Laser light scattering as a method for measuring the magnetic field direction in a plasma

The spectrum of light scattered by a plasma in a magnetic field consists of a sequence of peaks if the scattering vector k is almost perpendicular to the magnetic field B. Serious smearing of the peaks occurs when 2kνsub(e)cosphi >= ωsub(ce) where phi is the angle between k and B. Hence the scattered light will exhibit modulation only within a thin angular wedge. The angular location of this wedge can be determined by using a Fabry-Perot or a Michelson interferometer and from this the direction of the magnetic field can be determined. A generalized treatment is presented to allow for the interpretation of experimental results from any light scattering arrangement or plasma field configuration. The limits imposed on this diagnostic technique, such as the laser beam divergence and field uniformity, are also derived. For a representative Tokamak plasma of Bsub(tor) = 3T, Bsub(pol) = 0.3T and Tesub(e) = 1 keV, the poloidal field must be uniform within 0.015T in the scattering volume for 300 scattering by a ruby laser beam whose divergence must be less than 3 mrad to obtain an accuracy of about 5% in the determination of the ratio Bsub(pol)/Bsub(tor)