A. Carlson

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.

Volume recombination in divertor I of ASDEX Upgrade

High density operation in the ASDEX Upgrade divertor I with horizontal target plates is reported. Density rampup experiments were carried out to characterize detached plasma conditions in the divertor. During the detached phases, hydrogen continua and spectral line emission from high-n shells were observed in the divertor due to the volume recombination. The spectroscopic measurements provide a consistent picture of the evolution of the divertor plasma parameters during the density ramp. By means of the ADAS atomic physics program package, the rate of volume recombination was evaluated, including the effect of opacity. The relative importance of volume recombination in comparison with the target plasma sink is discussed. Observations indicating differences in volume recombination between the two divertor legs are presented, and the connection of volume recombination to divertor detachment is addressed.

Experimental investigation of marfes and the density limit in the ASDEX Upgrade

In the ASDEX Upgrade, X-point marfe formation and its behaviour up to the density limit is investigated in gas-fuelled ohmically-heated single-null discharges over a wide range of parameters: Ip=0.6-1.2 MA, BTF=1.35-2.4 T, a plasma elongation of 1.6 and Zeff<2. The standard-ion Del B drift is directed towards the X-point. At medium electron densities inevitably a marfe develops in the vicinity of the active X-point. The marfe formation is consistent with a model of thermal instabilities in the radiating edge plasma. Moreover, stable steady-state operation is demonstrated with marfes which can extend significantly into the bulk plasma. The density limit is always connected with quick marfe expansion and movement followed by mode-locking leading to a major disruption. The limit scales linearly with Ip and is in good agreement with the Greenwald density limit scaling. The resulting experimental Hugill limit is neRq95/BTF=2.8*1020) m-2 T-1. Reversal of the ion Del B drift direction away from the target has a detrimental effect on the density limit.

Electric currents in the scrape-off layer in ASDEX Upgrade

Abstract Scrape-off layer (SOL) currents are measured by means of Langmuir probes and shunts in the divertor of ASDEX Upgrade. They consist of the overlayed contributions of thermoelectric and Pfirsch–Schluter (PS) currents. The SOL currents exhibit a drastic decrease when the line-averaged density approaches the Greenwald density in the H-mode. An analytical model is presented which reproduces the measured thermoelectric current quantitatively. Matching of the analytical model with the measured current scaling yields information about divertor temperatures and SOL e-folding lengths.

Recent results from divertor operation in ASDEX upgrade

The results of divertor studies on ASDEX Upgrade, at currents of up to 1.2 MA and heating powers up to 10 MW are described, with emphasis on the ELMy H-mode. The spatial and temporal characteristics of their heat load, and the simulation of ELMs by a time-dependent scrape-off layer code are described. High gas puff rates were found to lead to a large increase in divertor neutral pressure, at modest changes in ne, and to a strong reduction in time-averaged power flow and complete detachment from both target plates in between ELMs. Using pre-programmed puffs of neon and argon, the radiative power losses could be raised to 75% of the heating power, in H-regime discharges, and the regime of enhanced divertor neutral pressure was found also to lead to an improved pumping of recycling impurities.

Confinement regime transitions in ASDEX

The authors give an overview of the different confinement regimes observed on ASDEX and compare the changes during the transition phases with qualitative tendencies suggested by theoretical models. The transitions discussed are those between purely Ohmic heating and additional heating in the L-regime between the L- and the H-regime and between discharges with flat and peaked electron density profiles.

Impurity Accumulation in Plasma Regimes with High Energy Confinement

Investigations of impurity accumulation phenomena in ASDEX are reviewed. There are four different operating regimes where pronounced accumulation is observed and these regimes are also characterized by improved energy confinement. In particular, medium-Z metallic ions are involved in accumulation processes whereas low-Z ions appear almost unaffected. The rapid accumulation observed in the case of metallic ions may be explained by neoclassical inward drifts if we assume that the anomalous diffusion is sufficiently suppressed, some indication of this being found from laser blow-off studies. The present results, however, can only be partly explained by neoclassical theory, according to which accumulation of low-Z impurities should also occur. The temporal behaviour of accumulation and the retarding effect of proton dilution for collision dominated transport are also discussed. Finally, we conclude that the full benefits of improved energy confinement can be achieved only if the impurity influxes are kept to a sufficiently low level. Expressed in terms of concentrations under low confinement conditions we have to postulate, for ASDEX, concentrations ≲ 10−4 for metals and ≲ 2% for all light impurities.

Long-Pulse Heating of ASDEX Plasmas

The Divertor Tokamak ASDEX, its neutral injection system and its ICRH system have been modified to permit additional heating with a power of 6 MW for pulse lengths up to 10 s. The paper summarizes the arguments for long-pulse heating, describes the technical modifications of the divertor performed, their effect on the operational behaviour of the tokamak and presents a few typical results of recent experiments exploiting the long-pulse heating facilities.

Characterizing the edge plasma of different Ohmic confinement regimes in ASDEX

To compare different Ohmic confinement regimes in ASDEX, the edge conditions are analyzed in detail. The results show that the improved Ohmic confinement coincides with a drop of the separatrix density. This drop allows the density profile to peak and seems to be the trigger of a change in the transport. A universal scaling between the electron temperature and the electron density at the separatrix prevails for all Ohmic scenarios. In addition, the total particle flux across the separatrix is evaluated and found to be strongly correlated to the separatrix density. Thus, the convective energy loss contributes less to the total energy losses when the confinement is improved. Since the correlations between the edge parameters do not change in different Ohmic confinement regimes of ASDEX, the edge physics appears to remain the same. Improved Ohmic confinement is characterized by an optimum separatrix density which provides a sufficiently high edge temperature together with low particle fluxes. These optimum conditions yield the maximum particle confinement.

Spectroscopic study of the radiation in divertor I of ASDEX Upgrade at high density

Radiation losses in divertor I of ASDEX Upgrade have been studied at high density when the divertor is detached. Spectroscopic measurements in the visible and VUV spectral range allow a characterization of both the impurity source and radiation. Despite very low target temperatures of 1 eV, carbon is released from the target with high rates by chemical sputtering. Power losses are estimated from the resonance lines of deuterium and the carbon ions. A comparison with bolometry shows that 80% of the emission can be ascribed to carbon and deuterium. The composition of the carbon emission is compared with results obtained on JT-60 and DIII-D. Using the Braams-Eirene model of the detached divertor the influence of transport on the power losses is investigated. By means of a realistic radiative loss function we find that the enhancement of maximum power input into the divertor by non-coronal effects is not more than a factor of two.