J. Schweinzer

H-mode power threshold and transition in ASDEX Upgrade

Global and local H-mode threshold analyses are presented and discussed. The density window was extended up to the density limit towards which the power threshold exhibits a dramatic increase. First results with the new divertor are compared to the previous threshold scaling. Local edge measurements at the L-H transition show that the temperature decreases slightly with density and increases with magnetic field, and that the ion collisionality at the plasma edge is always clearly above unity, between 5 and 15. Experiments at high power threshold exhibit improved L-mode confinement and suggest that two mechanisms are required for the L-H transition.

Scrape-off layer radiation and heat load to the ASDEX Upgrade LYRA divertor

In 1997 the new `LYRA divertor went into operation at ASDEX Upgrade and, in parallel, the neutral beam heating power was increased to 20 MW by installation of a second injector leading to a P/R value of 12 MW/m. Experiments have shown that the ASDEX Upgrade LYRA divertor is capable of handling such high heating powers. There is an overall reduction of the maximum heat flux in the LYRA divertor by about a factor of 2 compared with the previous open divertor Div I. This reduction is mainly due to increased radiative losses inside the divertor region, which are caused by an effective reflection of hydrogen neutrals into the hot separatrix region. The main channel of radiative loss is carbon radiation, which cools the divertor plasma down to a few electronvolts, where hydrogen radiation losses become significant. The radiative losses preferentially reduce the power flux at the separatrix, leading to early detachment around the strike point position. With increasing density, the detached region extends upwards on the vertical target. The power fraction radiated in the LYRA divertor is around 45% and nearly independent of the heating power. This value is a factor of 2 higher than the typical radiation fraction in Div I. B2-EIRENE modelling of the performed experiments supports the experimental finding and refines the understanding of loss processes in the divertor region.

Stationary advanced scenarios with internal transport barrier on ASDEX Upgrade

Steady-state discharges with improved core confinement and H-mode edge with edge localized modes (ELMs) are investigated. In plasmas with an upper triangularity top close to zero an H-factor of HITER89-P = 2.7 and N = 2.2 could be maintained for 1 s and HITER89-P = 2.4 and N = 2.0 for 6 s, the latter corresponding to 40 confinement times or 2 1/2 resistive time scales for current redistribution, only limited by the duration of the possible discharge length. At a line averaged density of 4 × 1019 m-3 the central temperatures reach values of Ti = 10 keV and Te = 6.5 keV. The stationarity of the current profile is explained by magnetic reconnection driven by strong (m = 1, n = 1) fishbones, which, in the absence of sawteeth, also expel energy and impurities. Further increasing the pressure, is limited by neoclassical tearing modes. Raising the density by edge gas fuelling and the simultaneous increase of the neutral beam power, HITER89-P remained unchanged up to ne = 5.5 × 1019 m-3, accompanied by a substantial reduction of Zeff. Increasing top to 0.2, both confinement and -limit improved reaching values of HITER89-P = 3.0 and N = 2.4 at densities above ne = 5 × 1019 m-3. This resulted in the highest fusion product of nD,0Ti,0E = 0.9 × 1020 keV s m-3 so far observed in ASDEX Upgrade.

Closed divertor operation in ASDEX Upgrade and JET

Operation in ASDEX Upgrade and JET with different divertor geometries is compared with emphasis upon energy and particle exhaust and the impact of the divertor on core plasma performance. Closing the divertor increases the ratio of the neutral flux density in the divertor to that in the main chamber and facilitates improved pumping. The influence of closure on H-mode performance turns out to be weak, the main chamber physics dominates. The electron density/neutral hydrogen flux in the scrape-off layer is found to be a key parameter for particle and energy confinement as well as for the radiative losses. The density in the outer scrape-off layer is related to main chamber recycling, while the separatrix density is primarily affected by divertor geometry. Reduced target plate power load due to an increased divertor radiation level is obtained in ASDEX Upgrade Div II. The radiated power fraction in JET is systematically lower, which is partly explained by the lower edge densities. Core plasma Zeff and carbon concentrations are similar in both machines and do not depend on divertor geometry.

Effects of type-I edge-localized modes on transport in ASDEX upgrade

Particle and energy losses due to type-I edge-localized modes (ELMs) and the effect of H-mode pedestal parameters on global confinement are studied for H-Mode discharges in ASDEX Upgrade. ELM frequency as well as particle and heat losses during ELMs are strongly affected by gas puffing, impurity injection, variation of plasma current and heating power. However the average power loss due to ELMs P-ELM = f(ELM)Delta W shows much less variation, and cannot account for the variations of stored energy encountered, e.g. in a plasma current scan. A good correlation between pedestal pressure and stored energy is found for type-I ELMy discharges.

The relation of edge confinement to global confinement in ASDEX upgrade (Axially Symmetric Divertor Experiment)

Experimental evidence is presented from the ASDEX Upgrade (Axially Symmetric Divertor Experiment) tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1993 (International Atomic Energy Agency, Vienna, 1994), Vol. I, p. 127] of a robust relation between the edge radial pressure gradient and the global confinement of the plasma. This relation transcends the power flowing across flux surfaces near the edge and thus suggests that the usual model of cross-field heat transport, where local gradients increase with increasing local power flow, is not appropriate.

Characteristics of edge localized modes in ASDEX upgrade

Characteristics of the various types of edge localized modes (ELMs) observed during the H-mode of the ASDEX Upgrade tokamak are presented. Magnetic high-mode-number ELM precursor activity is found for type I ELMs and type III ELMs, the former only during neutral beam injection in the counter direction. The effect of ELMs on electron temperature and density profiles is discussed.