The precision of electron cyclotron emission measurements from DITE Tokamak

Corrections for the systematic errors associated with the measurements of electron cyclotron emission from DITE Tokamak using a Michelson interferometer are considered under the following headings. (a) Finite optical depth an isotropic reflection model is developed leading to a line-integral expression for the emission intensity which depends on the optical depth and the wall reflection coefficient. (b) Finite density and refraction: the error due to the distortion of the instrument antenna pattern is calculated from results of ray-tracing computations. (c) Relativistic resonance layer width: an expression is derived for the corresponding spatial error for emission perpendicular to the magnetic field. (d) Magnetic field corrections: an expression for the spatial error due to the poloidal and diamagnetic field contributions is given and its effect on the observed Shafranov shift is investigated. (e) Frequency response: the general principles of quasi-optics are applied to the Michelson interferometer demonstrating that a small-aperture blackbody oven may be used to calibrate the system. (f) Spatial resolution of the viewing optics: the equivalence of lens optics and waveguide antenna is shown both theoretically and experimentally and expressions are given for the resolution perpendicular to the viewing axis. (g) Frequency resolution: it is shown that the resolution of a Michelson interferometer can be better than that suggested by application of the Rayleigh criterion. These corrections are applied to experimental emission spectra and the derived electron temperature profiles compared with laser scattering measurements in a discharge where the plasma equilibrium is changing. The Shafranov shift of the corresponding temperature surfaces is shown to be in good agreement with that expected for the magnetic flux surfaces. Using the wall reflection model, the electron density profile is derived from the emission profile of the third harmonic cyclotron frequency.