Dynamics of Low-Intermediate–High-Confinement Transitions in Toroidal Plasmas

Abstract

The dynamic features of the low-intermediate-high-(L-I-H) confinement transitions on HL-2A tokamak are presented. Here we report the discovery of two types of limit cycles (dubbed type-Y and type-J), which show opposite temporal ordering between the radial electric field and turbulence intensity. In type-Y, which appears first after an L-I transition, the turbulence grows first, followed by the localized electric field. In contrast, the electric field leads type-J. The turbulence-induced zonal flow and pressure-gradient-induced drift play essential roles in the two types of limit cycles, respectively. The condition of transition between types-Y and -J is studied in terms of the normalized radial electric field. An I-H transition is demonstrated to occur only from type-J.

Low-to-High Confinement Transition Mediated by Turbulence Radial Wave Number Spectral Shift in a Fusion Plasma

“…Some supporting experimental evidence has been reported recently [2][3][4][5][6][7][8][9]. However, there are still some observations that cannot be interpreted in terms of zonalflow-based transition theory [10,11]. A question raised by this body of work is whether normal, single-step, fast L-H transitions merely compress the oscillatory behavior into a short time, i.e., just one cycle of the oscillation with the Reynolds stress overshot behavior or whether the process is skipped entirely and a different process occurs.…”

“…There has been a large body of work in the past few years, focusing on slow L-H transitions near the power threshold, where a transitory intermediate oscillatory phase can be identified [2][3][4][5][6][7][8][9][10][11]. A model based on turbulence-driven zonal flows has been proposed to explain the oscillatory behavior and transition trigger [12,13].…”

“…Direct evidence for the turbulence radial wave number spectral shift and turbulence structure tilting prior to and across the L-H transition has newly been obtained at plasma edge in EAST using a 3 × 4 probe array. The probe array has four layers of tips spaced radially by Δr ¼ 2.5 mm with three tips spaced poloidally by 6 mm for each layer [9,10], mounted on a reciprocating probe system at the outboard midplane. The tips in the first and third layers are operated as triple probes providing the measurements of T e , n e , and plasma potential ϕ p ¼ ϕ f þ 2.5T e .…”

Turbulence elasticity—A new mechanism for transport barrier dynamics

“…Since the flip of LCO happens during the I-H transition, it is a signature of the formation of H-mode. 5 It is argued that if the delay time is reduced below a critical value by the mean shear flow, the DW-ZF system will be "locked" to the Hsolution. Therefore, turbulent elasticity is a critical "controller" of transport barrier dynamics.…”

“…Recently, experiments 4 with high spatial and temporal resolution suggest that the floating radial electric field (associated with the ZF), rather than the mean electric field (associated with ion pressure gradient), is central to triggering the transport barrier formation. 5 LCOs often occur in a "intermediate" regime, where the ZF shear is stronger than the turbulence decorrelation rate, but is not sufficiently strong to fully quench the DW turbulence. Though the LCO-like phenomena in DW-ZF system were noted in the extended predator-prey systems, such as the 2 predator þ 1 prey system composed of ZF ("predator"), mean shear flow ("predator"), and DW ("prey"), 6 the existing studies are limited to qualitative descriptions, and do not present predictions, e.g., state under what circumstance the LCO onset or stop?…”