<i>L</i>–<i>H</i> transition trigger physics in ITER-similar plasmas with applied <i>n</i>  =  3 magnetic perturbations

In this work, the <em>L – H</em> transition power threshold <em>Ρ</em>_LH is observed to increase with applied <em>n</em> = 3 Resonant Magnetic Perturbations (RMP) in ITER-similar-shape (ISS) plasmas with balanced neutral beam torque injection in DIII-D. The increase is most pronounced with added ECH: <em>Ρ</em>_LH increases with decreasing edge plasma collisionality as <em>Ρ</em>_LH/<em>Ρ</em>_LH-08 ~ (<em>v</em>*)^-0.5, where <em>Ρ</em>_LH-08 is the 2008 ITPA multi-machine power threshold scaling. This result raises concerns for H-mode access at low edge collisionality in ITER, where resonant magnetic perturbations (RMP) may have to be applied before the <em>L – H</em> transition to safely suppress the first ELM. Non-axisymmetric modifications with RMP include a simultaneous reduction of the radial electric field (<em>E</em>_r) well depth and <em>E &times; B</em> shear. This can be attributed to increasing edge toroidal co-current rotation, and is consistent with substantially increased local long-wavelength turbulence [measured via Beam Emission Spectroscopy (BES)]. At high RMP perturbation strength the edge electric field <em>E</em>_r reverses sign locally (becomes positive), with changes in dominant turbulence modes. Edge magnetic stochasticity provides an attractive explanation of the observed modifications, and the observed changes in toroidal rotation and <em>E</em>_r are consistent with a simple fluid model describing radial electron current flow along stochastic fieldlines. The observed collisionality dependence of the L-mode edge electric field with applied RMP is also qualitatively consistent with this model. Reflectometry data indicate a significant reduction of the normalized L-mode radial density gradient <em>a/L_n</em> at high RMP field with simultaneous increase in radial particle flux and electron thermal flux from power balance analysis. We conjecture that the increase of <em>Ρ</em>_LH with RMP results from the combined effects of reduced <em>E &times; B</em> flow shear (increasing turbulent transport levels) and toroidal/poloidal flow modulation due to edge stochasticity. Initial experiments indicate that non-resonant <em>n</em> = 3 magnetic perturbations lead only to relatively small changes in <em>E</em>_r, <em>E &times; B</em> shear and fluctuation characteristics, and have less impact on the <em>L – H</em> transition power threshold. In conclusion, this motivates further exploration of the RMP spectrum dependence of <em>Ρ</em>_LH for possible mitigation of the observed threshold increase.