A method for measuring the direction of the magnetic field in a Tokamak plasma by laser light scattering

For scattering wavenumbers k such that alpha =(1/k lambda D )<<1 and 1/k perpendicular to rho e <<1, the profile of the spectrum of light scattered from a magnetised plasma is similar to the profile of the electron velocity distribution. However, for k perpendicular and close to perpendicular to B, the light is concentrated within this profile, in peaks which are separated from the incident frequency by multiples of the cyclotron frequency Omega e . For a Maxwellian velocity distribution, these peaks are Gaussian in shape with half width k/sub ///a. The degree of modulation of the spectrum is very sensitive to the value of k/sub ///a/ Omega e . For plasmas typical of those in Tokamak devices, n e =10 13 to 10 14 cm -3 , T e =100 to 2000 eV, B=10 to 50kg, there is significant modulation, with ruby laser light, only when k is within about one degree of the perpendicular to B. The shape of the spectrum may therefore in principle be used as an indicator of the direction of B. However, for ruby laser light the modulation is on such a fine scale, k perpendicular to a>> Omega e , that resolution of the spectrum shape by conventional means is not possible. It is shown in this paper that the peaks are superimposed when the scattered light is passed through a Fabry-Perot etalon of free spectral frequency range equal to the electron cyclotron frequency. The resulting modulation, and consequently the effect of the magnetic field, can then be detected even with a ruby laser source.