Search

In-vessel inspections of AUG reveal arc tracks at different locations. This paper presents investigation of arcing at the inner transition and retention module where arc tracks were found around the whole toroidal circumference. A representative tile was investigated with profilometry, SEM, EDX, RBS, and colourimetry. Type 2 arc tracks with an area fraction of 12% were found in a 10 mm wide region near to the leading edge. Here the tungsten coating (3–4 μm) is removed and a small amount of carbon (max. 1–2 μm) is eroded. Droplets are detected at the surface. The region of the inner divertor where the arcs are observed shows a local enhancement of plasma density, n e ≈ 10 20 m −3 , and neutral pressure, 0.1 mbar, favouring the arc ignition. A high sheath potential at the beginning of an ELM might trigger the arc. The arc pattern and the target load as measured with a fast IR-camera coincide. 2 g of tungsten are eroded in the inner divertor. If 10% of the material is eroded as droplets, 10 8 droplets (5 μm diameter) are ejected.

The filamentary structure in the SOL of ASDEX Upgrade during type-I ELMs was investigated by combined measurements of reciprocating Mach probe, filament probe – a combination of Langmuir probes and magnetic sensor for the radial component of the magnetic field, and IR-thermography of the reciprocating probe head. A correlation along field lines between the Langmuir probes was found, confirming the field aligned structure of the observed filaments. The ELM radial e-folding lengths of particle and heat flux are comparable (about 2 cm). This may be explained by an approximately constant ion temperature well above the electron temperature in the far SOL. The ion temperature estimated from the comparison of heat and particle flux is about 30–60 eV.

Large molybdenum mirrors have been exposed in the SOL of TEXTOR in order to simulate conditions relevant for ITER optical components. Distortions of the reflectivity - increase as well as decrease - are found in the erosion and deposition dominated areas, respectively. The changes are most pronounced in the near UV and level off in the IR and can partly be attributed to observed surface changes. A novel periscope system was installed and mirrors exposed in a pilot experiment to simulate the transmission of light to distant sensors in ITER.

Numerous traces of arcs have been found on W-covered graphite tiles of ASDEX Upgrade after exposure. The distributions of number density, lengths and orientation are calculated and compared to pure graphite tiles at comparable locations. It was established that arcs perforate a 1 μm tungsten layer down to the carbon substrate. The amount of removal should rise with arc current, but a surface fraction of about 8% is eroded at 10 A already. At tiles of the divertor baffle the layer is continuously removed along the entire track pointing to higher currents. The carbon of the stripped parts is subject to subsequent erosion processes. The distribution of materials in and around arc tracks was investigated by sputter depth profiling (SIMS and AES) and the characteristic geometry was studied using an electron microscope. Observations are interpreted using results from laboratory vacuum arcs on the same material.

Plasma sprayed B4C-layers considered as wall coatings for the W7X stellarator have been studied during and after exposure to TEXTOR and after arcing experiments in vacuum. Arcing through the B4C layer occurred favoured by high power fluxes and not restricted to less stable phases. But this arcing implies an especially noisy scrape-off layer (SOL). Instead of moving retrograde in the external magnetic field, the arc spot on the B4C-layer sticks to the same location for its whole lifetime. Consequently, the arc erodes the entire B4C layer, finally burning down to the Cu substrate. In the neighbourhood of craters the surface contains Cu originating from those craters. This material, hauled to the surface by the arc, is subject to subsequent erosion, transport, and redeposition by the SOL-plasma. The behaviour of arcs on B4C is most probably caused by the peculiar temperature dependences of the electrical and heat conductivity of B4C. 2003 Elsevier Science B.V. All rights reserved.

Analysis of Type I ELMs from present experiments shows that ELM energy losses decrease with increasing pedestal plasma collisionality ( ν ∗ ped ) and/or increasing τ Front ∥ , where ( τ ∥ Front =2π Rq 95 / c s ,ped ) is the typical ion transport time from the pedestal to the divertor target. ν ∗ ped and τ Front ∥ are not the only parameters that affect the ELMs, also the edge magnetic shear influences the plasma volume affected by the ELMs. ELM particle losses are influenced by this ELM affected volume and are weakly dependent on other pedestal plasma parameters. ‘Minimum’ Type I ELMs, with energy losses acceptable for ITER, where there is no change in the plasma temperature profile during the ELM, are observed for some conditions in JET and DIII-D. The duration of the divertor ELM power pulse is well correlated with τ Front ∥ and not with the duration of the ELM-associated MHD activity. Similarly, the time scale of ELM particle fluxes is also determined by τ Front ∥ . The extrapolation of present experimental results to ITER is summarised.

One of the key unresolved issues in the design of a future fusion reactor is the power handling capability of the divertor target plates. Earlier we reported on the existence of narrow power deposition profiles in JET, obtained mainly from Langmuir probe measurements. We repeated these measurements in the MkI, MkII and MkIIGB divertor configurations with an upgraded probe system, which allowed us to study the profile shape in more detail. The main results of this study are: In NB heated discharges the electron temperature and power flux at the outer target show a distinct peak of ∼5 mm half-width near the separatrix strike point. The corresponding profiles on the inner target do not show a similar feature. The height of the narrow peak increases with NB heating power and decreases with deuterium and impurity gas puffing. Ion orbit losses are suggested as a possible explanation of the observed profile shape.

We found a complete extinction of the CD molecule emission in the inboard divertor of ASDEX Upgrade at high density with the transition to a recombining divertor plasma. The electron temperature in the recombining divertor plasma ranges from 1.2 down to 0.3 eV. Two effects may explain the extinction at such low temperatures, (1) a steep decay of the yield for chemical sputtering as theoretically predicted and (2) a suppression of the optical emission while the methane molecules dissociate unchanged via charge exchange.

In this paper we describe impurity line emission measurements in the divertor of ASDEX Upgrade during high power neutral beam heated discharges. We focus on detached conditions where the dominating part of the radiation comes from the X-point region. Spatially resolved line emission in the VUV and visible spectral region of the intrinsic carbon and additionally puffed impurities (neon and nitrogen) is presented. A simple interpretation of the line emission profiles is given and they are also compared to the results of bolometry.

In this paper we present first measurements of the plasma potential close to the separatrix in ASDEX Upgrade using a self emitting tip. The probe was inserted into the edge plasma of AUG using the midplane manipulator. Assuming Maxwellian plasmas, the observations agree with the predicted voltage drop in the plasma sheath, V pl ps − V fl = 2.5 T e , where V pl ps is the plasma potential at the presheath boundary and V fl the floating potential. Applying this technique a rapid change of the plasma potential was observed close to the separatrix during Ohmic discharges. From the gradient we derive a radial electric field E r of about −5 kV / m close to separatrix. Further out the field strength changes sign and we find up to +7 kV / m in the SOL.

The erosion and redeposition of silicon and tungsten evaporated onto the divertor plates of ASDEX Upgrade have been investigated. For comparison with the experimental results the computer program, ERO, has been used. The erosion of Si markers is smaller than expected from plasma ion sputtering, but still exceeds the W erosion by more than an order of magnitude. The observed erosion can only be explained by assuming a protective effect of carbon ions coming from the plasma which are deposited on the marker surfaces, thus reducing the marker erosion. A redeposition of the sputtered tungsten atoms has been found close to the original marker spots mainly because of prompt redeposition during the first gyration.