Plasma density fluctuation measurements from coherent and incoherent microwave reflection

Using the spatial coherency present in a reflected microwave signal (Conway et al 1994 Rev. Sci. Instrum. 65 2920) it is possible to measure. coherent, Gamma(c), and an incoherent, Gamma(i), reflection coefficient (proportional to the radar cross section) from a turbulent plasma cutoff layer. Results acquired with a 17 GHz reflectometer from the STOR-M tokamak edge region (r/a approximate to 0.8) give significant Gamma(c) and Gamma(i), which suggests two-dimensional structure in the reflection layer. Using a 'distorted-mirror' model for the plasma fluctuations, estimates of an effective radial width, sigma, and poloidal correlation length, L(p), can be derived from the reflection coefficients. STOR-M results typically give a a of a few millimetres and an L(p) of a couple of centimetres.