Charge exchange collision cross sections for tungsten ions

One of the urgent technical issues in materializing larger fusion devices is a need for using heavier elements in plasma-facing components. Tungsten element is attracting particular interests these days because of its specific aspects of high melting-point, good thermo-mechanical properties, low tritium retention, low sputtering rates, and so on. Since tungsten causes a serious problem of radiation power-loss brought by its high atomic number, charge-state resolved transport phenomena of tungsten ions in fusion plasma have become notably important feature in larger fusion devices. Use of tungsten impurities for diagnostics is also an important issue to consider. To accomplish these issues, charge exchange collision (electron capture and loss) cross sections involving ions of tungsten become essential in both low-energy, up to 20 keV for tungsten projectile ions, and high-energy regions, up to 1 MeV NBI hydrogen atoms colliding with tungsten ions at rest. We have experimentally produced total electron capture cross sections for low-q heavy-ions Be^q+, B^q+, C^q+, Fe^q+, Ni^q+, and W^q+ (q = 1,2) ions colliding with atomic and molecular gas targets of He, Ne, Ar, Kr, H₂, CH₄, C₂H₆, C₃H₈, CO, CO₂, and N₂ at collision energy between 4 and 25 keV in the Van de Graaff accelerator facility in Kyoto University. We have extensively derived scaling laws for single-electron-capture cross sections for these collision systems using ΔIP, i.e., the first ionization potential of the target atom/molecule subtracted by that of the projectile ion after electron capture. There used to exist some scaling formulae predicting electron capture cross sections for high-q projectile ions at low-energy limit, but no such scaling behaviour had never been introduced for low-q projectile ions at low-energy until ours. We are also performing charge exchange cross section production at high-energy for diagnostic purposes using the Tandem accelerator facility in Japan Atomic Energy Agency (JAEA.) To produce cross sections of electron loss and transfer process for 1 MeV NBI hydrogen beam by tungsten impurity ions, cross sections of an equivalent electron capture process for 184 MeV (1 MeV/u) tungsten ions from atomic hydrogen target will be measured. In this measurement, ionization cross section of tungsten ions by NBI hydrogen beam will be obtained at the same time by analyzing the outgoing charge-states of the projectile tungsten ions. The target thickness is normalyzed using iron projectile ions, whose cross sections are well-resolved. (author)