Stationary QH-mode plasmas with high and wide pedestal at low rotation on DIII-D

Abstract

A stationary, quiescent H-mode (QH-mode) regime with a wide pedestal and improved confinement at low rotation has been discovered on DIII-D with reactor relevant edge parameters and no ELMs. As the injected neutral beam torque is ramped down and the edge E × B rotation shear reduces, the transition from standard QH to the wide pedestal QH-mode occurs. At the transition, the coherent edge harmonic oscillations (EHO) that usually regulate the standard QH edge cease and broadband edge MHD modes appear along with a rapid increase in the pedestal pressure height (by ⩽60%) and width (by ⩽50%). We posit that the enhanced edge turbulence-driven transport, enabled by the lower edge E × B flow shear due to lower torque reduces the pedestal gradient and, combined with the high edge instability limit provided by the balanced double-null plasma shape, permits the development of a broader and thus higher pedestal that is turbulence-transport-limited. Even with the significantly enhanced pedestal pressure, the edge operating point is below the peeling ballooning mode stability boundary and thus without ELMs. Improved transport in the outer core region (0.8 ⩽ ρ ⩽0.9) owing to increased E × B flow shear in that region and the enhanced pedestal boost the overall confinement by up to 45%. These findings advance the physics basis for developing stationary ELM-free high-confinement operation at low rotation for future burning plasma where similar collisionality and rotation levels are expected.

Analytic study on low- external ideal infernal modes in tokamaks with large edge pressure gradients

“…It is found that in the case of ideal wall boundary conditions the growth rates become larger with increasing n (Zheng et al 2013b;Liu et al 2015). This might be relevant for QH regimes with zero injected torque (Burrell et al 2016;Chen et al 2017). In such a case a weakly quasi-coherent MHD (broadband) activity with a cessation of the EHO is observed.…”

“…2016; Chen et al. 2017). In such a case a weakly quasi-coherent MHD (broadband) activity with a cessation of the EHO is observed.…”

Scaling trends of the critical E × B shear for edge harmonic oscillation onset in DIII-D quiescent H-mode plasmas

“…The average inner shear layer showed little trend with the predicted model parameters, and therefore the outer shear layer is presented due to the more interesting observed correlation. It should be noted that for wide pedestal QH-mode experiments with broadband MHD, it is observed that the local ExB shear on the inside of the Er well plays a role[22] in determining the turbulent state, indicating further research is needed to understand the local versus global pedestal ExB shear requirements in QH-mode.…”

A Riccati solution for the ideal MHD plasma response with applications to real-time stability control

“…qp ð1Þ W P (27) comprises the plasma response matrix. It may be regarded as the plasma permeability, or the "conjugate momentum" created per unit of plasma displacement-i.e., W P $ p=q.…”

“…4 the ten smallest energy eigenvalues of the plasma response matrix over the course of DIII-D shot #163518. 27 This QHmode shot was chosen because its lack of ELMs allowed for a fair comparison between the two methods-(i.e., one without ELM-induced reconstruction issues). The accurate, rapid analysis of such a shot's time-varying dW-which corresponds to the smallest plotted energy eigenvalue-would be FIG.…”