The H-mode density limit in the full tungsten ASDEX Upgrade tokamak

Abstract

The high confinement mode (H-mode) is the operational scenario foreseen for ITER, DEMO and future fusion power plants. At high densities, which are favorable in order to maximize the fusion power, a back transition from the H-mode to the low confinement mode (L-mode) is observed. In present tokamaks, this H-mode density limit (HDL) occurs at densities on the order of, but below, the Greenwald density.In gas ramp discharges at the fully tungsten covered ASDEX Upgrade tokamak (AUG), four distinct operational phases are identified in the approach towards the HDL. These phases are a stable H-mode, a degrading H-mode, the breakdown of the H-mode and an L-mode. They are reproducible, quasi-stable plasma regimes and provide a framework in which the HDL can be further analyzed. During the evolution, energy losses are increased and a fueling limit is encountered. The latter is correlated to a plateau of electron density in the scrape-off layer (SOL). The well-known extension of the good confinement at high density with high triangularity is reflected in this scheme by extending the first phase to higher densities.In this work, two mechanisms are proposed, which can explain the experimental observations. The fueling limit is most likely correlated to an outward shift of the ionization profile. The additional energy loss channel is presumably linked to a regime of increased radial filament transport in the SOL. The SOL and divertor plasmas play a key role for both mechanisms, in line with the previous hypothesis that the HDL is edgedetermined.The four phases are also observed in carbon covered AUG, although the HDL density exhibits a different dependency on the heating power and plasma current. This can be attributed to a changed energy loss channel in the presented scheme.

Heat flux pattern in detached L-modes and ELM mitigated H-modes with rotating magnetic perturbations in ASDEX Upgrade

“…The line integrated edge density n edge ( Fig. 2 (c)), measured along the interferometry Line-Of-Sight (LOS) H-5 [20], increases during that phase from 1.1 to 3.2 × 10 19 m −3 . In Fig.…”

Plasma particle sources due to interactions with neutrals in a turbulent scrape-off layer of a toroidally confined plasma

“…It is possible to further illuminate how the interactions with neutrals affect blobs, and how blobs affect neutrals, with a model that include both dynamical neutrals and plasma fields. Moreover a range of experimentally observed phenomena that are most likely due to neutrals influencing the plasma fields, such as shoulder formation 30 and the fuelling triggered H-mode density limit 31 , are expected to at least be partially explained by future simulations with this model. …”

“…Had elastic collisions with neutrals been included in the model, a yet another first order drift term would be added to (31). This is the so-called Pedersen drift, and details on the derivation of the term are found in 8 .…”

Dependence of alpha-particle-driven Alfvén eigenmode linear stability on device magnetic field strength and consequences for next-generation tokamaks

“…An H-mode can thus vary its Greenwald fraction below this limit, but not above it; if the tokamak seeks to maximize its density, the density will be determined by this limit. Regressions over a very limited magnetic field range (roughly 1 T to 3 T) have found that the Greenwald fraction corresponding to this H-mode density limit declines sharply with magnetic field, but the full magnetic field behavior has not been explored [26,27].…”

“…The width of an AE may be estimated by taking (27) and solving for a mode of the form ξ ⊥ = exp [i (nφ − ωt)] m ξ ⊥,m (r) exp (−imθ). Stating the result to zeroth order in inverse aspect ratio gives [29][30][31] d dr…”