W. Schneider

ELM pace making and mitigation by pellet injection in ASDEX upgrade

In ASDEX Upgrade, experimental efforts aim to establish pace making and mitigation of type-I edge localized modes (ELMs) in high confinement mode (H-mode) discharges. Injection of small size cryogenic deuterium pellets (~(1.4?mm)2 ? 0.2?mm ? 2.5 ? 1019?D) at rates up to 83?Hz imposed persisting ELM control without significant fuelling, enabling for investigations well inside the type-I ELM regime. The approach turned out to meet all required operational features. ELM pace making was realized with the driving frequency ranging from 1 to 2.8 times the intrinsic ELM frequency, the upper boundary set by hardware limits. ELM frequency enhancement by pellet pace making causes much less confinement reduction than by engineering means like heating, gas bleeding or plasma shaping. Confinement reduction is observed in contrast to the typical for engineering parameters. Matched discharges showed triggered ELMs ameliorated with respect to intrinsic counterparts while their frequency was increased. No significant differences were found in the ELM dynamics with the available spatial and temporal resolution. By breaking the close correlation of ELM frequency and plasma parameters, pace making allows the establishment of fELM as a free parameter giving enhanced operational headroom for tailoring H-mode scenarios with acceptable ELMs. Use was made of the pellet pace making tool in several successful applications in different scenarios. It seems that further reduction of the pellet mass could be possible, eventually resulting in less confinement reduction as well.

Tokamak edge modeling and comparison with experiment in ASDEX

A survey of edge modeling and its application to running experiments is given with emphasis on poloidal divertors. The basic edge structure in axisymmetric and weakly perturbed tokamaks is first discussed. The ongoing modeling activities and the status of model validation are outlined. ASDEX data are mostly used for comparison, since sufficiently detailed and coherent edge measurements are not available in the literature for most experiments. Edge physics issues discussed in more detail are the basic model equations, parallel and perpendicular transport coefficients, thermoelectric effects, edge density limit and three-dimensional perturbations including magnetic field ergodization.

Frequency control of type-I ELMs by magnetic triggering in ASDEX Upgrade

Magnetic triggering of edge localized modes (ELMs) was reported first from TCV in ohmic plasmas showing type-III ELMs. This method, showing successful locking of the ELM frequency to an imposed vertical plasma oscillation, has now also been demonstrated in the ITER-relevant type-I ELM regime in ASDEX Upgrade. Our experiments showed the ELM frequency becoming identical to the driving frequency in steady state for an applied motion of only about twice the value caused by an intrinsic ELM event. Triggered ELMs still showing clear type-I features were found when the plasma down-shift velocity reached its maximum, corresponding to the lowest edge current value. This is the opposite of the behaviour expected from the peeling–ballooning nature attributed to the ELM boundary and to TCV observations. The reason for this behaviour is not yet clear.

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.

A numerical study of high-beta stellarator equilibria

A numerical study of surface- and diffuse-current, high-beta stellarator equilibria is described. Numerical results are obtained from an algorithm for the solution of the ideal magnetohydrodynamic equations in three dimensions as an initial- and boundary-value problem. Equilibria are obtained for diffuse-current plasmas just as for surface-current plasmas. The equilibrium conditions for diffuse-current plasmas are quantitatively different from those for surface-current plasmas, especially at high beta for l = 0, 1 systems. In contrast, the equilibrium conditions scale according to small-δ theory even to δ ~ 1. Results are obtained for a range of dimensionless parameters (beta, aspect ratio, etc.) and compared to an asymptotic equilibrium theory and experiments. Good agreement among the two theories and the experiments is shown.

First Results of Ion Cyclotron Resonance Heating on ASDEX Upgrade

ASDEX Upgrade is equipped with an ICRH system consisting of 4 generators of 2 MW power each and 4 double loop antennas. The generators, tuneable in frequency from 30 to 120 MHz, cover several heating scenarios over a wide range of magnetic fields (1 T<Bt<3.9 T): minority heating of H and He3 and second harmonic heating of H and D. ICRH‐heated discharges in ASDEX Upgrade were so far carried out mainly at 30 MHz and a magnetic field of 2 T (H minority in D and He). Peak powers of 2.4 MW and pulse length up to 2.5 s were achieved (total energy 3.75 MJ). In L‐mode, the density on turn‐on of the ICRH stays constant, or even decreases. The ratio of radiated power to total input power is unchanged (60% in an unboronized machine, 30% in a freshly boronized machine) between Ohmic and ICRH phases. The electron temperature increases with 0.9 MW from 1 to 1.25 keV, the loop voltage drops. Transitions to the H‐mode were easily and reliably achieved with ICRH alone (necessary ICRH power as low as 0.9 MW) and the length of the ELMy H‐mode phases was limited only by the applied ICRH pulse length (ELMy H‐mode phases of up to 2 s were achieved). The paper presents further results on heating and confinement in L and H‐mode, antenna and edge studies and on divertor measurements. Preliminary experiments, performed with a combination of H minority heating (30 MHz) and H second harmonic (60 MHz) in 600 kA He and D discharges (H minority in the 5 to 20% range) at 2 T, and with non‐resonant heating (30 MHz and 60 MHz at 1.35 T) are briefly discussed.