Increased understanding of neoclassical internal transport barriers in CHS

We report recent progress in the study of neoclassical internal transport barriers (N-ITBs) in the compact helical system (CHS) experiment. N-ITBs have been observed in electron cyclotron (EC) heated NBI plasmas. The improved confinement region for electrons is expanded compared with that of previous N-ITB plasmas heated by only EC heating. Moreover, the ion temperature is found to be increased simultaneously by two to three times (T_i(0) ∼ 400-500 eV), with a steep gradient region at ρ ∼ 0.6. From measurement of the averaged peak energy intensity using a soft x-ray CCD camera, it is confirmed that the impurity confinement is also improved with N-ITBs. The radial electric field is observed to bifurcate into ion and electron root (E_r ∼ 15 kV m^-1), and a rather large electric field shear (dE_r/dr ∼ -300 kV m^-2) is produced in the layer between ion and electron root.