Electron Cyclotron Resonance Hydrogen/helium Plasma Characterization and Simulation of Pumping in Tokamaks

Electron Cyclotron Resonance (ECR) plasmas have been employed to simulate the plasma conditions (i.e., ion energy and fluence) at the edge of a tokamak in order to investigate hydrogen/helium uptake in thin metal films. The process of microwave power absorption, important to characterizing the ECR plasma source, was investigated by measuring the electron density and temperature with a Langmuir probe and optical spectroscopy as a function of the magnetic field gradient and incident microwave power. A novel diagnostic, carbon resistance probe, provided a direct measure of the ion energy and fluence while measurements from a Langmuir probe were used for comparison. The Langmuir probe gave a plasma potential minus floating potential of 30 +/- 5 eV, in good agreement with the carbon resistance probe result of ion energy <=40 eV. The measured ion energy was consistent with the ion energy predicted from a model based upon divergent magnetic field extraction. Also, based upon physical sputtering of the carbon, the hydrogen (ion and energetic neutrals) fluence rate was determined to be 1 times 10^ {16} /cm^2-sec for 50 Watts of incident microwave power. ECR hydrogen/helium plasmas were use to study preferential pumping of helium in candidate materials for tokamak pump-limiters: nickel, vanadium, aluminum, and nickel/aluminum multi-layers. Nickel and vanadium exhibited similar pumping capacities whereas aluminum showed a reduced capacity due to increased sputtering. A helium retention model based upon ion implantation ranges and sputtering rates agreed with the experimental data. A new multilayer/bilayer pumping concept showed improved pumping above that for single element films. Also, deuterium/helium trapping site competition is shown to be more important in vanadium than in nickel. When heated above 400^ circC, vanadium and nickel are observed to preferentially pump helium and are excellent candidate materials for future tokamak pump limiters.