Outer Divertor Strike Point Research Articles

Effect of the divertor configuration on the JET edge radial electric field

The role of the divertor configuration and divertor plasma physics on the L–H transition is poorly understood, leading to large uncertainties in predicting the L–H power threshold in future devices. This contribution reports on edge perpendicular plasma flow measurements by Doppler backscattering in JET L–H transition experiments with the outer divertor strike-point at different positions: horizontal target (HT), vertical target (VT), and in the corner configuration (between the HT and VTs). The edge perpendicular flow was found to be significantly affected by changes in the divertor configuration in the region inside the separatrix. Our results do not show evidence for the existence of a critical edge flow shear needed to achieve H-mode for different divertor configurations, with a larger shear observed for the VT configuration. No significant change in the shear flow and in the density fluctuation level is measured preceding the L–H transition in the region just inside the separatrix. The dynamics of the L–H transitions are also influenced by the divertor configuration with divertor oscillations (DOs) observed only in the HT configuration. Interestingly, DOs are associated with marked changes in the edge perpendicular flow around the separatrix.

Conceptual design of reciprocating probes and material-testing manipulator for tokamak COMPASS Upgrade

Three new in-vessel manipulators are designed and built for the new COMPASS Upgrade tokamak with uniquely high vessel temperature (250–500 °C) and heat flux density (perpendicular to divertor surface q ⊥ ∼ 80 MW/m2 and q ∥ ∼ GW/m2 at separatrix), which challenges the edge plasma diagnostics. Here we show their detailed engineering designs supported by heat conduction and mechanical models. Deep reciprocation of electrostatic probes near the separatrix should be possible by optimizing older concepts in (a) the head and probe geometry, (b) strongly increasing the deceleration up to 100× gravity by springs and strengthening the manipulator mechanical structure. One reciprocates close to the region of edge plasma influx (the outer midplane), the other at the plasma sink (between the outer divertor strike point and X-point), for studying the plasma divertor (impurity-seeded) detachment and liquid metal vapor transport. Both probe heads are equipped with a set of ball-pen and Langmuir probes, measuring reliably and extremely fast (10−6 s) local (1 mm resolution) plasma potential, density, electron temperature and heat flux and even ion temperature with 10−5 s resolution. The divertor manipulator (without reciprocation) will place various material test targets at the outer divertor. Unique will be its capability to increase 15× the surface heat flux with respect to the surrounding tungsten tiles just by controllable surface inclination of the test targets. We plan to test liquid metal targets where such inclined surface was found critical to achieve the desired mode with lithium vapor shielding. Even in the conservative expected performance of COMPASS Upgrade, we predict to reach and survive the EU DEMO relevant heat fluxes.

Modelling dust transport in DIII-D with DTOKS-Upgrade

Comprehensive upgrades to the dust transport code Dust in TOKamaks (DTOKS) that extend the plasma-dust interaction model are presented and compared with recent measurements of dust transport in DIII-D. Simulations incorporating variation in physical properties of graphite dust with temperature and size in a stationary plasma background suggest a substantial decrease in lifetimes due principally to thermal expansion. The trajectories of 53 dust grains identified from analysis of visible camera data taken across two similar shots were used to measure the dust particle velocity distributions. Dust tracks terminated mostly at the outer divertor strike point having a mean observation time of 2.1 ± 0.4 ms. Stochastic modelling of 200 graphite dust particles in the DIII-D tokamak performed with DTOKS-U using plasma simulations generated by OEDGE found similar behaviour, with particles ablating rapidly after acquiring a positive charge in the region close to the outer strike point, creating an acute source of neutral carbon atoms. The simulated mean lifetime, 11 ± 2 ms, showed approximate agreement with experimental observation when corrected by accounting for dust visibility and ignoring the longest trajectories 1.5 ± 0.2 ms. Synthetic diagnostic data generated from coupling the results of DTOKS-U with the visualisation software Calcam offers a powerful new tool for validation of simulations and predictive calculations of dust dynamics.

Evidence of near-SOL tungsten accumulation using a far-SOL collector probe array and OEDGE modelling in the DIII-D metal rings L-mode discharges

Abstract Experimental evidence is presented of near-scrape off layer (SOL) tungsten accumulation near the crown of lower single-null L-mode discharges in the DIII-D Metal Rings Campaign, based on a peripheral-SOL collector probe (CP) array and OEDGE modelling. Such accumulation has been long-theorized due to parallel force balance in the SOL dominated by the ion temperature gradient force [ 1 , 2 ] but direct experimental evidence has been lacking. Impurity accumulation at this location is undesirable since it largely sets the boundary condition for impurity levels in the confined plasma. Toroidally symmetric rings of 5 cm wide tungsten-coated tiles were installed in the outer divertor of DIII-D. A CP array having multiple-diameter, dual-facing collector rods with axes in the radial direction, was inserted into the peripheral-SOL near the outer midplane to measure the plasma W content. In many cases, more W was deposited on the largest rod on the side facing toward the inner target along the field lines, despite the location of the W source being at the outer divertor strike point. Interpretation of the W deposition profiles on the CP rods support the long-theorized impurity accumulation hypothesis; however, more definitive conclusions will require further experimentation and modelling effort.

Measurement and modeling of aluminum sputtering and ionization in the DIII-D divertor including magnetic pre-sheath effects

We present analysis and modeling of Al sputtering and ionization in attached, low-power L-mode plasmas near the outer divertor strike point of the DIII-D tokamak. Al serves as a useful proxy for Be, the low-Z main wall material for ITER and JET that will undergo significant divertor plasma contact upon migrating from the first wall to the divertor. Al is easily distinguishable from background sources in DIII-D (namely C and B), has a high physical sputtering yield similar to Be, and has a long ionization mean free path compared to its gyro radius (). Using neutral Al emission imaging techniques, we measured a toroidal and radial asymmetry in the shape of the photo-emission plumes of sputtered neutral Al that was consistent with previously observed asymmetry in the distribution of redeposited Al in these experiments. We propose that the main cause of the emission and redeposition asymmetry is due to a sputtering anisotropy caused by near-grazing angle incident ions. The observed emission asymmetry was reproduced using a simple emission/ionization model that included full angular distributions of sputtering yield and energy calculated by SDTRIM.SP, but not when symmetric, mono-energetic cosine sputtering distributions were assumed. We used an ion orbit tracking model to calculate the distributions of ion impact energies through the potential gradient in the magnetic pre-sheath and Debye sheath. We found that with the magnetic field pitch angle (1.5°–2° with respect to the surface plane), the majority of ions strike the surface at <15° with respect to the surface plane, leading to angular sputtering yield and energy distributions with significant forward-scattering bias. We also observed surface microstructure consistent with directional sputtering and ion flux shadowing expected from the calculated ion incidence angles.

Characterizing Low-Z erosion and deposition in the DIII-D divertor using aluminum

We present measurements and modeling of aluminum erosion and redeposition experiments in separate helium and deuterium low power, low density L-mode plasmas at the outer divertor strike point of DIII-D to provide a low-Z material benchmark dataset for tokamak erosion-deposition modeling codes. Coatings of Al ∼100nm thick were applied to ideal (smooth) and realistic (rough) surfaces and exposed to repeat plasma discharges using the DiMES probe. Redeposition in all cases was primarily in the downstream toroidal field direction, evident from both in-situ spectroscopic and post-mortem non-spectroscopic measurements. The gross Al erosion yield was estimated from film thickness change measurements of small area samples, and was found to be ∼40–70% of the expected erosion yield based on theoretical physical sputtering yields after including sputtering by a 1–3% carbon impurity. The multi-step redeposition and re-erosion process, and hence the measured net erosion yield and material migration patterns, were found to be influenced by the surface roughness and/or porosity. A time-dependent model of material migration accounting for deposit accumulation in hidden areas was developed to reproduce the measurements in these experiments and determine a redeposition probability distribution function for sputtered atoms.

Post mortem analysis of a tungsten coated tile from the outer divertor strike point region of ASDEX upgrade

In the present study, the structure and the composition of co-deposited layers developed at the outer divertor strike point tile 1 in ASDEX Upgrade during three campaigns from 2009 to 2013 were examined. The samples were cut from representative locations which differed in received flux: private flux, strike point, the highest plasma fluence and “moderate" flux regions. High resolution scanning (SEM) and transmission electron microscopy (HRSTEM) combined with energy-dispersive X-ray spectroscopy (EDS) and optical profilometry have been used to identify deposits composition and morphology as well as to assess coating degradation. The aim of the post-mortem analyses was the evaluation of materials mixing and evidence of plasma-induced damage.Our results confirm that the outer divertor of AUG is generally a net erosion region for tungsten, however, the strong deposition of eroded tungsten takes place close to the outer strike point (private flux region). The observations on cross-sections of the tungsten coatings revealed micro-cracks around the strike point region. These cracks are caused by thermal stresses due to periodic heating and cooling of the layer during ELMs.

Local effects of biased electrodes in the divertor of NSTX

The goal of this paper is to characterize the effects of small non-axisymmetric divertor plate electrodes on the local scrape-off layer plasma. Four small rectangular electrodes were installed into the outer divertor plates of NSTX. When the electrodes were located near the outer divertor strike point and biased positively, there was an increase in the nearby probe currents and probe potentials and an increase in the Li I light emission at the large major radius end of these electrodes. When an electrode located farther outward from the outer divertor strike point was biased positively, there was sometimes a significant decrease in the Li I light emission at the small major radius end of this electrode, but there were no clear effects on the nearby probes. No non-local effects were observed with the biasing of these electrodes.

High resolution scanning transmission electron microscopy (HR STEM) analysis of re-deposited layer on ASDEX Upgrade tile

Erosion and re-deposition of plasma-facing components (PFCs) is one of the most important issues in fusion devices and as such it is an area of interest for many research groups. However, the structure and composition of re-deposited layers as well as the mechanism and condition of their formation are not yet fully described and understood. In the present study, the structure and the composition of co-deposited layers, which developed at the outer divertor strike point tiles in ASDEX Upgrade during the 2009 campaign were examined. High resolution scanning transmission electron microscopy (HRSTEM) combined with energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) have been used to identify deposits composition and morphology. Tungsten foam like structure and co-deposits rich in tungsten, oxygen, carbon, boron and nitrogen were observed.

Carbon balance and deuterium inventory from a carbon dominated to a full tungsten ASDEX Upgrade

The evolution of carbon/boron deposition and the deuterium inventory were determined during the transition from a carbon dominated to a full tungsten ASDEX Upgrade. In the carbon dominated machine about 17 g of carbon were deposited at the inner divertor and in remote areas during one standard discharge campaign. Main carbon sources were the ICRH antennae protection limiters in the main chamber. After coating these limiters with tungsten the carbon deposition decreased to 3–5 g. The remaining carbon originated mainly from erosion at the outer divertor strike point. Transition to a full tungsten machine resulted in a further decrease of the carbon deposition to about 1 g. 1.3–1.7 g deuterium was trapped in codeposited carbon/boron layers in the divertor and in remote areas during the carbon dominated campaigns. The deuterium inventory decreased to 0.16–0.24 g in the full tungsten machine.

Carbon erosion and deposition on the ASDEX Upgrade divertor tiles

Carbon deposition and erosion were measured on ASDEX Upgrade divertor tiles and below the roof baffle during the operation period 2002/2003. The inner divertor is a net carbon deposition area, while a large fraction of the outer divertor is erosion dominated and the roof baffle tiles show a complicated distribution of erosion and deposition areas. In total, 43.7g B+C were redeposited, of which 88% were deposited on tiles and 9% in remote areas (below roof baffle, on vessel wall structures). 0.6g C was pumped out as volatile hydrocarbon molecules. Carbon sources in the main chamber are too low by a factor of more than ten to explain the observed carbon divertor deposition. Carbon erosion is observed at the outer divertor strike point tiles, but it is arguable if material can be transported from the outer strike point to the inner divertor.

Similarities in divertor erosion/redeposition and deuterium retention patterns between the tokamaks ASDEX Upgrade, DIII-D and JET

The similar poloidal patterns of divertor erosion andredeposition of the ASDEX Upgrade, DIII-D andJET tokamaks are examined by comparing hydrogen isotope retention at the surfacesof divertor tiles. The outer divertor strike point undergoes net erosion.The inner divertor is a region of net redeposition, with deuteriumco-deposition rates ∼20 μg m2 s-1 for all three devices; despite their differencesin geometry, size and plasma facing materials.This extrapolates to an unacceptably high tritium retention rate ina DT burning steady state tokamak.

Molybdenum erosion measurements in Alcator C-Mod

Erosion of molybdenum was measured on a set of 21 tiles after a run campaign of 1090 shots in the Alcator C-Mod tokamak. The net erosion of molybdenum was determined from changes in the depth of a thin chromium marker layer measured by Rutherford backscattering. Net Mo erosion was found to be approximately 150 nm near the outer divertor strikepoint and much less everywhere else. Gross erosion rates by sputtering were estimated using ion energies and fluxes obtained from Langmuir probe measurements of edge-plasma conditions. Predicted net erosion using calculated gross erosion with prompt redeposition agrees with measured net erosion within a factor of three. Sputtering by impurities, mainly boron, dominates erosion.

A Thomson scattering diagnostic for measurements in the divertor region of TCV

For future divertor studies on TCV measurements of electron temperature and density and their spatial gradients along open flux surfaces will be of major interest. A Thomson scattering system is proposed for this purpose, which takes advantage of the good access via horizontal ports on three vertical levels. The modular design of the existing Thomson scattering diagnostic on TCV offers the possibility to modify it for simultaneous measurements in the main plasma and in the divertor region using the same laser. Arranging for a second passage of the laser beam at a radial position shifted towards the inside, measurements can be made along the open flux surfaces between the X point and the outer divertor strike point. The optical system is designed to share the camera lens with the existing system. After installation of additional fiber bundles and another set of filter spectrometers, the system will be capable of measuring temperature and density at ten selected locations in the divertor zone with a resolution in the vertical direction of 15 mm. Using numerical simulations, a set of interference filters has been selected to cover the range from 100 to 5 eV.

Evidence for volume recombination in JET detached divertor plasmas

Evidence for volume recombination in the JET divertor has been obtained from the population ratios of the excited states of neutral hydrogen. The ratio of the Balmer series lines Dγ/Dα has been used to identify recombination at both the inner and the outer divertor target. At the outer target the onset of recombination is well correlated with the plasma detachment as observed with Langmuir probes in the divertor target. While Te obtained from the target probe characteristics falls to ∼2 eV, Te from the population distribution of the excited states is 0.8±0.1 eV. The peak plasma density viewing across the outer divertor leg just above the target exceeds 2.5 × 1020 m-3. When the plasma becomes partially detached from the outer target, the radial distribution of recombination shows a maximum close to the outer divertor strike point.