Faraday rotation measurements in high-β fusion experiments

The application of the rotation measurement to a linear magnetic-plasma system is described in which the integral n/sub e/(l)dl is determined by interferometric measurements and combined with the measured rotation integral B/sub i/(l)n/sub e/(l)dl to give the average value of the internal magnetic field B/sub i/(r) which exits over the plasma length l at the radius r. The measurements of B/sub i/(r) are combined with those of the external magnetic field B/sub e/ to determine the average plasma beta as a function of radius, β = 1 - B2/sub i/(r)/B2/sub e/. The Faraday rotation measurement uses a 1.5-mm diameter beam of visible radiation (6328 A) from a He--Ne gas laser as a plasma-magnetic field probe and a calcite Wollaston prism as the polarization analyzer with silicon diode detectors. The rotation is a small effect at optical wavelengths even for substantial densities and fields. The rotation is proportional to the square of the probing wavelength which suggests the use of infrared laser wavelengths. However, density gradients perpendicular to the beam direction cause beam deflections that also scale with lambda2. The size and spatial sensitivity of detectors is also an important consideration in the selection of the probing wavelength