PFC / JA - 89 - 57 ICRF - Edge Plasma Interaction Studies at MIT

The Alcator C-Mod experiment will study high power ICRF heating of elongated, diverted plasmas at high density in a compact, high field tokamak. ICRF heating will be carried out in the minority heating regime by fast wave excitation in a deuterium plasma with a 'He or H minority. Because of compact size of the tokamak, high loading will be required to couple the power at acceptable voltages and there will be high power density at the antenna surface. High edge densities and steep edge gradients are expected, making the ICRF-edge plasma interactions a key issue. Several studies have been undertaken to better understand these effects. Measurements have been made of the short wavelength perturbations to the rf fields caused by various Faraday shield geometries. Magnetic field probes show a perturbation which is largest near the ends and between the shield elements where the rf magnetic field is mostly radial. Electric field probes show a perturbation which is larger in the center of the elements and close to each element. A test antenna is being installed in the Versator II tokamak to study ICRF-edge plasma effects. The rf will be excited in the IBW polarization with ku chosen for interaction with edge electrons. The properties of different Faraday shield coatings, including titanium carbide and boron nitride, will be explored. It will also be possible to bias the protection limiters of this antenna in order to study the effect of biasing on the plasma density in the vicinity of the Faraday shield and resultant impurity generation.