L-H Transition Dynamics in ITER-Similar Plasmas with Applied n=3 Resonant Magnetic Perturbations

In ITER-similar plasmas in DIII-D (<n_e>=1.5-5x10¹⁹m^-3, B_t=1.9-2T, I_p=1.5MA, q₉₅~3.6), the L-H threshold power P_LH with n=3 Resonant Magnetic Perturbations (RMP) is found to increase with decreasing collisionality [P_LH~(nu*)^-0.3]. This is a concern for H-mode access in primarily ECH-heated ITER plasmas since RMP may be applied before the L-H transition to safely suppress the first ELM. Non-axisymmetric modifications of the L-mode shear layer with RMP include a substantial local reduction of the E_r well and ExB shear (including E_r reversal at high RMP field), and spatially modulated low-wavenumber turbulence amplitude, increasing substantially on field lines mapped to high RMP field. Two-fluid M3D-C1 modeling [1,2] shows that the normalized gradients a/L_n, a/L_Te are toroidally modulated and periodically increased on the outboard midplane with RMP. The increase in P_LH is atttributed to the combined effects of reduced (and non-axisymmetric) ExB flow / flow shear, and locally enhanced instability drive (via increased kinetic gradients). [1] N.M. Ferraro,Phys. Plasmas 19 056105 (2012); [2] R.S. Wilcox et al. Phys. Plasmas 25 056108 (2018).