Evolution of the current density profile in advanced tokamak scenario plasma

B.S. Victor, C.T. Holcomb, S.L. Allen, K.E. Thome, J.R. Ferron, C.C. Petty, J.M. Park, E. Schuster, W. Wehner, B.A. Grierson, F.M. Poli

Division of Plasma Physics (DPP) Annual Meeting of the American Physical Society (APS)

Portland, OR, USA, November 5-9, 2018

Recent experiments on DIII-D have been performed to study the evolution of the current density profile in high qmin plasmas (qmin > 2). Modifying the plasma current (Ip) 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 qmin. Changing the Ip 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 qmin. Increased co-current NBI power will benefit the high qmin scenario through higher plasma rotation. The research goal is to achieve fully non-inductive operations with βn > 4, and zero Ohmic current density across the profile after the DIII-D upgrade.