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The turbulence and flows at the plasma edge during the L-I-H, L-I-L and single-step L-H transitions have been measured directly using two reciprocating Langmuir probe systems at the outer midplane with several newly designed probe arrays in the EAST superconducting tokamak. The E× B velocity, turbulence level and turbulent Reynolds stress at ∼1cm inside the separatrix ramp-up in the last ∼20ms preceding the single-step L-H transition, but remain nearly constant near the separatrix, indicating an increase in the radial gradients at the plasma edge. The kinetic energy transfer rate from the edge turbulence to the E×B flows is significantly enhanced only in the last ∼10ms and peaks just prior to the L-H transition. The E×B velocity measured inside the separatrix, which is typically in the electron diamagnetic drift direction in the L-mode, decays towards the ion diamagnetic drift direction in response to fluctuation suppression at the onset of the single-step L-H, L-I-L as well as L-I-H transitions. One important distinction between the L-I-H and the L-I-L transitions has been observed, with respect to the evolution of the edge pressure gradient and mean E×B flow during the I-phase. Both of them ramp up gradually during the L-I-H transition, but change little during the L-I-L transition, which may indicate that a gradual buildup of the edge pedestal and mean E×B flow during the I-phase leads to the final transition into the H-mode. In addition, the transition data in EAST strongly suggest that the divertor pumping capability is an important ingredient in determining the transition behaviour and power threshold.

The turbulence and flows at the plasma edge during the L-I-H, L-I-L and single-step L-H transitions have been measured directly using two reciprocating Langmuir probe systems at the outer midplane with several newly designed probe arrays in the EAST superconducting tokamak. The E× B velocity, turbulence level and turbulent Reynolds stress at ∼1cm inside the separatrix ramp-up in the last ∼20ms preceding the single-step L-H transition, but remain nearly constant near the separatrix, indicating an increase in the radial gradients at the plasma edge. The kinetic energy transfer rate from the edge turbulence to the E×B flows is significantly enhanced only in the last ∼10ms and peaks just prior to the L-H transition. The E×B velocity measured inside the separatrix, which is typically in the electron diamagnetic drift direction in the L-mode, decays towards the ion diamagnetic drift direction in response to fluctuation suppression at the onset of the single-step L-H, L-I-L as well as L-I-H transitions. One important distinction between the L-I-H and the L-I-L transitions has been observed, with respect to the evolution of the edge pressure gradient and mean E×B flow during the I-phase. Both of them ramp up gradually during the L-I-H transition, but change little during the L-I-L transition, which may indicate that a gradual buildup of the edge pedestal and mean E×B flow during the I-phase leads to the final transition into the H-mode. In addition, the transition data in EAST strongly suggest that the divertor pumping capability is an important ingredient in determining the transition behaviour and power threshold.

The intermediate oscillatory phase during the L-H transition, termed the I-phase, is studied in the EAST superconducting tokamak using a newly developed dual gas puff imaging (GPI) system near the L-H transition power threshold. The experimental observations suggest that the oscillatory behaviour appearing at the L-H transition could be induced by the synergistic effect of the two components of the sheared m, n = 0 E × B flow, i.e. the turbulence-driven zonal flow (ZF) and the equilibrium flow. The latter arises from the equilibrium, and is, to leading order, balanced by the ion diamagnetic term in the radial force balance equation. A slow increase in the poloidal flow and its shear at the plasma edge are observed tens of milliseconds prior to the I-phase. During the I-phase, the turbulence recovery appears to originate from the vicinity of the separatrix with clear wave fronts propagating both outwards into the far scrape-off layer (SOL) and inwards into the core plasma. The turbulence Reynolds stress is directly measured using the GPI system during the I-phase, providing direct evidence of kinetic energy transfer from turbulence to ZFs at the plasma edge. The GPI observations strongly suggest that the SOL transport physics and the evolution of pressure gradient near the separatrix play an important role in the L-I-H transition dynamics. To highlight these new physics, the previous predator-prey model is extended to include a new equation for the SOL physics. The model successfully reproduces the L-I-H transition process with several features comparing favourably with GPI observations. © 2014 IAEA, Vienna.

The intermediate oscillatory phase during the L-H transition, termed the I-phase, is studied in the EAST superconducting tokamak using a newly developed dual gas puff imaging (GPI) system near the L-H transition power threshold. The experimental observations suggest that the oscillatory behaviour appearing at the L-H transition could be induced by the synergistic effect of the two components of the sheared m, n = 0 E × B flow, i.e. the turbulence-driven zonal flow (ZF) and the equilibrium flow. The latter arises from the equilibrium, and is, to leading order, balanced by the ion diamagnetic term in the radial force balance equation. A slow increase in the poloidal flow and its shear at the plasma edge are observed tens of milliseconds prior to the I-phase. During the I-phase, the turbulence recovery appears to originate from the vicinity of the separatrix with clear wave fronts propagating both outwards into the far scrape-off layer (SOL) and inwards into the core plasma. The turbulence Reynolds stress is directly measured using the GPI system during the I-phase, providing direct evidence of kinetic energy transfer from turbulence to ZFs at the plasma edge. The GPI observations strongly suggest that the SOL transport physics and the evolution of pressure gradient near the separatrix play an important role in the L-I-H transition dynamics. To highlight these new physics, the previous predator-prey model is extended to include a new equation for the SOL physics. The model successfully reproduces the L-I-H transition process with several features comparing favourably with GPI observations. © 2014 IAEA, Vienna.

Experiments on HL-2A, DIII-D and EAST show that turbulence just inside the last closed flux surface acts to reinforce existing sheared E × B 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 favourably with a reduced predator-prey type model. © 2013 IAEA, Vienna.

A new turbulence-flow cycle state has been discovered after the formation of a transport barrier in the H-mode plasma edge during a quiescent phase on the EAST superconducting tokamak. Zonal-flow modulation of high-frequency-broadband (0.05-1MHz) turbulence was observed in the steep-gradient region leading to intermittent transport events across the edge transport barrier. Good confinement (H98y,2∼1) has been achieved in this state, even with input heating power near the L-H transition threshold. A novel model based on predator-prey interaction between turbulence and zonal flows reproduced this state well. © 2012 American Institute of Physics.

A new turbulence-flow cycle state has been discovered after the formation of a transport barrier in the H-mode plasma edge during a quiescent phase on the EAST superconducting tokamak. Zonal-flow modulation of high-frequency-broadband (0.05-1MHz) turbulence was observed in the steep-gradient region leading to intermittent transport events across the edge transport barrier. Good confinement (H98y,2∼1) has been achieved in this state, even with input heating power near the L-H transition threshold. A novel model based on predator-prey interaction between turbulence and zonal flows reproduced this state well. © 2012 American Institute of Physics.

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. © 2012 American Institute of Physics.

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. © 2012 American Institute of Physics.

A quasiperiodic Er oscillation at a frequency of

A quasiperiodic Er oscillation at a frequency of