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Dynamics of L-H transition and I-phase in EAST

  • Author(s): Xu, GS
  • Wang, HQ
  • Xu, M
  • Wan, BN
  • Guo, HY
  • Diamond, PH
  • Tynan, GR
  • Chen, R
  • Yan, N
  • Kong, DF
  • Zhao, HL
  • Liu, AD
  • Lan, T
  • Naulin, V
  • Nielsen, AH
  • Juul Rasmussen, J
  • Miki, K
  • Manz, P
  • Zhang, W
  • Wang, L
  • Shao, LM
  • Liu, SC
  • Chen, L
  • Ding, SY
  • Zhao, N
  • Li, YL
  • Liu, YL
  • Hu, GH
  • Wu, XQ
  • Gong, XZ
  • et al.
Abstract

© 2014 IAEA, Vienna. 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.

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