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After vertical displacement events (VDEs) in the JET-ILW tokamak, debris is produced in liquid form and ejected from the vessels beryllium upper dump plates (UDPs). Most of the droplets splash near the UDPs outboard tile 8 as documented by an in-vessel photographic survey. Moreover, the systematic collection of 'dust' from the main chamber and the divertor region provides evidence of small solid spherical Be particles and flat splats (with fairly circular, 'pancake-like', or elongated shapes) related to melting events (with a diameter of few mu m prior the deposition). The migration of metallic Be is studied with the numerical code DUSTTRACK, capable of describing the full droplet dynamics in a non-stationary tokamak plasma configuration. The description of the JET plasma during the VDE proposed here allows for a sufficiently consistent reconstruction of the transient conditions of the background plasma, relying on actual physical measurements. Due to the lack of experimental data related to the initial properties of the ejecta, the statistically relevant sample of 4x10(5) particles was considered in the DUSTTRACK simulations. A reasonable fit of the initial parameters for the ejected droplets, as well as the time-dependent plasma parameters for the VDE resulted in good qualitative agreement with the post-mortem experimental findings from dust collectors and with the Be deposition pattern near UDPs on the low-field side.

(CH4)-C-13 was injected through graphite and tungsten spherical limiters in reproducible TEXTOR discharges. These materials were chosen, as they represent the actual compromise for the plasma facing components in the ITER divertor. C-13 was used in order to distinguish injected and intrinsic carbon in the layer deposited on the limiter surface. Shot-by-shot video recordings show a continuous growth of the deposit near the injection hole. A pronounced difference in the C-13 deposition pattern on the graphite and tungsten limiters was observed. Post-mortem surface analysis showed that the ratios of the locally deposited to the injected amount of carbon are 4% for graphite and 0.3% for tungsten. The margins of the carbon layer deposited on tungsten are significantly steeper in comparison with the graphite limiter case. The large difference in the C-13 deposition efficiency can be explained by direct reflection of carbon from tungsten and the enhanced sputtering of carbon on the tungsten substrate. Nucleation is suggested to be an important mechanism for carbon deposition on tungsten. Monte Carlo impurity transport calculations by the ERO code reproduce reasonably the experimental results for the graphite limiter.

Transport of hydrocarbon impurities in a high-density (>10 20 m−3), low-temperature (

A Monte-Carlo code called SURO was developed to study the influence of surface roughness on the impurity deposition characteristics in fusion experiments. SURO uses the test particle approach to describe the impact of background plasma and the deposition of impurity particles on a sinusoidal surface. The local impact angle and dynamic change of surface roughness as well as surface concentrations of different species due to erosion and deposition are taken into account. Coupled with the three-dimensional Monte-Carlo code ERO, SURO was used to study the impact of surface roughness on 13C deposition in 13CH4 injection experiments in TEXTOR. The simulations showed that the amount of net deposited 13C species increased with surface roughness. Parameter studies with varying 12C and 13C fluxes were performed to gain insights into impurity deposition characteristics on the rough surface. Calculations of the exposure time needed for surface smoothing for TEXTOR and ITER were also carried out for different scenarios.

The Monte-Carlo neutral transport code 3D-GAPS is described. The code models impurity transport and deposition in remote areas, such as gaps between cells of castellated plasma-facing surfaces. A step-by-step investigation of the interplay of different processes that may influence the deposition inside gaps, namely particle reflection, elastic neutral collisions, different particle sources, chemical erosion and plasma penetration into gaps, is presented. Examples of modelling results in application to the TEXTOR experiment with a castellated test limiter are provided. It is shown that only with the assumption of the presence of species with different reflection probabilities, do simulated carbon deposition profiles agree with experimental observations for side surfaces of the gaps. These species can be attributed to different particle sources, e. g. carbon atoms and hydrocarbon radicals. Background carbon ions and atoms have low and moderate values of the reflection coefficient (R = 0.9). Deposition at the bottom of the gaps cannot be adequately reproduced unless extreme assumptions on particle sources and reflection properties are imposed. Elastic neutral collisions and ionization of neutrals escaping the gaps have no significant influence on the results. Nevertheless, particle-in-cell simulations of plasma penetration into gaps are essential for estimating the incoming ion flux and leading to a better quantitative agreement with experimental observations.

The 3D Monte Carlo code ERO has been used to simulate dedicated TEXTOR experiments in which methane 13CH4 and ethene 13C2H4 were injected into the plasma through graphite and tungsten spherical limiters to investigate local carbon transport and deposition. Spectroscopy was used to follow the observable hydrocarbon break-up products, and the local 13C deposition efficiency was measured by post-mortem surface analysis. The experimental observations showing the dependence of the 13C deposition on substrate and gas have been modelled. The main uncertain input parameters for the modelling, as 12C concentration in the background plasma, effective sticking coefficient for hydrocarbons and enhanced erosion of redeposited carbon, have been defined by benchmarking between modelling and experiment. With the same set of parameters, the modelled 13C deposition efficiencies are in good agreement with the experimental ones (0.8–2.1%). Also, the observed distribution of 13C deposition on the limiter surfaces is well reproduced by modelling. The higher amount of 13C deposition on graphite compared with tungsten limiter is mainly due to the higher reflection of carbon on tungsten and the higher physical sputtering of carbon film on top of the tungsten compared with graphite. The larger amount of 13C deposition for 13C2H4 instead of 13CH4 injection can be explained by more 13C returning to the limiter surface from 13C2H4 injection than 13CH4 injection, which is mainly determined by the different masses and reaction rate coefficients.

Reaction rate coefficients for higher hydrocarbon molecules (ethane family) have been implemented into the 3D Monte Carlo code ERO, which is used for modelling of hydrocarbon transport and light emission of spectroscopically observable radicals in the edge plasma of fusion experiments. For benchmarking of the ERO modelling, dedicated experiments were carried out at TEXTOR in which different hydrocarbon molecules (CD4, C2D4 and C2D6) were injected into the plasma near the last closed flux surface. The light emission of the hydrocarbon break-up products CD, C2 and C+ was observed. The modelled penetration depths of the light emission from the CD Ger?-band, C2 Swan band and C?II are in good agreement with the experiments. The effective inverse photon efficiencies (D/XB values) for CD and C2, which include the dissociation chains of the initial molecules, are more dependent on local electron temperature than on electron density. The calculated D/XB values are in fair agreement with the experimental ones.

The 3D Monte-Carlo code ERO, which calculates erosion processes, impurity transport and deposition, has been coupled to the Monte-Carlo code SDTrimSP to simulate material mixing processes in wall components more precisely. SDTrimSP calculates the transport of ions in solids by means of the binary collision approximation. It keeps track of the depth dependent material concentration caused by implantation of projectiles in the solid. Modelling with the coupled code ERO-SDTrimSP is compared with dedicated TEXTOR experiments, in which the formation of mixed surface layers has been studied. In these experiments, methane 13CH4 was injected through graphite and tungsten spherical limiters during plasma exposure and the local redeposition probability was measured post mortem by surface analysis. A significant difference in the carbon 13C deposition efficiency, i.e. the ratio of the locally deposited to the injected amount of 13C, between graphite and tungsten was found, 4% for graphite and 0.3% for tungsten. Modelling of these experiments with ERO-SDTrimSP reproduces the clear substrate dependence with about 2% deposition efficiency on graphite and less than 0.5% on tungsten in good agreement with the experiment. The reason for the substrate dependence is partly explained by the higher physical sputtering yield of a thin carbon film on top of a tungsten substrate compared with a graphite substrate. Surface roughness of the materials has been identified to be another important parameter for the interpretation of the results.