Observation of low impurity diffusivity inside the neoclassical transport barrier in the Compact Helical System

The radial profiles of titanium Kα spectra are measured with photon counting x-ray charge coupled device cameras for plasmas with a neoclassical internal transport barrier (N-ITB) in the Compact Helical System Heliotron/Torsatron [K. Matsuoka et al., Plasma Phys. Controlled Fusion 42, 1145 (2000)]. The Kα lines are dominated by the resonance line of He-like titanium in the plasma core where a thermal transport barrier results in a high electron temperature of ∼3 keV. The impurity diffusion coefficients are evaluated from the ionization levels of titanium. The diffusion coefficient inside the neoclassical ITB is <0.03 m2/s, which is one order of magnitude lower than that of the plasma without neoclassical ITB.