Comparison of theory and experiment on electron cyclotron emission from spherical tori

Summary form only given. The characteristic low magnetic field and high plasma density of a spherical Tokamak does not permit the typical radiation of O and X modes from the first five electron cyclotron harmonics. Thus only electron Bernstein waves, (modes not subject to a density limit) can be responsible for ECE radiation upon which mode convert into electromagnetic waves in the upper hybrid resonance region can be responsible for ECE radiation. A detailed 3 D model of the MAST equilibrium plasma is determined from the EFIT code and high spatial resolution Thomson scattering data. Taking into account the horn, thin lens two mirrors and a window associated with the RF antenna, a comparison is made between measured ECE and theoretical predictions from a Gaussian 3 D ray tracing code. For each ray we determine the spot on the separatrix and also the corresponding EBW-X-O mode conversion efficiency in the transport barrier. Using the reciprocity between EBW emission and EBW absorption, the EBW rays are launched from the upper hybrid resonance (UHR) towards the plasma center. Since N/spl par/ strongly oscillates for rays launched near the equatorial plane, nonlocal absorption characteristics must be considered. The effective radiation temperature is determined by solving the ray equations coupled to the radiative transfer equation, using the Rayleigh-Jeans black body radiation law. At oblique incidence, the antenna beam is partially blocked (and reflected) by the finite size of the window. This, and similar geometric effects, are responsible for the enhancement of the low frequency part of the ECE spectra (larger visible area) and suppression of the higher frequency parts of the spectrum (blocked by the window). Taking these effects into account, the comparison between theory and experimental data is good.