M. Weinlich

H mode discharges with feedback controlled radiative boundary in the ASDEX Upgrade tokamak

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment

Spectroscopic measurements of tungsten erosion in the ASDEX Upgrade divertor

Tungsten erosion in the ASDEX Upgrade divertor has been measured by observing the WI emission at 400.9 nm. The sputtering yield is in the region of atoms/ion for typical divertor plasma conditions. By comparison of the measured sputtering yield with modelled sputtering yields one can conclude that tungsten erosion is dominated by plasma impurities. Adding impurities without cooling the boundary plasma increases the sputtering yield due to the additional impact energy gained by the particle in the sheath potential caused by its higher net charge. Cooling the boundary plasma by adding more impurities or increasing the plasma density leads to a strong decrease in the tungsten erosion. By comparison of the tungsten influx with the measured net erosion the prompt redeposition was determined. The redeposited fraction of tungsten increases significantly with a higher divertor electron density as expected.

High Density Operation Close to Greenwald Limit and H Mode Limit in ASDEX Upgrade

The L mode and H mode density operational window in the vicinity of the density limit has been investigated with a combination of gas puff refuelling and improved fine tuning of neutral beam injection (NBI) heating power. In this way, a novel strategy is achieved by means of a parallel increase of density and heating power. As the density limit is approached, H modes degrade into L modes independently of heating power; this is in contrast to the generally accepted L to H mode threshold scaling PheatL-H varies as neB. Furthermore, contrary to the well known heating power independent Greenwald limit, the L mode density limit increases moderately with rising heating power, neDL varies as Pheat0.3+or-0.1, if a simple power law is assumed. The power dependence becomes more obvious when analysed in terms of edge densities and powers flowing across the separatrix into the scrape-off layer, nesep varies as Psep0.6+or-0.2. The corresponding H mode studies show that before an H mode quenches into an L mode the maximum achievable density (i.e. The H mode density limit) is practically independent of the heating power, as observed on many machines

A fast scanning Langmuir probe system for ASDEX-Upgrade divertor

Abstract A fast scanning Langmuir probe system (LPS) for ASDEX-Upgrades divertor plasma investigations was designed, manufactured and operated. Profiles of ion saturation current density, Jsat, electron temperature, Ted, electron density, Ned, floating potential, Vfl, and Mach number have been recorded for ohmic (OH) and low/medium power neutral beam heated (NI) discharges. In these cases the probe can access both divertor legs, allowing comparison of plasma parameters in the two divertors and investigations of the private flux region. Strong divertor asymmetries, complex plasma flow patterns and changes in the power flow to the divertor targets, depending on ion gradB drift direction, density and divertor geometry, have been observed.

Study of Gross and Net Erosion in the ASDEX Upgrade Divertor

Abstract Erosion of carbon and tungsten in the divertor of ASDEX Upgrade was investigated by surface analysis of target tile probes. The probes were exposed in the divertor using a manipulator system. Erosion of carbon was measured by graphite probes covered with a 150 nm layer of 13 C isotope to distinguish from the intrinsic carbon content. Tungsten erosion was measured by similar probes with 1–20 nm tungsten markers evaporated on the surface. From the measured erosion, sputtering yields were determined using plasma parameters at the target plates obtained from flush mounted Langmuir probes.

The tungsten experiment in ASDEX Upgrade

Abstract Tungsten coated tiles, manufactured by plasma spray on graphite, were mounted in the divertor of the ASDEX Upgrade tokamak and cover almost 90% of the surface facing the plasma in the strike zone. Over 500 plasma discharges, among which around 300 were heated with heating powers up to 10 MW, were performed up to now. The tungsten flux in the divertor was monitored by a WI line at 400.8 nm. In the plasma centre an array of spectral lines at 5 nm emitted by ionisation states around W XXX was monitored. Under normal discharge conditions W-concentrations of around 10 −5 or even lower were found. The influence on the main plasma parameters was negligible. In a few low power discharges accumulation of tungsten occurred and the temperature profile was flattened. The concentrations of the intrinsic impurities carbon and oxygen are comparable to the discharges with graphite divertor. Furthermore, the density-limits and the β-limits remained unchanged and no negative influence on the energy confinement as well as on the H-mode threshold was found.

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.

Energy transport to the divertor plates of ASDEX-Upgrade during ELMy H-mode phases

Abstract The energy flux to the ASDEX-Upgrade divertor plates is routinely measured by thermography and Langmuir probes. The thermographically observed power decay length at the target plate is about 1 cm near the inboard separatrix. During an edge localized mode (ELM) of type I the density profiles are significantly, changed; an additional contribution occurs characterized by a power decay length in the order of 10 cm outside the separatrix and additional power is deposited into the private flux region. It is supposed that this is due to the changing, contribution of energy conduction versus convection. Results of ELM-modelling using the coupled B2-EIRENE code reproduce the main features of the experimental observations. The sheath transmission factor is calculated by combining thermography and Langmuir probe data.

Divertor heat and particle flux due to ELMs in DIII-D and ASDEX-upgrade

The authors characterize the divertor target plate heat and particle fluxes that occur due to Edge-Localized-Modes (ELMs) during H-mode in DIII-D and ASDEX-Upgrade. During steady-state ELMing H-mode the fraction of main plasma stored energy lost with each ELM varies from 6% to 2% as input power increases above the H-mode power threshold. The ELM energy is deposited near the strikepoints on the divertor target plates in a fast time scale of {le} 1 ms. The spatial profile of the ELM heat pulse is flatter and broader, up to about a factor of 2, than that of the heat flux between ELMs. On ASDEX-Upgrade the inboard strike-point receives the greatest fraction, {ge} 75%, of ELM divertor heat flux, while on DIII-D the in/out split is nearly equal. The toroidal asymmetry of the heat pulse has produced a peaking factor on DIII-D of no more than 1.5. The particle flux, as measured by Langmuir probes, has also been found to be localized near the divertor strike-points. The increased particle flux during ELMs is a significant fraction of the total time-integrated divertor plate particle flux.

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.