It is shown that the current distribution in a typical Tokamak plasma can be measured by a light scattering technique. The direction of the total magnetic field is measured accurately enough that the magnitude of the small poloidal component can be found. The field direction is measured by observing the scattered frequency spectrum of CO2 laser light. The usual Gaussian spectrum becomes modulated at the electron cyclotron frequency when the difference between the incident and scattered wave vectors is nearly perpendicular to the magnetic field. The harmonics can be superimposed with a Fabry-Perot interferometer and their collective width resolved as the scattering direction is changed. The SNR is high only when the detector is shielded against background radiation.
Norton Bretz, "Proposed Method of Measuring the Current Distribution in a Tokamak Plasma," Appl. Opt. 13, 1134-1140 (1974)