Evolution of the current density profile in advanced tokamak scenario plasmas*

Recent experiments on DIII-D have been performed to study the evolution of the current density profile in high q_min plasmas (q_min > 2). Modifying the plasma current (I_p) ramp rate and the timing and power of the neutral beam (NBI) and electron-cyclotron current drive (ECCD) changes the distribution of Ohmic current and the shape of the current density profile. NBI drives current in the inner-half radius of the plasma and increases the pressure gradient, which increases the bootstrap current. Applying ECCD earlier in the discharge increases mid-radius current leading to a broader current density profile and higher q_min. Changing the I_p ramp rate has the largest effect on the edge loop voltage. This knowledge is then applied to predict effects of the ongoing DIII-D upgrade, which will increase the off-axis NBI current drive and power in the co-current direction. Increased off-axis NBI current drive will broaden the current density profile and raise q_min. Increased co-current NBI power will benefit the high q_min scenario through higher plasma rotation. The research goal is to achieve fully non-inductive operations with β_n > 4, and zero Ohmic current density across the profile after the DIII-D upgrade.