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

Abstract

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 ∼1 cm inside the separatrix ramp-up in the last ∼20 ms 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 ∼10 ms 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.

Axisymmetric oscillations at L–H transitions in JET: M-mode

“…Similar phenomena have been reported and discussed at JET and elsewhere [2,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], usually concentrating on electrostatic fluctuation measurements in similar frequency ranges. This is a rapidly advancing field, references [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] may not include all the latest or relevant results, but are representative.…”

“…This is a rapidly advancing field, references [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] may not include all the latest or relevant results, but are representative. Reference [20] presents a good global view of previous studies. The many observations reported are variously described as L-H dithering transitions, Limit Cycle Oscillations or small ELMs.…”

Study on the L–H transition power threshold with RF heating and lithium-wall coating on EAST

“…Two types of L-H transitions, the single-step L-H transition and dithering L-H transition, have been identified. The dynamics of these L-H transitions have been discussed in [10] recently, exhibiting similar features of turbulence-flow interactions at the plasma edge evidenced by dedicated probe measurements. In the 2010 campaign with the C wall, the L-H transitions were mostly single-step L-H transitions with a heating power very marginal to the transition threshold predicted by the international tokamak scaling.…”

Mean flows and blob velocities in scrape-off layer (SOLT) simulations of an L-mode discharge on Alcator C-Mod

“…1,2 Efforts to understand the low (L) mode to high (H) mode confinement transition in tokamaks have further increased interest in this topic. [3][4][5][6][7][8][9][10][11] A closely related subject is that of understanding the saturation mechanism for edge and SOL turbulence. 12,13 In addition to contributing to L-H mode transition physics, some models predict that turbulence in the vicinity of the separatrix could be sufficiently large so as to impact the width of the heat flux channel.…”