L–H transition and pedestal studies on MAST

Abstract:An overview is given of recent experimental results in the areas of innovative confinement concepts, operational scenarios and confinement experiments as presented at the 2010 IAEA Fusion Energy Conference. Important new findings are presented from fusion devices worldwide, with a strong focus towards the scientific and technical issues associated with ITER and W7-X devices, presently under construction.

Section: Empirical Studies Of L To H Transitionssupporting

confidence: 88%

“…A new Motional Stark Effect diagnostic has been made operational on MAST to measure the edge toroidal current density [41]. These measurements have been compared with neoclassical calculations of the bootstrap current as shown in figure 40.…”

Section: Pedestalmentioning

confidence: 99%

23rd IAEA Fusion Energy Conference: summary of sessions EX/C and ICC

Hawryluk¹

2011

“…Energy-transfer coefficients well in excess of one have been confirmed immediately preceding the transition, as required for rapid turbulence quench. We wish to point out, however, that several experiments do not reveal a clear separation between flow generation and pressure-profile modification/ increase in diamagnetic flow and flow shear across the L-H transition, pertaining to fast L-H transitions [101][102][103] but also L-mode-LCO transitions [57,104]. It is unknown at this time, which characteristics lead to these observed differences.…”

Section: Discussionmentioning

confidence: 92%

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

Schmitz

2017

This scaling reflects only the dependence of the L-H transition power threshold on plasma density ) − n (10 m e 203 , toroidal magnetic field φ B (T), and the plasma surface area S (m 2 ). However, the threshold power has been shown to depend on additional parameters such as isotopic plasma composition, plasma shape, divertor geometry, and the beam-induced and intrinsic torque [4][5][6][7][8]. A physics-based model of the L-H transition threshold power is therefore needed to confidently extrapolate to auxiliary heating requirements for ITER and future burning plasma experiments. It was recognized early on that the H-mode edge barrier forms as fluctuations are suppressed due to E × B flow shear [9][10][11] in a narrow (few cm wide) radial layer just inside the last closed flux surface (LCFS) [12][13][14][15]. While the paradigm of Nuclear Fusion

“…Recently, there is renewed interest in the L-H transition physics and power threshold [22][23][24][25][26][27][28][29][30][31][32][33], stimulated by the ITER requirement for H-mode operation in the initial non-active 5 phases with limited power available [3], as well as remarkable progress in high-resolution diagnostic capability at the plasma edge that allows us to gain deeper insights into the physics of the L-H transition [34]. Several powerful new diagnostics, such as multichannel Doppler Reflectometry [35][36][37][38], gas puff imaging (GPI) [39][40][41], beam emission spectroscopy (BES) [42,43], and reciprocating Langmuir probes [44][45][46], have recently been employed in several tokamaks [37][38][39][40][41][42][43][44][45] and a stellarator [35,36] to study the L-H transition physics near the threshold conditions, with some experimental results comparing favorably with the L-H transition model [15].…”

Section: Introduction and Brief Review Of The L-h Transition Studymentioning

confidence: 99%

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

Shao

Liu

et al. 2013

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.