Study of the L–I–H transition with a new dual gas puff imaging system in the EAST superconducting tokamak

Abstract

Abstract:The intermediate oscillatory phase during the L-H transition, termed I-phase, has been studied in the EAST superconducting tokamak by employing a newly developed dual gas puff imaging (GPI) system near the L-H transition power threshold. The experimental observations suggest that the oscillatory behavior 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 (EF). The latter arises from the neoclassical 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 level decays and recovers periodically. The turbulence recovery appears to originate from the vicinity of the separatrix with clear wave fronts propagating both outward into the far scrape-off layer and inward into the core plasma. The Reynolds work done by the turbulence on the ZFs has been directly measured using the GPI system in the experiments, providing a direct evidence of kinetic energy transfer from turbulence to ZFs, thus driving the ZFs at the plasma edge. The ZFs are damped shortly after turbulence suppression, due to the 2 loss of turbulent drive, which then leads to the subsequent recovery of the turbulence level, initiating the next dithering cycle, or followed by a final transition into the H-mode, as the EF shear is strong enough to maintain turbulence suppression, even without the assistance of the ZF shear. A new self-consistent zero-dimensional model, incorporating the evolution of the EF and ZF shear, as well as the parallel transport in the scrap-off layer, has been developed and successfully reproduced the L-I-H transition process with many features comparing favorably with the experimental observations.

Magnetic structure and frequency scaling of limit-cycle oscillations close to L- to H-mode transitions

“…This suggests that the M-mode from JET is the same as the I-phase at AUG. The results are also similar to the findings during I-phase at EAST [62] and HL-2A [16]. It was proposed that the magnetic m = 1 structure during the I-phase could simply be a consequence of the reorganisation of Pfirsch-Schlüter flows/currents during modulations of the pressure gradient (cf.…”

Linking the micro and macro: L-H transition dynamics and threshold physics

“…This evolution can occur via an extended cyclic I-phase or in a single burst of shear flow growth. [19][20][21] Recent fluctuation measurements 16 and transition model studies 22 support this two-step scenario. However, the improved understanding of dynamics so far has not yielded an improved understanding of the power threshold.…”

Invited Review Article: Gas puff imaging diagnostics of edge plasma turbulence in magnetic fusion devices

“…86 The Experimental Advanced Superconducting Tokamak (EAST) in China has installed a dual GPI system with two views located ±50 • poloidally with respect to the outer midplane, in order to evaluate the up/down symmetry of the turbulence. 51,[87][88][89] The glass fiber bundles are shielded by molybdenum tubes of 10 mm thickness to prevent darkening due to the large hard x-ray flux during lower hybrid current drive (LHCD).…”