H-mode Barrier Research Articles

Internal transport barrier discharges on ASDEX upgrade:progress towards steady state

A stationary advanced tokamak scenario with an internal transportbarrier (ITB) for ions and electrons, particles and momentum incombination with an H-mode barrier and flat shear(q0≈1) wasmaintained for 40 confinement times and several internal skintimes with HITERL-89P βN≈5. Raising thedensity by edge gasfuelling close to 50% of the Greenwald density to integrateproper exhaust conditions causes an increase of the thresholdpower to sustain an ITB and a decrease of Zeff below 2. Incontrast, a density increase caused by improved core particleconfinement at more triangular plasma shapes does not change theITB onset conditions. No temporal impurity accumulation evenwith high-Z Ar puffing was observed despite of peaked impuritydensity profiles.MHD modes contribute to the stationarity ofthe shear profile. In the ITB/H-mode scenario (1,1) fishboneswith a large reconnection area `clamp' the q-value in thevicinity of one and avoid sawteeth during the wholehigh-performance phase. In ITB scenarios with reversed shear(qmin ⩾2)fishbones can clamp the current profile development near theq = 2 surface without deteriorating energy confinement, whereasdouble-tearing modes, acting in a similar form, lead tosubstantial confinement losses.Applying central ECRF heatingand current drive to beam heated reversed-shear ITB dischargesshows an substantial effect on MHD stability, affecting thepassage of the q-profile through qmin = 2, and degrading orprolonging the reversed-shear phase depending on the CDdirection. Moreover, reactor relevant Te⩾Ti operation withtemperatures in excess of 10 keV was achieved with internaltransport barriers for both electrons and ions simultaneously.

Divertor tokamak operation at high densities on ASDEX Upgrade

Densities achievable in ASDEX Upgrade discharges are restricted by a disruptive limit in the L-mode caused by an edge-power imbalance which is linking divertor detachment, Marfe formation and the separatrix density. The attainable average densities depend then on the internal particle sources and the core transport and can exceed the empirical Greenwald density. In H-mode an upper density limit is found which represents a non-disruptive H - L back transition, which is preceded by the occurrence of type-III ELMs. Close to the Greenwald limit this H - L transition cannot be avoided at any power flux across the separatrix and - at high external neutral gas fluxes - confinement compared with ITER H-92P scaling degrades even before the back transition. The H-mode operational window is determined by local edge-barrier parameters and their gradients, respectively. The boundaries are represented by the L - H transition-temperature threshold, the ideal ballooning edge-pressure gradient limit, the upper temperature limit for type-III ELMs and an upper H-mode barrier density limitation. The cause for the last limitation is not yet identified; it may be due to resistive ballooning modes or the separatrix density limit. Despite the limited edge densities the Greenwald density could be surpassed by a factor of three with pellet refuelling from the low magnetic-field side. Pellet injection from the high-field side gains from the strong increase of fuelling efficiency due to the assisting toroidal outward drift of the formed high- ablatant. Higher densities are achievable in H-mode compared with low-field side injection and diminished convective losses avoid confinement degradation up to the Greenwald density. In gas-puffed type-I ELMy H-modes the plasma thermal energy and the edge-pressure gradients, which are limited by ballooning stability, are linked via a robust temperature-profile stiffness and the flat density profiles resulting from dominant edge refuelling at high densities. Their confinement does not improve with increasing density (and neutral gas fluxes) and may even slightly degrade. Therefore, the superior confinement of type-I ELMy H-modes compared with type-III ELMy ones at medium densities is actually offset at densities close to the Greenwald density. In contrast to the temperature-profile resilience density profiles can be changed both by deep refuelling (with pellets) and intrinsic transport improvements connected with density peaking (observed in CDH-modes), which offers the combination of high confinement and high density operation. The possible alliance with radiation cooling, divertor detachment and divertor compatible type-III ELMs could solve the power exhaust problem.

The H-mode in the ASDEX tokamak

The L-H transition in the ASDEX tokamak was investigated. The level of fluctuations both in the scrape-off layer and in a narrow region inside the separatrix drops on a 100 mu s timescale. Density turbulence coming from the edge and magnetic turbulence, as measured with magnetic probes, show a similar behavior. Before and during the transition no frequency shift in the fluctuation spectra could be detected which would indicate the rapid appearance of a radial E-field in the region of the H-mode barrier. Within milliseconds after the L-H transition new fluctuations propagating in the electron diamagnetic direction are destabilized which however exhibit no marked influence on transport across the barrier. In the bulk of the plasma fluctuation spectra show a frequency shift which is dominated by toroidal plasma rotation due to neutral beam injection. The initial transport barrier is deduced to lie inside the separatrix with a width of <or=3 cm.