Far-infrared interferometry/polarimetry on the ISX-B tokamak

Theoretical analyses have shown that the current density distribution in a tokamak plasma can be obtained indirectly by the measurement of the poloidal magnetic field created by the current flowing in the plasma discharge. The distribution of this field may be determined by projecting linearly polarized far-infrared (FIR) laser beams through the plasma and measuring the Faraday rotation of the polarization. Since the rotation angle of the polarization vector is proportional to the line integral of electron density times the poloidal magnetic field along the path, the electron density profile must also be measured simultaneously in order to unfold the current distribution. The amount of Faraday rotation expected in a typical plasma experiment is calculated to be on the order of 5 to 10 degrees. Therefore the authors have constructed a multichord modulated FIR polarimeter/interferometer that measures both the phase delay of four laser beams passing through a plasma discharge and the instantaneous polarization of these same beams. The phase shift of each of the beams is directly proportional to the line-averaged electron density and the polarization rotation is directly proportional to the line-averaged density times poloidal field