Advances towards QH-mode viability for ELM-stable operation in ITER

Abstract

The application of static, non-axisymmetric, nonresonant magnetic fields (NRMFs) to high beta DIII-D plasmas has allowed sustained operation with a quiescent H-mode (QH-mode) edge and both toroidal rotation and neutral beam injected torque near zero. Previous studies have shown that QH-mode operation can be accessed only if sufficient radial shear in the plasma flow is produced near the plasma edge. In past experiments, this flow shear was produced using neutral beam injection (NBI) to provide toroidal torque. In recent experiments, this torque was nearly completely replaced by the torque from applied NRMFs. The application of the NRMFs does not degrade the global energy confinement of the plasma. Conversely, the experiments show that the energy confinement quality increases with lower plasma rotation. Furthermore, the NRMF torque increases plasma resilience to locked modes at low rotation. These results open a path towards QH-mode utilization as an edge-localized mode (ELM)-stable H-mode in the self-heated burning plasma scenario, where toroidal momentum input from NBI may be small or absent.

Changes in particle transport as a result of resonant magnetic perturbations in DIII-D

“…Previous work on non-resonant magnetic fields (NRMF) in QH-mode discusses the same phenomenon. 33 In this paper, neoclassical toroidal viscosity (NTV) is playing a critical role in modifying the changes in toroidal rotation. Future modeling would be required to address the changes in toroidal rotation as a result of RMPs versus NRMFs.…”

From Phase Locking to Phase Slips: A Mechanism for a QuiescentHmode

“…(11), the approximation jk x j ≃ 1=Δx was employed. Using the radial force balance relation for the ions enE ¼ −V ϕ B θ þ ∂ x hPi (V ϕ the ion's toroidal rotation velocity; we have assumed ion's poloidal rotation in the H-mode pedestal is low [11]), one has V 0 E×B ¼ −V 0 ϕ B θ =B þ ðhPi 0 =enBÞ 0 . Therefore, there are two ways to facilitate accessing the QH mode: enhancing the steepness of the edge pressure profile (which requires more external power input) and increasing the toroidal rotation shear (which is more feasible in practice, currently).…”

“…There is welldocumented experimental evidence [11] that E × B shear (V 0 E×B -driven by radial electrostatic field shear) is a central ingredient in determining which type of H mode the plasma system stays in. The most obvious aspect of accessing the QH mode is that it requires V 0 E×B to exceed a critical value [11]. The critical role of V 0 E×B for the L → H transition has been extensively studied both by experiment and theory.…”

Edge-localized mode avoidance and pedestal structure in I-mode plasmas

“…As such, recent efforts have focused on mitigating or avoiding large ELMs in high-performance regimes, either via engineering solutions (e.g., pellet pacing [10] or resonant magnetic perturbations [11][12][13][14]) or via alternate highperformance scenarios which inherently avoid large ELMs, e.g. the EDA H-mode [15,16] or QH Mode [17][18][19].…”