Effect of radial magnetic perturbations on turbulence-flow dynamics at the L-H transition on DIII-D

Low-k density fluctuation amplitudes measured with beam emission spectroscopy (BES) leading up to the L-H transition are altered in a toroidally dependent manner when RMPs are applied, potentially contributing to the resulting increase of the L-H power threshold. In the 50 ms period leading up to L-H transitions in ITER-shaped low-torque plasmas with applied n=3 magnetic perturbations, two turbulence instabilities coexist in the ρ=0.95–1 region: a low-frequency (~5 kHz) mode peaking at the separatrix and a high-frequency (~30 kHz) mode peaking at ρ≈0.95. RMPs raise the low-frequency mode amplitude at different toroidal phases, but only raise the high-frequency mode amplitude at one toroidal phase, implying the turbulence is toroidally-modulated by the RMPs. RMPs also destabilize a high-frequency (~80 kHz), counter-propagating mode. In contrast, non-resonant fields, which do not raise P_LH, do not significantly affect turbulence. Zonal flow structures in the turbulence poloidal flow spectrum are somewhat surprisingly found to have higher amplitude when RMPs are applied, since RMPs have been predicted to damp zonal flows.