Impurity transport measurements in beam heated low-confinement mode discharges in the national spherical torus experiment

An experimental technique base don ultrasoft x-ray imaging in multiple spectral bands was used to estimate the low-Z impurity diffusivity in beam heated NSTX plasmas. A time averaged diffusion coefficient in the neoclassical range was obtained for the core of slow evolving L-mode discharges, with a higher level of transport observed in the outer plasma. The results showed that there were still some uncertainties on whether the observed region of low ion energy and particle transport represents an internal transport barrier as seen in conventional tokamaks, or was a phenomenon intrinsic to the low aspect-ratio, high beta plasma.