Validation of Density Pump-out by Magnetic Island Formation in KSTAR and DIII-D Tokamaks

Nonlinear TM1 simulations reveal characteristics of density pump-out by pedestal-foot island formation when applying n=1 RMPs, including: 1) bifurcation in pump-out with low RMP coil current threshold, 2) sensitivity of pump-out magnitude on q₉₅, and 3) the magnitude of pump-out scales linearly with the island width. Dedicated experiments are carried out in KSTAR and DIII-D to validate these features and we find that: 1) staircase bifurcation in density pump-out is observed when slowly ramping up the n=1 RMP current, and the current threshold is ~1kAt when using one single row coils; 2) the magnitude of density pump-out becomes weaker when decreasing q₉₅ from 5.5 to 4.5, and the density recovery is observed when q₉₅ is ramped down to lower than 4.9; 3) analysis of DIII-D database of n=3 RMP experiment finds that the magnitude of density pump-out is proportional to the square root of RMP coil current Δn_e/n_e∝I_RMP^0.5. These experimental observations are all consistent with TM1 simulations. The validated model also predicts that n=2-4 RMP in the baseline scenario of ITER plasma will cause 5-20% density pump-out, and high-n RMP leads to weaker pump-out. This indicates high-n RMPs are favorable for minimizing confinement degradation in RMP ELM control.