Reduced L-H Transition Power Threshold in ITER-Similar-Shape DIII-D Hydrogen Plasmas*

Recent DIII-D experiments in low-torque, ITER-similar-shape (ISS) hydrogen plasmas (q₉₅ ~3.6) show that the L-H transition power threshold P_LH can be reduced substantially (~ 25-30%) via moderate (≤ 20%) Helium admixtures. P_LH has also been effectively reduced at low ion edge collisionality via applied n=3 Non-Resonant Magnetic Perturbations (NRMF), producing local edge counter-current torque via the Neoclassical Toroidal Viscosity (NTV). CER measurements confirm a significant increase in toroidal (Carbon) edge rotation shear with applied NTV before the L-H transition. Techniques for reducing P_LH are very important for ITER, in particular for hydrogen plasma operations during the PFPO-1 campaign with marginal auxiliary heating (20-30 MW of ECH).  Control of L-mode E×B shear via Helium seeding, or applied NRMF/NTV can open up a path for reducing P_LH in burning plasma experiments. Linear plasma response simulations for ITER indicate that the 3-D internal coil set (with n=3) can be used to generate large edge NTV, favored by the low collisionality expected in the ITER L-mode edge. *This work was supported by the US Department of Energy under DE-SC0020287, DE-FG02-08ER54984, DE-AC05-00OR22725, DE-FG02-08ER54999, DE-FC02-04ER54698, DE-FG02-07ER54917, and DE-AC02-09CH11466.