Advances in the physics basis of the hybrid scenario on DIII-D

Experiments on the DIII-D tokamak have developed a long duration, high performance discharge that is an attractive operating scenario for future burning plasma devices. While this 'hybrid scenario' has a type-I ELMy H-mode edge, like the standard H-mode scenario, hybrids differ by having suppressed sawteeth, a higher β limit to the 2/1 NTM, and remarkably good transport properties. Recent experiments on DIII-D have advanced our physics understanding of the hybrid scenario by extending the plasma conditions towards the burning plasma regime by separately incorporating strong electron heating, low torque injection, and ELM suppression from resonant magnetic perturbations (RMPs). Recent hybrid experiments have replaced some of the co-NBI with 2.4 MW of ECH to reduce the toroidal rotation and raise Te closer to Ti. At fixed beta, raising Te/Ti reduces the confinement factor by 10% and increases turbulence. Additional experiments on DIII-D found that the beneficial characteristics of the hybrid scenario (no sawteeth, benign NTMs, high beta) are maintained when shifting from co-NBI to nearly balanced-NBI. As the toroidal rotation velocity is reduced by an order of magnitude, the confinement factor is reduced from H98y2 = 1.4 to H98y2 = 1.1 modelling using TGLF shows that the increase in transport with lower NBI torque is consistent with changes in the E x B flow shear. For the first time, type-I ELMs have been suppressed in hybrid plasmas by applying edge RMPs with toroidal mode number n = 3. This is an important advance in developing hybrids as a baseline-operating scenario for ITER since such ELMs may cause unacceptable divertor erosion. In ITER-shaped plasmas with normalized β of 2.5 and edge safety factor of 3.6, the confinement factor drops from H98y2 = 1.3 to H98y2 = 1.0 during RMP ELM suppression owing to an increase in both the electron and ion thermal diffusivities. Additionally, high performance hybrid operation has been demonstrated with reduced frequency of wall conditioning. In both the 2006 and 07 campaigns on DIII-D, over 7000 s of plasma operation were conducted with no intervening boronizations. The maintenance of good wall conditions over these campaigns is seen in the impurity line emission and fueling/exhaust data from daily reference shots. (author)