflected from a polarizer. Alternatively, single detector techniques have been developed, where the incident polarization is modulated to generate an amplitude-modulation on the detected signal, from which the Faraday rotation can be derived.(Abstract shortened by ProQuest.).