Observation of a new turbulence-driven limit-cycle state in H-modes with lower hybrid current drive and lithium-wall conditioning in the EAST superconducting tokamak

Abstract

The first high confinement H-mode plasma has been obtained in the Experimental Advanced Superconducting Tokamak (EAST) with about 1 MW lower hybrid current drive after wall conditioning by lithium evaporation and real-time injection of Li powder. Following the L–H transition, a small-amplitude, low-frequency oscillation, termed a limit-cycle state, appears at the edge during the quiescent phase with good energy and particle confinement. Detailed measurements by edge Langmuir probes show modulation interaction and strong three-wave coupling between the low-frequency oscillations and high-frequency-broadband (80–500 kHz) turbulences that emerge after the L–H transition or in the inter-ELM phase. The potential fluctuations at the plasma edge are correlated with the limit-cycle oscillations, and the fluctuations in the floating potential signals at different toroidal, poloidal and radial locations are strongly correlated with each other, with nearly no phase differences poloidally and toroidally, and finite phase difference radially, thus providing strong evidence for zonal flows. The growth, saturation and disappearance of the zonal flows are strongly correlated with those of the high-frequency turbulence. And the measurements demonstrate that the energy gain of zonal flows is of the same order as the energy loss of turbulence. This strongly suggests the interactions between zonal flows and high-frequency turbulences at the pedestal during the limit-cycle state.

Edge-coherent-mode nature of the small edge localized modes in Experimental Advanced Superconducting Tokamak

“…EAST is a medium-sized tokamak with major radius R 0 ¼ 1.88 m, minor radius a ¼ 0.45 m, toroidal magnetic field B T < 3 T, plasma current I p < 1 MA, elongation ratio j ¼ 1.2-2, and triangularity d 0.8. 17 H-modes, with the normalized electron collisionality at the pedestal top e * > 1, 18 are usually obtained with a combination of lower hybrid current drive (LHCD) and ion cyclotron resonance heating (ICRH) accompanying with lithium wall conditioning. 19 Both LHCD and ICRF (ion cyclotron range of frequency) heating do not provide direct momentum injection, in contrast to the Neutral Beam Injection (NBI), which is the main heating source for the achievement of the type-II ELMy H-mode in ASDEX-Upgrade.…”

Compound sawtooth in EAST LHCD plasma: An experimental study

“…EAST is a superconducting tokamak with major radius R 0 = 1.70 m, minor radius a = 0.40 m, toroidal magnetic field B T < 3 T, plasma current I p ≤ 1 MA, and elongation ratio κ = 1.8-2. It was designed for long-pulse diverter operations up to 1000 s [17,18] and achieved diverter discharges exceeding 400 s during the last experimental campaign. The X-ray tomographic system used extensively for this paper is located in port C, and the line arrangement is shown in Fig.…”

Comparison of bifurcation dynamics of turbulent transport models for the L-H transition

“…These smooth transitions from L-mode to H-mode without clear bifurcation are also seen in low-density discharges at JET, [31][32][33] and also "smoother" transitions are observed at ASDEX Upgrade. 34 Additionally, Carlstrom points out the "dithering" transitions observed at ASDEX Upgrade; 35 however, nowadays there is lots of investigation into all types of oscillatory transitions, at DIII-D, 36 ASDEX Upgrade, 37,38 EAST, 39,40 JET, 41 TJ-II, 42 and HL-2A. 43 Most probably, some of these oscillations are caused by the same mechanism, but there could also be multiple mechanisms at play leading to oscillatory behaviour around the L-H transition.…”