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In a tokamak the plasma current and the poloidal magnetic field which it induces are essential both for the confinement and for the heating of the plasma, but not enough is known about the current density distribution in tokamaks. The polarimetric measurement technique developed for the TEXTOR tokamak in Jülich enables the internal magnetic field structure to be determined for a wide range of parameters and to be compared to theoretical models. The method is based on the fact that the vibration planes of linearly polarised laser beams passing vertically through the plasma are twisted as a consequence of the Faraday effect, the degree of twisting being in proportion to the parallel component of the poloidal magnetic field. The physical basis and the apparatus of the polarimetric method are described in order to illustrate its potential for diagnostics and its limitations. The method is applied to a number of problems posed by the physics of the tokamak. The results are shown to have implications for the theoretical description of tokamak plasmas, requiring a reassessment of widely accepted theoretical models of the structure of the magnetic fields and their magnetohydrodynamic stability, particularly in the area of the hot plasma core. In future the polarimetric measurement technique will be of use in the diagnosis of the dynamic processes which occur, for example, when the total current is altered or when auxiliary heating is applied to the plasma.

Additional information

Authors: SOLTWISCH H, Institut für Plasmaphysik, Kernforschungsanlage Jülich GmbH, Jülich (DE)
Bibliographic Reference: Report: Jül-2339 DE (1989) 111 pp.
Availability: Available from Zentralbibliothek der Kernforschungsanlage Jülich GmbH, Postfach 1913, 5170 Jülich (DE)
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