Experiments on HL-2A, DIII-D and EAST show that turbulence just inside the last closed flux surface (LCFS) acts to reinforce existing sheared ExB flows in this region. This flow drive gets stronger as heating power is increased in L-mode, and leads to the development of a strong oscillating shear flow which can transition into the H-mode regime when the rate of energy transfer from the turbulence to the shear flow exceeds a threshold. These effects become compressed in time during an L-H transition, but the key role of turbulent flow drive during the transition is still observed. The results compare favorably with a reduced predator-prey type model.

The kinetic energy transfer between shear flows and the ambient turbulence is investigated in the Experimental Advanced Superconducting Tokamak during the L-H transition. As the rate of energy transfer from the turbulence into the shear flow becomes comparable to the energy input rate into the turbulence, the transition into the H-mode occurs. As the observed behavior exhibits several predicted features of zonal flows, the results show the key role that zonal flows play in mediating the transition into H-mode. V C 2012 American Institute of Physics. [http://dx.doi.org/ 10.1063/1.4737612] INTRODUCTION The transition from low (L-mode) to high confinement (H-mode) regime in magnetized confined fusion devices occurs very rapidly at a critical condition, similar, in a sense, to leaning slightly over the side of a canoe causing only a small tilt of the craft, but leaning slightly more may roll you and the craft into the lake. Rapid threshold transitions between distinctly different stable states then require a triggering event, akin to leaning out too far from the canoe. 1 However, the physics that triggers the transition into Hmode is not understood, and thus predictions of the conditions for the transition into the H-mode regime-which are critical for the operation of ITER in the burning plasma regime-are based on empirical scalings with a wide range of uncertainty. Relative to the conditions found in low confinement regimes, H-mode plasmas are characterized by a reduced turbulence level and strong radial electric field (E r ) shear. 2,3 Azimuthally symmetric, bandlike, time-varying, turbulent generated shear flows called zonal flows (ZFs) also appear to be associated with the L-H transition. 4,5 Therefore, the interaction between micro and macroscale turbulent fluctuations has developed into one of the most active research topics in the physics of magnetized plasmas. The main focus has been placed on the generation of zonal flows and the reduction of the ambient turbulence via the nonlinear exchange, or transfer, of energy from the smaller scaled higher frequency turbulent fluctuations into the large scale, low frequency ordered zonal flow. 6-14 SELF-REGULATION OF TURBULENCETheory predicts that the L-H transition can be explained by an intermediate, quasi-periodic transient stage, where turbulence, zonal flow, mean shear flow, and the pressure gradient are coupled. 15,16 In this model, as the input power increases the pressure gradient also increases, resulting in stronger instabilities and fluctuation levels. The turbulence level grows and begins to nonlinearly drives the zonal flow until the zonal flow drive can overcome the flow damping. A finite zonal flow then begins to grow and extract kinetic energy from the turbulence and thereby acts to suppress the turbulence amplitude. Zonal flows can trigger the transition by regulating the turbulence level and associated transport until the mean shear flow is high enough to suppress the remaining turbulence and associated transport, causing the pressure gradient to i...

A bifurcative step transition from low-density, high-temperature, attached divertor conditions to high-density, low-temperature, detached divertor conditions is experimentally observed in DIII-D tokamak plasmas as density is increased. The step transition is only observed in the high confinement mode and only when the B×∇B drift is directed towards the divertor. This work reports for the first time a theoretical explanation and numerical simulations that qualitatively reproduce this bifurcation and its dependence on the toroidal field direction. According to the model, the bifurcation is primarily driven by the interdependence of the E×B-drift fluxes, divertor electric potential structure, and divertor conditions. In the attached conditions, strong potential gradients in the low field side (LFS) divertor drive E×B-drift flux towards the high field side divertor, reinforcing low density, high temperature conditions in the LFS divertor leg. At the onset of detachment, reduction in the potential gradients in the LFS divertor leg reduce the E×B-drift flux as well, such that the divertor plasma evolves nonlinearly to high density, strongly detached conditions. Experimental estimates of the E×B-drift fluxes, based on divertor Thomson scattering measurements, and their dependence on the divertor conditions are qualitatively consistent with the numerical predictions. The implications for divertor power exhaust and detachment control in the next step fusion devices are discussed.

The ‘Super H-Mode’ regime is predicted to enable pedestal height and fusion performance substantially higher than standard H-Mode operation. This regime exists due to a bifurcation of the pedestal pressure, as a function of density, that is predicted by the EPED model to occur in strongly shaped plasmas above a critical pedestal density. Experiments on Alcator C-Mod and DIII-D have achieved access to the Super H-Mode (and Near Super H) regime, and obtained very high pedestal pressure, including the highest achieved on a tokamak (p  ped ~ 80 kPa) in C-Mod experiments operating near the ITER magnetic field. DIII-D Super H experiments have demonstrated strong performance, including the highest stored energy in the present configuration of DIII-D (W ~ 2.2–3.2 MJ), while utilizing only about half of the available heating power (P heat ~ 7–12 MW). These DIII-D experiments have obtained the highest value of peak fusion gain, Q DT,equiv ~ 0.5, achieved on a medium scale (R  <  2 m) tokamak. Sustained high performance operation (β N ~ 2.9, H98 ~ 1.6) has been achieved utilizing n  =  3 magnetic perturbations for density and impurity control. Pedestal and global confinement has been maintained in the presence of deuterium and nitrogen gas puffing, which enables a more radiative divertor condition. A pair of simple performance metrics is developed to assess and compare regimes. Super H-Mode access is predicted for ITER and expected, based on both theoretical prediction and observed normalized performance, to allow ITER to achieve its goals (Q  =  10) at I p  <  15 MA, and to potentially enable more compact, cost effective pilot plant and reactor designs.