Simulation Study of Electrostatic Potential Generated by NBI and its Effect on the Neoclassical Transport of Carbon Impurity Ions in LHD

Full text: Electrostatic potential Φ_NBI generated by the neutral-beam-injection (NBI) to the plasma, and its effect on the neoclassical transport of carbon impurity ions in the Large Helical Device (LHD) are investigated for the first time by means of the global drift-kinetic simulations. The ripple-trapped beam ions of the perpendicular NBI (40 keV) have been found to generate Φ_NBI in the order of 0.01-0.02T_e when n_e = 3 x 10¹⁹/m³, T_e = T_i = 3 keV, and the injection power is 5 MW. The global neoclassical transport simulations taking into account Φ_NBI have shown that the diffusion coefficient of C⁶⁺ impurity ions decreased by 14% and the radially inward convection velocity decreased by 22% in the presence of Φ_NBI of the 5 MW injection. These new findings suggest that Φ_NBI may have a non-negligible impact on the neoclassical impurity transport in LHD, especially in the impurity-hole plasma with high-power NBI heating. (author)