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Sputtering by ions with low near-threshold energies is investigated. Experiments and simulations are conducted for tungsten sputtering by low-energy, 85-200 eV Ar atoms. The angular distributions of sputtered particles are measured. A new method for molecular dynamics simulation of sputtering taking into account random crystallographic surface orientation is developed, and applied for the case under consideration. The simulations approximate experimental results well. At low energies the distributions acquire "butterfly-like" shape with lower sputtering yields for close to normal angles comparing to the cosine distribution. The energy distributions of sputtered particles were simulated. The Thompson distribution remains valid down to near-threshold 85 eV case. (C) 2017 Elsevier B. V. All rights reserved.

The beryllium erosion by helium plasma irradiation is studied at the PISCES-B linear plasma device and interpreted using the accompanying simulations by the ERO code. The influence of plasma conditions and varying negative biasing of the Be plasma target on BeI and BeII absolute line intensities are reproduced in detail by the simulations. The synthetic axial line intensity shapes and line ratios match with experiment. This indicates that atomic data are quite accurate. The initial population state of quasi-metastable 3P level in BeI is found to be MS:GS= 0.33:1 for all conditions. The yields determined by the modeling interpretation are compared to the SDTrimSP code simulations in the binary collision approximation. Keywords: Beryllium, Erosion, Helium, ERO code, PISCES-B, Spectroscopy

International audience; We present the observation of peculiar non-monotonic photon energy dependencies of the count rates and of the rocking curves of parametric down-conversion of x rays into ultraviolet far from any atomic resonances. The observations cannot be explained by models that consider only atomic or bond charge responses and suggest that collective phenomena contribute to the effect. We propose an interpretation that includes nonlinear interactions with plasmons, which can explain the existence of peaks in this energy range. Our work implies that nonlinear interactions between x rays and either ultraviolet or visible radiation can be utilized as a powerful atomic scale probe for collective effects in solids.

Proceedings of the 22nd International Conference on Plasma Surface Interactions 2016, 22nd PSI Series of experiments on tungsten (W) erosion and transport in Argon (Ar) plasma were conducted at the linear plasma device PSI-2. W erosion was measured with three independent methods: WΙ spectroscopy, mass loss and quartz micro-balance (QMB) deposition sensor. Consistent set of data produced in these experiments was interpreted using the 3D ERO code simulations, which have reproduced all the main trends observed. Influence of the physical model assumptions (e.g. energy and angular distributions of sputtered particles) was demonstrated. The effect of WΙ effective quasi-metastable (MS) state population dynamics on spectroscopy measurements is shown; the characteristic relaxation time is determined. The measured physical sputtering yields for W are close to the simulated data obtained in the binary collision approximation (BCA) approach (SDTrimSP code). The remaining discrepancies between simulations and the experiment, mostly in spectroscopy, are accounted to the uncertainties in the plasma parameters and atomic data.

We present experimental data on the rate coefficient for electron impact ionisation of W atoms at electron temperatures between 5 and 10 eV. Our results undercut different theoretical data sets based on the Born approximation and the distorted wave approximation, respectively, by more than a factor of 3. Better agreement in absolute values is found with a more recent work by Goswami et al. The present data match well with earlier measurements by Steinbrink obtained with a different method.

Beryllium (Be) erosion has been studied in dedicated limiter discharges in JET with the recently installed ITER-like wall [1] . Passive spectroscopy [2] in the vicinity of the solid Be limiter is used for measurement of the Be physical sputtering as the main erosion mechanism. To consider the 3D configuration of plasma parameters and the electromagnetic field, the actual limiter shape and the local transport affecting the fraction of Be coming into the observation volume, a detailed modelling with the ERO code has been applied to interpret the experimental data. The observed dependence of BeI and BeII line intensities on plasma parameters during the density scan and various line ratios are used to validate the model and the underlying data including the recently introduced assumptions for Be physical sputtering, the very same which were used before for ITER predictive modelling [3] .

To quantify tungsten (W) sputtering measurements by spectroscopy, two experiments have been carried out in the tokamak TEXTOR. The erosion of a W limiter in the plasma edge was studied spectrocopically under different plasma conditions. Conversion of the photon fluxes of neutral W lines was performed [1] with the aid of inverse photon efficiencies, measured in situ in a second experiment with WF6 injection to realise a controllable W source. Penetration depths, particle velocities and line ratios of W I were determined and compared between the two sources of W. The velocities of injected and sputtered W differ by a factor of 3 as the analysis of the penetration depth shows, while the line ratios for different W I lines are comparable. A comparison of the e-folding length with the model code GKU was performed which reveals a deviation of at least a factor of 6. Inverse photon efficiencies at Te = 40 eV were determined to be about 44 for W I (400.88 nm) and about 63 for W I (429.46 nm).

Local beryllium (Be) I and Be II line intensities were measured in the plasma–wall interaction region near an ICRH antenna in JET. The intent was to use these intensities as a measure of the formation of local Radio Frequency (RF) sheath potentials, through RF sheath rectification and potential build up at the end of field lines passing in front of the antenna. Experimentally, it was found that the Be I and Be II emission increase when using the antenna local to the spectroscopic measurement, and increase even more when using a remote antenna that is magnetically connected to the observation point. Magnetic field mapping indicates a magnetic connection between the observation location and the top corner region of the remote antenna and/or its protection limiter. These measurements can be used in support of RF sheath modeling that is an important part of the optimization of antenna design for next generation fusion energy devices, including ITER.

On the TEXTOR tokamak various experiments aimed at investigation of tungsten erosion and transport are performed. In one experiment a spherical W/C twin limiter positioned close to the last-closed flux surface in the near scrape-off layer was exposed to a number of comparable plasma discharges with stepwise variations of edge plasma parameters. Spatial distribution of tungsten and carbon light emission was recorded with two dimensional CCD cameras and spectrometer systems with high spectral and spatial resolution. Penetration depths, tungsten sputtering fluxes and erosion yields were measured. Comparison between experimental data and the results of modelling with the 3D Monte-Carlo code ERO is performed. The main objective of this study was to test the adequacy of the existing atomic data for neutral tungsten. The modelled penetration depths of the light emission of tungsten are a factor of 2–3 smaller than in experiment, which may indicate the overestimation of ionization rates.

Hydrocarbon injection experiments on molybdenum targets facing high-density plasmas in Pilot-PSI were simulated with the 3D Monte Carlo impurity transport and PSI code ERO. Impurity transport and calculation of redeposition profiles were decoupled by calculating carbon redistribution matrices with ERO. Redeposition was found to be strongly dependent on the electron density: greater densities result in a much more localized redeposition pattern. The calculated average number of recycling events of hydrocarbon molecules on the surface went up from 1.5 for n e = 5 × 10 19 m - 3 to 19.2 for n e = 4 × 10 20 m - 3 ; at the latter density, only 2.4% of the hydrocarbon molecules escapes the simulated plasma beam without returning to the target at least once. Agreement with experimental deposition profiles in argon was fair. The results in hydrogen point towards a strong gradient in chemical erosion yield along the target.

We describe an experiment demonstrating the possibility to generate collinear indistinguishable x-ray photon pairs with a highly suppressed background. By choosing angles near 90 degrees between the detectors and the pump beam, where the pump polarization is in the scattering plane, we improve the signal-to-noise ratio by 2 orders of magnitude. We measure the pure coincidence spectrum without any energy discrimination with a bandwidth of 1.5 keV at the full width at half maximum.

During high-power, ion cyclotron resonance heating (ICRH), RF sheath rectification and RF induced plasma-wall interactions (RF-PWI) can potentially limit long-pulse operation. With toroidally-spaced ICRH antennas, in an ITER-like wall (ILW) environment, JET provides an ideal environment for ITER-relevant, RF-PWI studies. JET pulses combining sequential toggling of the antennas with q95 (edge safety factor) sweeping were recently used to localize RF-enhanced Be I and Be II spectral line emission at outboard poloidal (beryllium) limiters. These measurements were carried out in the early stages of JET-ILW and in ICRF-only, L-mode discharges. The appearance of enhanced emission spots was explained by their magnetic connection to regions of ICRH antennas associated with higher RF-sheath rectification [1]. The measured emission lines were the same as those already qualified in ERO modelling of inboard limiter beryllium erosion in JET limiter plasmas [2]. In the present work, we revisit this spectroscopic study with the focus on obtaining estimates of the impact of these RF-PWI on sputtering and on net erosion of the affected limiter regions. To do this, the ERO erosion and re-deposition code [2] is deployed with the detailed geometry of a JET outboard limiter. The effect of RF-PWI on sputtering is represented by varying themore » surface negative biasing, which affects the incidence energy and the resulting sputtering yield. The observed variations in line emission, from [1], for JET pulse 81173 of about factor 3 can be reproduced with ~ 100 200 V bias. ERO simulations show that the influence of the respective E-field on the local Be transport is localized near the surface and relatively small. Still, the distribution of the 3D plasma parameters, shadowing and other geometrical effects are quite important. The plasma parameter simulated by Edge2D-EIRENE [3] are extrapolated towards the surface and mapped in 3D. These initial modelling results are consistent with the range of potentials anticipated through RF sheath rectification (see, e.g., [4]). Shortcomings from both the modelling and experimental side will be discussed, as will be plans for improvements in both areas method for the upcoming 2015 - 2016 JET campaign. [1] C.C. Klepper et al., J. Nucl. Mater. 438 (2013) S594 S598 [2] D. Borodin et al., Phys. Scr. T159 (2014) 014057 [3] M. Groth et al., Nucl. Fusion 53 (2013) 093016 [4] Jonathan Jacquot et al., Phys. Plasmas 21 (2014) 061509 *Corresponding author: presently at CCFE (UK) tel.: +44 1235 46 4304, e-mail: kleppercc@ornl.gov **See the Appendix of F. Romanelli et al., Proc. of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia Work supported, in part, by US DOE under Contract DE-AC05-00OR22725 with UT-Battelle, LLC.« less

Use of new 3-strap ICRF antennas with all-tungsten (W) limiters in ASDEX Upgrade results in a reduction of the W sources at the antenna limiters and of the W content in the confined plasma by at least a factor of 2 compared to the W-limiter 2-strap antennas used in the past. The reduction is observed with a broad range of plasma shapes. In multiple locations of antenna frame, the limiter W source has a minimum when RF image currents are decreased by cancellation of the RF current contributions of the central and the outer straps. In JET with ITER-like wall, ITER-like antenna produces about 20% less of main chamber radiation and of W content compared to the old A2 antennas. However the effect of the A2 antennas on W content is scattered depending on which antennas are powered. Experiments in JET with trace nitrogen (N-2) injection show that a presence of active ICRF antenna close to the midplane injection valve has little effect on the core N content, both in dipole and in -90 degrees phasing. This indicates that the effect of ICRF on impurity transport across the scape-off-layer is small in JET compared to the dominant effect on impurity sources leading to increased impurity levels during ICRF operation. (C) 2016 The Authors. Published by Elsevier Ltd. For complete list of authors see http://dx.doi.org/10.1016/j.nme.2016.10.026

At FOM Rijnhuizen, linear plasma generators are used to investigate plasma-material interactions under high-density (≤1021 m -3), low-temperature (≤5 eV) plasma bombardment. Research into carbon-based materials has been focused on chemical erosion by hydrogen plasmas. Results from plasma exposure to high-flux (>1023 H +/m2 s) and low-temperature hydrogen plasma indicate silicon carbide has a lower relative rate of gross erosion than other carbon-based materials (e.g. graphite, diamond, carbon-fiber composites) by a factor of 7-10. Hydrogenic retention is the focus of research on tungsten and molybdenum. For target temperatures of 700-1600 K, the temperature dependence of hydrogenic retention is the dominant factor. Damage to the surface by heavy ion irradiation has shown to enhance retention by a factor of 2.5-4.1. Thermal stressing of W via. e-beam thermal cycling also enhances hydrogenic retention by a factor of 2.1 ± 0.2, likely due to the introduction of thermal defects, which act as trapping sites for implanted hydrogenic isotopes.

The mitigation of carbon chemical erosion observed in PISCES-B due to the presence of a minor beryllium concentration in plasma can be determined to a large extent by formation of carbide (Be2C) on the plasma-facing surface. To study this, dedicated beryllium seeding experiments in the linear plasma simulator PISCES-B have been simulated using the Monte-Carlo code ERO. The spectroscopy is used to benchmark the simulations against observations. A temporal dependent evolution of the beryllium carbide formation on the carbon surface of the target is introduced on the basis of characteristic time scales. The influence of carbide formation on the effective mitigation time for chemical erosion is investigated.

A one-dimensional fluid model, which calculates the modification of density and temperature along the magnetic field and the parallel electrical field in the presence of local impurity sources, has been implemented into the ERO code. The influence of impurity source strength on the local plasma parameters and resulting changes in impurity transport and deposition has been studied. Dedicated TEXTOR experiments of 13 CH 4 injection through roof-like test limiters are modelled for comparison. Modelling with high injection rates (larger than about 4 × 10 18 s −1 ) results in too localized light emission pattern near to the injection hole, and therefore indicates an underestimation of the reduction of electron temperature in the model. Besides, the preliminary results indicate that possible modification of local plasma conditions cannot significantly reduce the modelled 13 C deposition and therefore cannot explain the measured low 13 C deposition efficiency.

A molecular dynamics technique was used to study the sputtering characteristics of co-deposited carbon (C) and to investigate subsequent effects of introducing beryllium (Be) impurities. Amorphous C layers containing hydrogen (H) with atomic ratios of H/(C+H)

In order to investigate the redeposition characteristics of hydrocarbons released by chemical sputtering, a Monte Carlo simulation of the transport in edge plasmas is performed, where the reflection and dissociation efficiencies at tungsten (W) and carbon (C) surfaces are calculated using molecular dynamics simulation. The redeposition probability for W and W-C mixed material is strongly suppressed due to strong break-up of hydrocarbons on the surface. Sticking coefficient for ion species increases with increasing plasma temperature, whereas for low plasma temperature (

Impurity transport within the inner JET divertor has been modelled with ERO to estimate the transport to and the resulting deposition at remote areas. Various parametric studies involving divertor plasma conditions and strike point position have been performed. In JET-ILW (beryllium main chamber and tungsten divertor) beryllium, flowing from the main chamber into the divertor and then effectively reflected at the tungsten divertor tiles, is transported to remote areas. The tungsten flux to remote areas in L-Mode is in comparison to the beryllium flux negligible due to small sputtering. However, tungsten is sputtered during ELMs in H-Mode conditions. Nevertheless, depending on the plasma conditions, strike point position and the location of the remote area, the maximum resulting tungsten flux to remote areas is at least ∼3 times lower than the corresponding beryllium flux. Modelled beryllium and tungsten deposition on a rotating collector probe located below tile 5 is in good agreement with measurements if the beryllium influx into the inner divertor is assumed to be in the range of 0.1% relative to the deuterium ion flux and erosion due to fast charge exchange neutrals is considered. Comparison between JET-ILW and JET-C is presented.

The close coupling (CC) equations in the impact parameter (IP) representation are considered. Calculations for transitions in the 3s–3p–3d He atom system induced by collisions with single charged ions were carried out. Results are compared with those obtained in the frame of the Born approach. The effects of stepped transitions, normalization, channel coupling and their dependence on interaction strength are discussed.

The Monte Carlo code ERO simulates the three-dimensional transport of impurity particles in a background plasma using the test particle approximation. It treats the interaction of the plasma ions, atoms and of the impurities with the plasma facing components (PFC). ERO modelling for ITER estimates the lifetime of the inner target (erosion of 0.5 cm) to about 7000 shots with a shot length of 400s. The maximum allowed tritium retention due to eroded carbon not redeposited locally on the target plates would be already reached after 380-440 shots. Strike point sweeping might increase the target life time by a factor of about 4 while tritium retention seems not to be influenced. Apart from transient events especially material mixing effects of the elements foreseen for ITER as PFC (carbon, beryllium and tungsten) introduce uncertainties in the prediction. The injection of hydrocarbon ( 13 CH 4 ) through spherical shaped limiters (one made of tungsten, one of graphite) in TEXTOR reveal that carbon has a reduced deposition efficiency on tungsten compared to graphite. This effect has not been taken into account until now in the modelling by the ERO code. The surface model of the code has therefore been substituted by a customized version of the Monte Carlo Code SDTrimSP which will help to clarify the effects of material mixing. A first successful test simulation shows that the depth distributed concentration of PFC influences sputtering yields.

(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.

The prediction of the first wall deterioration and possible plasma contamination by impurities is a high priority task for ITER. 3D Monte-Carlo code ERO is a tool for modeling of eroded impurity transport and spectroscopy in plasma devices useful for experiment interpretation. Chromium (Cr) is a fusion-relevant reactor wall element (e.g. component of RAFM steels expected for use in DEMO). Linear plasma devices including PSI-2 are effective tools for investigations of plasma-surface interaction effects, allowing continuous plasma operation and good control over irradiation parameters. Experiments on Cr sputtering were conducted at PSI-2. In these experiments the Cr erosion was measured by three techniques: mass loss of the sample, quartz micro-balance of deposited impurities at a distance from it and optical emission spectroscopy. Experiments were modeled with the 3D Monte-Carlo code ERO, previously validated by application to similar experiments with tungsten (W). The simulations are demonstrated to reproduce the main experimental outcomes proving the quality of the sputtering data used. A significant focuses of the paper is the usage and validation of atomic data (resent metastable-resolved dataset from ADAS) for interpretation of Cr spectroscopy. Initial population of quasi-metastable state was fitted by matching the modeling with the experimental line intensity profiles.

Tungsten erosion and re-deposition at the upper outer divertor of the Experimental Advanced Superconducting Tokamak has been modelled using the 3D Monte Carlo code ERO. The measured divertor plasma condition in attached L mode discharges with upper single null configuration has been used to build the background plasma in the simulations. The tungsten gross erosion rate is mainly determined by carbon impurity in the background plasma. Increasing carbon concentration can first increase and afterwards suppress the tungsten erosion rate. Taking into account the material mixing surface model, the influence of eroded particles returning to the surface on sputtering has been studied. Sputtering by eroded particles returning to the surface can significantly enhance the gross erosion by reduction of the carbon ratio within the surface interaction layer and by increasing the erosion rate due to sputtering by both eroded tungsten and carbon particles. Modelling indicates that carbon deposition occurs on the dome pla...

We describe high energy-resolution measurements of parametric down-conversion of x-rays into ultraviolet radiation using a standard laboratory x-ray tube source with very minor modifications. We measure the effect in diamond and in lithium fluoride crystals in the ultraviolet range from 30 eV to 65 eV. We show that the effect depends strongly on the fluorine LI edge at 37 eV and on the K edge of the lithium at 55 eV. The comparison with theory reveals that the classical model that was previously used for the description of the effect agrees with the experimental results in diamond. However, the discrepancies of the model with the experimental results in lithium fluoride are prominent.

13th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, Peking Univ, Fus Simulat Ctr, Beijing, PEOPLES R CHINA, SEP 17-20, 2013; International audience; The effective sputtering yield of Be (Y-Be(tot)) was determined in situ by emission spectroscopy of low ionizing Be as function of the deuteron impact energy (E-in = 25-175 eV) and Be surface temperature (T-surf = 200 degrees C-520 degrees C) in limiter discharges carried out in the JET tokamak. Be self sputtering dominates the erosion at high impact energies (E-in > 150 eV) and causes Y-Be(tot) far beyond 1. Y-Be(tot) drops to low values, below 4.5%, at the accessible lowest impact energy (E-in similar or equal to 25 eV) achievable in limiter configuration. At medium impact energies, E-in = 75 eV, two contributors to the measured Y-Be(tot) of 9% were identified:two third of the eroded Be originates from bare physical sputtering (Y-Be(phys)) and one third from chemical assisted physical sputtering (Y-Be(chem)). The later mechanism has been clearly identified by the appearance of BeD A-X emission and quantified in cause of a temperature dependence at which the BeD practically vanishes at highest observed Be limiter temperatures. The recorded T-surf dependence, obtained in a series of 34 identical discharges with ratch-up of T-surf by plasma impact and inertial cooling after the discharge, revealed that the reduction of BeD is correlated with an increase of D-2 emission. The release mechanism of deuterium in the Be interaction layer is exchanged under otherwise constant recycling flux conditions at the limiter. The reduction of Y-Be(chem) with T-surf is also correlated to the reduction of the Be content in the core plasma providing information on the total source strength and Be screening. The chemical assisted physical sputtering, always present at the nominal limiter pre-heating temperature of T-surf = 200 degrees C, is associated with an additional sputtering channel with respect to ordinary physical sputtering which is surface temperature independent. These JET experiments in limiter configuration are used to benchmark the ERO code and verify ITER first wall erosion prediction. The ERO code overestimates the observed Be sputtering in JET by a factor of about 2.5 which can be transferred to ITER predictions and prolong the expected lifetime of first wall elements.

An estimation of the contribution of gaps to beryllium deposition and resulting tritium retention in the divertor of ITER is presented. Deposition of beryllium layers in gaps of the full tungsten divertor is simulated with the 3D-GAPS code. For gaps aligned along the poloidal direction, non-shaped and shaped solutions are compared. Plasma and impurity ion fluxes from Schmid (2008 Nucl. Fusion 48 105004) are used as input. Ion penetration into gaps is considered to be geometrical along magnetic field lines. The effect of realistic ion penetration into gaps is discussed. In total, gaps in the divertor are estimated to contribute about 0.3 mgT s(-1) to the overall tritium retention dominated by toroidal gaps, which are not shaped. This amount corresponds to about 7800 ITER discharges up to the safety limit of 1 kg in-vessel tritium; excluding, however, tritium release during wall baking and retention at plasma-wetted and remote areas.

Deposition/erosion measurements by means of a Quartz Micro Balance (QMB) located below the Load Bearing Septum Replacement Plate in the private flux region of the inner divertor of JET (with full carbon wall) revealed net deposition with the inner strike point located on the vertical tile and net erosion with the inner strike point on the horizontal tile [H.G. Esser et al., J. Nucl. Mater. 390–391, 148 (2009)]. ERO calculations show about 3.5 times larger flux entering the QMB aperture when the inner strike point is located on the vertical plate compared to the case when strike point is on the horizontal plate – thus indicating similar behavior. Using these fluxes from ERO as input, detailed modeling of erosion/deposition at the QMB itself considering the realistic geometry of the QMB housing has been performed with the 3D-GAPS code. The QMB measurements can be reproduced with combined ERO/3D-GAPS modeling if erosion due to deuterium atoms within the QMB housing is taken into account.

A new simplified analytical expression for the electromagnetic field in the Debye sheath in the presence of an oblique magnetic field including surface biasing effect is suggested. It is in good agreement with the numerical solution of the integral equation for the potential distribution in the Debye sheath. The energy and angular impact distributions and corresponding surface sputtering yields were analyzed in the presence of an oblique magnetic field and surface biasing. The analytical expression was used to estimate a) the effective sputtering yield of the W target with a varying negative voltage against plasma in PSI-2 linear device and b) erosion of the JET outer wall Be limiter near the ICRH antenna enhanced during RF emission.

Methane (CH4) was injected into the high density (n(e) similar to 10(20) m(-3)), low temperature (T-e similar to 1 eV) hydrogen plasma in Pilot-PSI to determine the CH A-X photon efficiency in this unexplored plasma regime. The effects of particle transport and particle reflection on the emission of directly excited CH under these plasma conditions were assessed with the 3D Monte Carlo code ERO. The simulations of the inverse photon efficiency showed a difference of similar to 20% between full hydrocarbon sticking or no sticking (reflection). In addition it predicts that particle transport may lead to more than a factor of 10 increase. The measured inverse photon efficiency is however constant at 100 +/- 30 for 0.1 < T-e < 1.0 eV. The constancy is consistent with dissociative recombination of CH4+, CH3+ and CH2+ to produce excited CH instead of direct excitation. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.

The simulations for linear devices using the 3D Monte-Carlo code ERO have a number of particularities of both physical and technical nature. Many physical effects introduced and tested for these simulations (elastic collisions, population of metastable states, etc.) can be relevant for tokamaks as well. The current status of these activities is summarized. As an example some simulations reproducing the experimental observations at PISCES-B and Pilot-PSI are presented. Main deviations and ideas for their origins are discussed. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Castellation is foreseen for the first wall and divertor area in ITER. The concern of the fuel accumulation and impurity deposition in the gaps of castellated structures calls for dedicated studies. Recently, a tungsten castellated limiter with rectangular and roof-like shaped cells was exposed to the SOL plasmas in TEXTOR. After exposure, roughly two times less fuel was found in the gaps between the shaped cells whereas the difference in carbon deposition was less pronounced. Up to 70 at.% of tungsten was found intermixed in the deposited layers in the gaps. The metal fraction in the deposit decreases rapidly with a depth of the gap. Modeling of carbon deposition in poloidal gaps has provided a qualitative agreement with an experiment. The significant anisotropy of C and D distributions in the toroidal gaps was measured.

Measurements of Si I- and Si II-emission lines were performed in the front of a test limiter under two different conditions: (i) silicon is sputtered from a silicon layer deposited on a graphite test limiter, (ii) silane is puffed into the plasma through a gas inlet in the limiter head, setting Si-atoms free from the dissociation of SiD4 molecules. It was shown that the relative intensities of the emission lines within the multiplets are in good agreement with theoretical data calculated in the SL-coupling scheme. The values for ‘ionization per photon’ were calculated for the electron temperature range below 100 eV using the GKU kinetic code and compared with measured ones. The measured and theoretical values for the Si II-emission lines are in reasonable agreement within a deviation of 30%. It was found that experimentally measured values for Si I lines match the calculated ones only when a LTE-population of 3p 2 SL terms ( 3 P, 1 D and 1 S) is assumed.

Spectra of the 011 ion were obtained with temporal and spatial (along the poloidal radius) resolution by injecting of oxygen-containing molecules through a block of the upper poloidal limiter of TEXTOR-94 into the boundary plasma. Three different wavelength regions (4660+/-100,4340+/-100, 3730+/-100 Angstrom) were observed during reproducible TEXTOR-94 discharges with n(e) = 1 x 10(18) m(-3), T-e = 90 eV at the last closed flux surface (LCFS). For calibration purposes the oxygen emission was simultaneously registered in all discharges with a 2D camera using the light from an O 11 line at 4416 Angstrom. Radial intensity distributions of lines corresponding to the transitions 2p(2)[P-3]3s-2p(2)[3P]3p (components of the multiplets P-4-S-4, P-4-P-4 P-4-D-4, P-2-D-2 and 2p(2)[D-1]3s-2p(2)[D-1]3p (doublet D-2-D-2) were measured. It is shown that the relative intensities of the lines inside the multiplets are in good agreement with theoretical data calculated in SL-coupling. An appropriate collisional-radiative model (the level list and the corresponding atomic database) has been developed and calculations have been carried out using the GKU kinetic code developed at the P.N. Lebedev Institute. Cross-sections of atomic processes have been calculated using the ATOM code by the K-matrix method. A comparison of theoretical and experimental data is presented. It is shown that the relative multiplet intensities have a weak dependence on electron density and temperature. The populations of the ground configuration states of O II were investigated. Absolute values for the 'ionization per photon' were measured and a comparison with modelled values is given.

When the ITER-like wall was installed in JET, one of the 218 Be inner wall guard limiter tiles had been enriched with 10Be as a bulk isotopic marker. During the shutdown in 2012-2013, a set of tiles were sampled nondestructively to collect material for accelerator mass spectroscopy measurements of 10Be concentration. The letter shows how the marker experiment was set up, presents first results and compares them to preliminary predictions of marker redistribution, made with the ASCOT numerical code. Finally an outline is shown of what experimental data are likely to become available later and the possibilities for comparison with modelling using the WallDYN, ERO and ASCOT codes are discussed.

Estimations of the ITER first wall (FW) lifetime, previously made using the three-dimensional Monte-Carlo ERO code (Borodin et al 2011 Phys. Scr. T145 014008), depend strongly on the assumptions of the physical sputtering yield for beryllium (Be). It is of importance to validate the respective model and data at existing devices including the JET ITER-like wall (ILW) as most ITER-relevant experiments. Applying the same sputtering input data in ERO as those used before in the ITER-predictions, the ERO simulations for the Be light intensity (using up to date atomic data from ADAS and measured plasma conditions) reveal a factor of 2 overestimation in the assumed yield even if the low estimate assuming 50% D surface content is used. This result indicates the preference of this assumption for plasma-wetted areas. It points to a possible necessity to correct (reduce) the respective estimates for the Be sputtering yield and, accordingly, re-visit the ITER FW lifetime predictions.

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

Tungsten is foreseen as the plasma-facing component material for baffles, the dome and strike-point area in the ITER divertor. Quantification of the W source, which is connected with the components lifetime and W plasma concentration, is one of the outstanding issues in the qualification process. A dedicated experiment in TEXTOR with the exposure of a W/C twin limiter to the near scrape-off layer plasma has been carried out in order to address the W sputtering and local material mixing in the electron temperature range between Te=30 and 85?eV, achieved with deuterium fueling in four steps. The Te range is comparable to the baffle region and the strike-point area during non-detached transient phases of the ITER divertor plasma. Quantification of the W sputtering yield and the impinging impurity fluxes was performed with the aid of optical spectroscopy, in particular by observation of WI and WII lines. As no inverse photon efficiencies in the plasma parameter range of the twin limiter experiment exist, we performed in a second experiment for the first time a calibration of WI and WII photon efficiencies with local injection of WF6 through a gas inlet into the TEXTOR edge plasma. The in situ determined effective inverse photon efficiency of about 85 for the most prominent WI line at 400.9?nm, which is in good agreement with GKU modelling for the covered Te range and 650 for the WII line at 434.8?nm, has been applied to the corresponding photon fluxes in the twin limiter experiment. The W sputtering yield decreases from 5.2 to 0.5%, thus by about one order of magnitude, with a reduction of Te from 85?eV down to 30?eV and a simultaneous increase of the impinging deuterium ion flux by 50% occurs. A lower limit for the prompt redeposition has been estimated at 50% by analyzing the WI to WII flux ratio. Local measurement of OII (441.6?nm) and CII (426.7?nm) provided impurity flux ratios of 0.6% for O and 5.2% for C related to the deuterium recycling, respectively ion flux. Both flux ratios remain constant for all phases of the discharge with plasma edge cooling. W erosion is predominantly caused by sputtering of higher ionization stages of O and C impinging on the W limiter half and not by the fuel species itself. Plasma cooling below the physical sputtering threshold could not be achieved without impurity seeding.

Ionization cross sections and rate coefficients of the atom W and the ion W+ by electron impact are calculated for the electron energies from thresholds to 300 eV. The Coulomb–Born method with exchange and normalization (by the reduced K-matrix method) is used. The contribution from excitation of autoionizing levels to the single ionization as well as of inner shell ionization to the double ionization is also calculated. Results for W+ are compared with experimental data and were found to be in agreement within 20%. A similar accuracy is expected for the W atoms.

Rate coefficients for the excitation and ionization of neutral as well as singly ionized particles and-–predominantly-–their ratios S(D)/XB, which are important for the conversion of photon into particle fluxes in ionizing fusion boundary plasma, have been modelled and experimentally determined in boundary plasmas for fusion-relevant species such as He I, Li I, C I, B I&II, O I&II, Si I&II, Mo I, W I&II, H2, CH(D), C2.

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.

ERO modelling of long-term tritium (T) retention has been done for the divertor of ITER with graphite target plates assuming a certain beryllium influx into the divertor, eroded from the main chamber. The divertor beryllium (Be) influx relative to the deuterium ion flux has been fixed at 0.1% for the outer divertor and 1.0% for the inner divertor. In addition to the original B2-Eirene plasma background, the influence of variations of temperature and density in the divertor has been studied. Moreover, assumptions for enhanced erosion of redeposited carbon and effective sticking for hydrocarbons have been analysed. With graphite target plates, long-term tritium retention is dominated by T co-deposition in deposits. Within the studied parameter range, the modelling yields 200?500 possible ITER discharges without cleaning before reaching the safety limit of 700?g of in-vessel retained tritium. Surface temperature-dependent tritium amounts in carbon and beryllium deposits have been taken into account.

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.

We report calculations of total (and absolute) electron-impact ionization cross sections (EICSs) for the fusion-relevant diatomic molecular species BeH, BeN, BeO, WH, WBe, WN, WO, O2, and N2 by means of the Deutsch-Mark and the binary-encounter-Bethe methods in the energy range from threshold to 10 keV. In addition, we discuss an empirical scheme to estimate partial cross sections from the total ones based on reaction energetics and empirical threshold laws and explore its accuracy by assessing available experimental data on total and partial EICSs. Finally, we also report parameters obtained by fitting the calculated cross sections to an expression commonly used in fusion edge plasma modeling.