Understanding the influence of current profile and RF heating on impurity transport in advanced tokamak scenarios

Recent DIII-D experiments show that the advanced tokamak hybrid scenario is compatible with a tungsten (W) divertor. The hybrid scenario, with on-axis electron cyclotron current drive (ECCD) and q$_{min}$~1, shows no degradation in $\beta_n$ or energy confinement compared to an all-carbon divertor. In contrast, a high q$_{min}$ scenario (q$_{min}\geq1.5$) with off-axis ECCD experiences on-axis W accumulation throughout the discharge. With the application of on-axis ECCD in the hybrid scenario, the velocity to diffusion ratio (V/D) calculated with STRAHL reverses sign in the core, producing an off-axis peak in the W density profile. These results indicate that plasmas with broader current density profiles and off-axis ECCD are more susceptible to high-Z impurity accumulation. Results from the hybrid plasmas show the feasibility of a steady-state scenario with a W divertor, thus improving the physics basis of Q=5 steady-state operation on ITER. *Supported by US DOE under DE-AC52-07NA27344 and DE-FC02-04ER54698.