Simulating the ITER plasma startup scenario in the DIII-D tokamak

DIII-D experiments have investigated the ITER startup scenario, including the initial phase where the plasma is limited on low field side (LFS) poloidal bumper limiters. Both the original ITER 'small-bore' (constant q95) startup and a 'large-bore' lower internal inductance (li) startup that avoids vertical disruption events (VDEs) have been simulated. In addition, li feedback control has been tested with the goal of producing discharges at the ITER design value, li = 0.85. These discharges have been simulated using the Corsica free boundary equilibrium code. High performance discharges (βN = 2 .8, H98y2 = 1 .4) have been obtained in a scaled ITER shape during the flattop phase after the ITER-like startup. These discharges have a figure of merit, G = βNxH89P /q952, up to 0.40, approaching the ITER value of 0.42 calculated for a fusion gain, Q 10. Important goals of this work are to demonstrate that the proposed ITER startup scenarios are feasible, and to benchmark modelling codes and to help develop future improvements to these scenarios. ITER startup presents unique challenges due to low inductive toroidal electric field (0.3 V/m), limits on poloidal flux due to power supply and poloidal field coil constraints, and plasma current ramp up near the n = 0 vertical stability limit. The original ITER startup scenario, beginning as a small volume and growing at constant q95, reached values of normalized internal inductance, li (3), much higher than the ITER design value of 0.85. A new scenario, beginning with a large volume plasma was developed that was stable and exhibited a lower li (3). Although the discharge was limited on LFS bumper limiters during the initial phase of the current ramp, no deleterious effects from impurities or excessive wall fueling were observed. In order to further control the current profile, li feedback was developed and successfully implemented using the plasma current ramp rate as the actuator. Electron cyclotron heating in the breakdown and current initiation phases has also been evaluated producing faster burnthrough of low Z impurities. (author)