J.D. Elder

Erosion/deposition issues at JET

Deposition and H-isotope retention in JET is highly asymmetric, with deposition predominantly in the inner divertor, where flaking deposits form on water-cooled louvres shadowed from the plasma. The asymmetry implies drift in the SOL of JET from outboard to inboard under normal operating conditions, which has been measured. In order to model the amounts of deposition, assumptions have to be made about the transport at the inner target. Analysis of divertor tiles shows that material from the main chamber travels along the SOL to the inner divertor wall, from where carbon is preferentially removed leaving a beryllium-rich film. The carbon travels to shadowed areas (such as the louvres) where deposits with high H-isotope content accrue. The analysis indicates that chemical processes must be important.

Calculation of observable quantities using a divertor impurity interpretive code, DIVIMP

Our picture of divertor edge plasma behaviour is largely theoretical with little experimental confirmation. Questions exist with regard to impurity production mechanisms, sputtering yields, plasma ion temperatures, parallel temperature gradients, the existence of plasma flow reversal, cross-field diffusion coefficients, etc. Spatial distributions of impurity ions of successive charge states in the edge, when interpreted with an impurity production/transport code, provide a basis for discriminating amongst the various theoretical possibilities. In this paper, for an example case, a wide range of models/assumptions about the divertor edge plasma behaviour is examined with regard to the effect on the spectroscopic intensities of C + through C 5+ in the vicinity of the divertor targets. It is shown that a single spectroscopic viewing location can be sufficient to discriminate amongst most of the models/assumptions made about the divertor plasma behaviour. More refined discrimination would require additional viewing locations and diagnostics, such as Doppler temperature measurements.

Interpretive modeling of simple-as-possible-plasma discharges on DIII-D using the OEDGE code

Abstract Recently a number of major, unanticipated effects have been reported in tokamak edge research raising the question of whether we understand the controlling physics of the edge. This report is on the first part – here focused on the outer divertor – of a systematic study of the simplest possible edge plasma – no ELMs, no detachment, etc. – for a set of 10 repeat, highly diagnosed, single-null, divertor discharges in DIII-D. For almost the entire, extensive data set so far evaluated, the matches of experiment and model are so close as to imply that the controlling processes at the outer divertor for these simple plasma conditions have probably been correctly identified and quantitatively characterized in the model. The principal anomaly flagged so far relates to measurements of Te near the target, potentially pointing to a deficiency in our understanding of sheath physics in the tokamak environment.

ERO code benchmarking of ITER first wall beryllium erosion/re-deposition against LIM predictions

Previous studies (Carpentier et al 2011 J. Nucl. Mater. 415 S165–S169) carried out with the LIM code of the ITER first wall (FW) on beryllium (Be) erosion, re-deposition and tritium retention by co-deposition under steady-state burning plasma conditions have shown that, depending on input plasma parameter assumptions and sputtering yields, the erosion lifetime and fuel retention on some parts of the FW can be a serious concern. The importance of the issue is such that a benchmark of this previous work is sought and has been provided by the ERO code (Pitts et al 2011 J. Nucl. Mater. 415 S957–S964) simulations described in this paper. Provided that inputs to the codes are carefully matched, excellent agreement is found between the erosion/deposition profiles from both codes for a given ITER-shaped FW panel. Issues regarding the difficult problem of the correct treatment of Be sputtering are discussed in relation to the simulations. The possible influence of intrinsic Be impurity is investigated.

Plasma fluxes to surfaces for an oblique magnetic field

The poloidal and toroidal spatial distributions of D α , He I and C II emission have been obtained in the vicinity of the TFTR bumper limiter and are compared with models of ion flow to the surface. The distributions are found not to agree with a model (the “cosine” model) which determines the incident flux density using only the parallel fluxes in the scrape-off layer and the projected area of the surface perpendicular to the field lines. In particular, the cosine model is not able to explain the significant fluxes observed at locations on the surface which are oblique to the magnetic field. It is further shown that these fluxes cannot be explained by the finite Larmor radii of impinging ions. Finally, it is demonstrated, with the use of Monte Carlo codes, that the distributions can be explained by including both parallel and cross-field transport onto the limiter surface.

Scrape-off layer transport and deposition studies in DIII-D

Trace {sup 13}CH{sub 4} injection experiments into the main scrape-off layer of low density L-mode and high-density H-mode plasmas have been performed in the DIII-D tokamak [Luxon{_}NF02] to mimic the transport and deposition of carbon arising from a main chamber sputtering source. These experiments indicated entrainment of the injected carbon in plasma flow in the main SOL, and transport toward the inner divertor. Ex-situ surface analysis showed enhanced {sup 13}C surface concentration at the corner formed by the divertor floor and the angled target plate of the inner divertor in L-mode; in H-mode, both at the corner and along the surface bounding the private flux region inboard of the outer strike point. Interpretative modeling was made consistent with these experimental results by imposing a parallel carbon ion flow in the main SOL toward the inner target, and a radial pinch toward the separatrix. Predictive modeling carried out to better understand the underlying plasma transport processes suggests that the deuterium flow in the main SOL is related to the degree of detachment of the inner divertor leg. These simulations show that carbon ions are entrained with the deuteron flow in the main SOL via frictional coupling, but higher charge state carbon ions may be suspended upstream of the inner divertor X-point region due to balance of the friction force and the ion temperature gradient.

Erosion/redeposition at the JET MkI divertor

Abstract The DIVIMP/NIMBUS codes are employed to simulate divertor carbon profiles and the net erosion measured by colorimetry using the most recent chemical sputtering data and models. The relative roles of physical and chemical sputtering are assessed in terms of erosion and impurity contamination. Beryllium photon fluxes are also modelled, showing much less sputtering than predicted by the pure beryllium sputtering data. In addition, the erosion of fixed divertor target Langmuir probe tips has been measured photographically and gives an effective sputtering yield of

Analysis of SOL behaviour in JET MkIIGB using an advanced onion-skin solver (OSM2)

Abstract Interpretative modelling of the JET SOL plasma using the onion-skin method (OSM) has provided valuable insights into relationships between edge plasma parameters and information about the anomalous cross-field energy transport coefficients. The method relies on solving the plasma transport equations along individual flux surfaces, and adjusting the cross-field source terms to best match the Langmuir target probe profiles. In this paper, an improved solver (OSM2) is described and three sets of results are presented: code–code comparison with EDGE2D, code–experiment comparison with reciprocating probe (RCP) data on JET MkIIGB, and detailed simulation of a 12 MW ELMy H-mode on MkIIGB.

A CFD onion-skin model for the interpretation of edge experiments

Abstract Earlier onion-skin modelling, O-SM, of divertor edge plasmas, which uses experimental target measurements of I sat and T e as boundary conditions, has shown good agreement with standard 2-D code solutions, except for detached plasma conditions. The inability to produce stable, detached solutions motivated the development of an improved O-SM solver using standard CFD techniques, which overcame earlier stability problems. A smooth transition between attached and detached regimes has been observed by reducing target T e with fixed I sat . In the case of strong detachment, typically with T e below 1 eV, it is found that upstream quantities are highly sensitive on the input target conditions. The solutions show the characteristic detachment profiles both along and across the magnetic field.

Simulation of gross and net erosion of high-Z materials in the DIII-D divertor

The three-dimensional Monte Carlo code ERO has been used to simulate dedicated DIII-D experiments in which Mo and W samples with different sizes were exposed to controlled and well-diagnosed divertor plasma conditions to measure the gross and net erosion rates. Experimentally, the net erosion rate is significantly reduced due to the high local redeposition probability of eroded high-Z materials, which according to the modelling is mainly controlled by the electric field and plasma density within the Chodura sheath. Similar redeposition ratios were obtained from ERO modelling with three different sheath models for small angles between the magnetic field and the material surface, mainly because of their similar mean ionization lengths. The modelled redeposition ratios are close to the measured value. Decreasing the potential drop across the sheath can suppress both gross and net erosion because sputtering yield is decreased due to lower incident energy while the redeposition ratio is not reduced owing to the higher electron density in the Chodura sheath. Taking into account material mixing in the ERO surface model, the net erosion rate of high-Z materials is shown to be strongly dependent on the carbon impurity concentration in the background plasma; higher carbon concentration can suppress net erosion. The principal experimental results such as net erosion rate and profile and redeposition ratio are well reproduced by the ERO simulations.