Dynamics of low–intermediate–high-confinement transitions in the HL-2A tokamak

Abstract

In recent experiments at the HL-2A tokamak, dynamic features across the low-intermediatehigh (L-I-H) confinement transition have been investigated in detail. Experimental evidence shows two types of opposite limit cycles (dubbed type-Y and type-J) between the radial electric field (E r ) and turbulence evolution during the intermediate I-phase. Whereas for type-Y the turbulence grows prior to the change in E r , for type-J the oscillation in E r leads turbulence. It has been found that the type-Y usually appears first after an L-I transition, followed by type-J before the transition to the H-mode phase. Possible roles played by zonal flows and the enhanced pressure-gradient-induced flow shear in suppressing turbulence, respectively, in the type-Y and type-J periods have been identified. In addition, during the I-phase of the L-I-H discharges a kink-type MHD mode routinely occurs and crashes rapidly just prior to the I → H transition. The mode crash evokes substantial energy release from the core to plasma boundary and further increases the edge pressure gradient and E r shear, which eventually results in confinement improvement into the H-mode.

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

“…In addition to so-called 'fast' or 'one-step' L-H transitions, much recent research has focused on transitions characterized by an intermediate oscillatory phase between L-and H-mode (dubbed I-phase or LCO (limit-cycle oscillation) phase). The intermediate phase is characterized by a periodic increase in sheared E × B flow velocity, and concomitant reduction in fluctuation levels, and has been observed in multiple devices (ASDEX and ASDEX-Upgrade (AUG) [29,[42][43][44], JET [45], TJ-II [28,31,46,47], DIII-D [32,36,[48][49][50][51][52], EAST [30,53,54], HL-2A [33,55,56], JFT-2M [57], and others). It is important to note that 'I-phase' is distinct from 'I-mode' which denotes an enhanced confinement regime with edge-temper ature pedestals similar to H-mode, but without a density pedestal.…”

Reversed rotation of limit cycle oscillation and dynamics of low-intermediate-high confinement transition

“…In order to deeply figure out the mechanism of the LH mode transition, some researches focused on the dynamics of LCO near the transition boundary in the I-phase. [17,19,20] The authors in Ref. [19] found two different rotations in the I-phase.…”

“…[16] The effect of energy transfer between turbulence and the zonal flow was observed in an LCO, where the zonal flow is estimated by the amplitude fluctuating of the radial electric field, and the ambient turbulence energy is measured by the electrons density fluctuations. [17] LCO was also used to analyze the dynamics of turbulence and the coherent structure in the transition, [18] but the detailed dynamic features of the LCOs, consisting of causality and conditions for the different instruments have not been identified. Experiments on the HL-2A tokamak studied the dynamic features in the LH transition, which showed that there are two different types of limit cycle, and reported the mechanism of zonal flow and pressure gradient induced shear flow in suppressing turbulence and the converting from type-Y LCO to the type-J one.…”

Favourable scenarios established by SMBI for the realization of the ELMy H-mode at HL-2A

“…[40][41][42] The backing pressure and the pulse duration of the SMBI can be changed in ranges of 1 bar-30 bar (1 bar = 10 −5 Pa) and 0.5 ms-5 ms, respectively. The injected deuterium molecular inventory is about 2×10 17 -6×10 19 and the velocity of the particles is about 1.3 km/s-1.8 km/s. Comparing with the ordinary gas puffing (GP), the increase of the fuelling efficiency for the SMBI is mainly due to the deep penetration and the edge cooling effect.…”

“…[14][15][16] Meanwhile, the influence of MHD activities on the L-H transition has also been discussed recently. [17] Therefore, the power needed for the L-H transition depends on many parameters. It is found that in many devices there are favourable plasma conditions by which the required power for the transition is relatively lowered.…”