[en] Full text: Tungsten,W, is the plasma-facing material of the JET-ILW divertor. W erosion by plasma and impurity impact determines the components lifetime as well as can influence the plasma performance by the W influx into the confined region. Certainly, the W screening by the divertor and the W transport into the plasma determines the W core content, but the W source itself impacts the process. Its quantification is essential to understand the interplay between the W impurity and the plasma. The JET-ILW provides access to a large set of W erosion-determining parameters permitting a detailed description of the source in the divertor closest to the ITER one. a) Effective sputtering yields and fluxes as function of impact energy of intrinsic (Be,C) and extrinsic (Ne,N) impurities as well as hydrogenic isotopes (H,D) are determined. This includes the interplay between intra- and inter-ELM W sources caused by the flux and energy distributions in these phases. The threshold behaviour and the spectroscopic composition analysis provide an insight in the dominating species and phases causing the erosion. b) The interplay between net and gross W erosion source will be elaborated considering prompt redeposition, thus, the return of W to the surface within one Larmor radius, and surface roughness, thus, the difference between smooth bulk-W and rough W-coating components. Both effects impact the balance equation of local W erosion and deposition. c) Postmortem analysis reveals the campaign-integrated net migration path identifying the W transport to remote areas. The transport is related to the plasma regime, e.g., H-mode with attached divertor and high impact energies of impinging species or detached operation, as well as to the magnetic configuration, e.g., corner with geometrical screening of W or ITER-like vertical target. d) The influence of parameters like surface temperature on the erosion, including the role of chemically assisted physical sputtering, is covered. JET-ILW permitted access to net W erosion in one magnetic configuration within a series of 151 subsequent discharges. Comparison of spectroscopy in the intra-ELM and inter-ELM phases with postmortem analysis of marker tiles provided a set of gross and net W erosion. ERO code simulations could reproduce the pattern as well as confirm high prompt W redeposition factors of more than 95% for the intra-ELM phase. (author)

[en] For simulation of plasma-facing component erosion in fusion experiments, an analytical expression for the ion velocity just before the surface impact including the local electric field and an optional surface biasing effect is suggested. Energy and angular impact distributions and the resulting effective sputtering yields were produced for several experimental scenarios at JET ILW mostly involving PFCs exposed to an oblique magnetic field. The analytic solution has been applied as an improvement to earlier ERO modelling of localized, Be outer limiter, RF-enhanced erosion, modulated by toggling of a remote, however magnetically connected ICRH antenna. The effective W sputtering yields due to D and Be ion impact in Type-I and Type-III ELMs and inter-ELM conditions were also estimated using the analytical approach and benchmarked by spectroscopy. The intra-ELM W sputtering flux increases almost 10 times in comparison to the inter-ELM flux.

[en] An important W erosion mechanism in JET divertor is the physical sputtering by both, impurity (e.g. Be) and hydrogenic ions hitting the W divertor with energies determined by the pedestal temperature during edge-localized modes (ELMs)—the so-called intra-ELM sputtering of W. The earlier developed analytical approach for the W divertor gross erosion estimation using the Langmuir probe measurements has been improved in this work taking into account the time-resolved pedestal temperature and density drop during the pedestal crash under intra-ELM conditions. The improved model allows reproducing the measured at the divertor tile particle and heat fluxes evolution at the effective magnetic connection length matched with the previous JET- ITER like wall (ILW) studies. The estimates for the tungsten sputtered flux in intra- and inter-ELM conditions for quasi-steady state plasmas executed at the end of the first year of JET-ILW operation (C30C experiment) show that Type I ELMs contribute significantly (∼85%) to the gross tungsten erosion which is in a good agreement with the divertor optical emission spectroscopy (W I 400.9 nm line). ELM filament radial propagation is considered based on the advective-diffusive model and JET-C experiment results. The estimation for the ELM-induced local Be main chamber erosion at JET-ILW reveals the increase of the Be sputtered flux by 30% under intra-ELM conditions. (topical issue article)

[en] Full text: Erosion will be one of the main factors determining the lifetime of the plasma-facing components (PFCs) in ITER, particularly the low-Z beryllium (Be) first wall (FW). This paper presents the tests of Be erosion data during experiments with the ITER-like wall (ILW) in JET and the corresponding revisiting of the predictive modelling for ITER. The key tool is the Monte Carlo 3D impurity transport and plasma-surface interaction ERO code. In this paper two fits for Be sputtering data are used, both based on simulated data including the molecular dynamic (MD) approach. The factor 3-4 lower one called 'ERO-min' implies large D content (50%) in a PFC surface. Chemically assisted sputtering (CAS) can contribute significantly (up to ∼50%) to Be erosion. According to MD data used in ERO, CAS varies with energy of impinging ions and surface temperature T_s. Benchmarking the ERO on results from the ILW is critical for gaining confidence in the modelling approach and the related data. Two Be erosion experiments have been performed in inner wall (IW) limited discharges. The T_s was found to have an influence on the molecular release fraction, which decreases to negligible values at 670 K. The plasma temperature in SOL (ion impact energies) was scanned whilst simultaneously monitoring the spectroscopic emission of BeI, BeII and BeD in the vicinity of the solid Be limiter. 3D ERO modelling allows the surface erosion to be characterized by the line-of-sight integrated emission. The 'EROmin' sputtering assumptions lead to the best match with experiments. ERO reproduces the BeD light emission trend and absolute value during the E_imp scan within 20%. Earlier ERO erosion predictions for the ITER FW panels have been revisited. The ILW benchmark shows that the previously calculated upper limit (based on the 'ERO-min' fit) of the FW panels lifetime estimation of ∼4200 ITER discharges (steady state erosion) is the most appropriate. However, the improved (analytical) approach for calculating ion movement just before the surface impact leads to a decrease of the corresponding lifetime by 30% to ∼3000 discharges. The CAS can lead to a further decrease, depending on T_s. However, these estimates are based on the most conservative assumptions regarding the background plasma and magnetic equilibrium expected for ITER. In reality, the Be FW panel lifetime is expected to be far greater. (author)

[en] 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. (paper)

[en] The surface morphology of plasma-facing components (PFCs) and its evolution during plasma irradiation has been shown to have a significant effect on the erosion and subsequent transport of sputtered particles in plasma. This in turn can influence the resulting lifetime of PFCs. A model for treatment of the effect of surface roughness on the erosion of PFCs has recently been incorporated into the three-dimensional Monte Carlo code ERO2.0. First simulations have confirmed a significant influence of the assumed surface roughness (for both regular and stochastic numerically constructed samples) on both the effective sputtering yields Y _eff and the effective angular distributions of sputtered particles. In this study, a series of experiments at the linear plasma device PSI-2 are conducted to test the effect of surface roughness on the sputtering parameters. Graphite samples prepared with a 100 nm molybdenum layer with various surface roughness characteristic sizes (R _a = 110 nm, 280 nm, 600 nm and R _a < 20 nm) were exposed to a helium plasma in the PSI-2 linear plasma device at a magnetic field B = 0.1 T. These PSI-2 experiments were simulated using ERO2.0 with a surface morphology model. Simulations are able to reproduce the experimentally observed significant suppression of erosion for higher R _a values. (topical issue article)

[en] Experiments at JET showed locally enhanced, asymmetric beryllium (Be) erosion at outer wall limiters when magnetically connected ICRH antennas were in operation. A first modeling effort using the 3D erosion and scrape-off layer impurity transport modeling code ERO reproduced qualitatively the experimental outcome. However, local plasma parameters—in particular when 3D distributions are of interest—can be difficult to determine from available diagnostics and so erosion / impurity transport modeling input relies on output from other codes and simplified models, increasing uncertainties in the outcome. In the present contribution, we introduce and evaluate the impact of improved models and parameters with largest uncertainties of processes that impact impurity production and transport across the scrape-off layer, when simulated in ERO: (i) the magnetic geometry has been revised, for affecting the separatrix position (located 50–60 mm away from limiter surface) and thus the background plasma profiles; (ii) connection lengths between components, which lead to shadowing of ion fluxes, are also affected by the magnetic configuration; (iii) anomalous transport of ionized impurities, defined by the perpendicular diffusion coefficient, has been revisited; (iv) erosion yields that account for energy and angular distributions of background plasma ions under the present enhanced sheath potential and oblique magnetic field, have been introduced; (v) the effect of additional erosion sources, such as charge-exchange neutral fluxes, which are dominant in recessed areas like antennas, has been evaluated; (vi) chemically assisted release of Be in molecular form has been included. Sensitivity analysis highlights a qualitative effect (i.e. change in emission patterns) of magnetic shadowing, anomalous diffusion, and inclusion of neutral fluxes and molecular release of Be. The separatrix location, and energy and angular distribution of background plasma fluxes impact erosion quantitatively. ERO simulations that include all features described above match experimentally measured Be I ($457.3$ nm) and Be II ($467.4$ nm) signals, and erosion increases with varying ICRH antenna’s RF power. However, this increase in erosion is only partially captured by ERO’s emission measurements, as most contributions from plasma wetted surfaces fall outside the volume observed by sightlines. (paper)

[en] A new analytical approximation for the electric potential profile in the presence of an oblique magnetic field and the analytical solution for the particle motion just before the impact with a plasma-facing surface are presented. These approximations are in good agreement with fluid solutions and the corresponding PIC simulations. These expressions were applied to provide effective physical erosion yields for Be, which have in a second step been used in ERO code simulations of spectroscopy at Be limiters of the JET ITER-like wall. These new analytical expressions lead to an increase of the effective physical sputtering yields of Be by deuteron impact up to 30% in comparison with earlier pure numerical simulations. (copyright 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] The ability to detect and control fuel isotopic content down to a 1% concentration level is greatly important for the upcoming JET DTE2 campaign, as well as its associated TT and DD phases. A reduction of H minority concentration even from 2% down to 1% is shown here to have significant impact on the effectiveness of ion cyclotron range of frequencies core heating, while the ability to maintain T or D concentration at or below 1% is critical to limiting fusion neutron generation in the DD and TT phases, correspondingly. The sub-divertor measurement of (global) isotopic concentration, based on Penning-activated optical spectroscopy, can deliver minimally this 1% detection for DTE2 as long as light collection from the Penning emission can be optimized and gradual window transmission deterioration can be minimized. This is simulated with a statistical analysis developed to understand the uncertainty sources in the JET DTE1 data, as well as to guide the optimization of an upgraded, fuel-isotopic content (and helium-ash concentration) gas analysis system for the JET divertor in preparation for DTE2. While this random error can be reduced to allow measurement substantially below 1% concentration, analysis also shows a systematic error of up to 1% understood to be due to plasma–surface interactions in the Penning excitation, suggesting that 1% may still be the low-end limit for the sub-divertor measurement, unless a Penning-source conditioning approach is also developed. (paper)

[en] ERO is a Monte-Carlo code for modeling plasma-wall interaction and 3D plasma impurity transport for applications in fusion research. The code has undergone a significant upgrade (ERO2.0) which allows increasing the simulation volume in order to cover the entire plasma edge of a fusion device, allowing a more self-consistent treatment of impurity transport and comparison with a larger number and variety of experimental diagnostics. In this contribution, the physics-relevant technical innovations of the new code version are described and discussed. The new capabilities of the code are demonstrated by modeling of beryllium (Be) erosion of the main wall during JET limiter discharges. Results for erosion patterns along the limiter surfaces and global Be transport including incident particle distributions are presented. A novel synthetic diagnostic, which mimics experimental wide-angle 2D camera images, is presented and used for validating various aspects of the code, including erosion, magnetic shadowing, non-local impurity transport, and light emission simulation. (paper)