JET Divertor Research Articles

Thermal and statistical analysis of the high-Z tungsten-based UFOs observed during the first deuterium high fluence campaign of the WEST tokamak

This paper summarizes the observations on the Unidentified Flying Object (UFO) observed during the first high fluence campaign performed in WEST. The campaign was characterized by high erosion and deposition rates, generating dusts and deposits prone to flaking during plasma operation. A total of 686 UFOs have been detected for 384 discharges executed, in which 133 UFOs lead to a disruption. These UFOs appear mostly during the low hybrid heating ramp-up early in the discharges. The mechanisms leading to a disruption is described as well as the delay available to react after the UFO entering in the plasma. The UFOs originate mostly from the high field side corresponding to the thick deposit area. These UFOs can be debris from a preceding disruption (≈27 %) or come from progressive loss of adherence of the thick deposits (≈55 %) under plasma cycling or more likely by disruption on this area. Finally, the thermal resistance of the poorly attached deposits has been calculated in the range from 2.10−4 °C m2/W to 1.10−3 °C m2/W coherent with previous observations in the high field side of the JET divertor.

Material transport from marker tiles in the JET divertor

Material transport from marker tiles in the JET divertor

Remote infrared view of JET divertor compatible with D-T operations

A remote viewing system located outside the biological shield wall to image the divertor region in JET is presented here. This paper focuses on the optical and mechanical designs of the installation of a line of sight which is capable of imaging the divertor over an optical distance of approximately 31 m. In addition, it includes the experience obtained during operations and it demonstrates its successful performance. This line of sight was the second viewing system used during the D-T campaign and it follows from the paper dedicated to the remote wide angle view line of sight which is also compatible with D-T operations and includes common elements to the construction of the two lines of sight. The main novel feature is the design in house of two glass reinforced plastic kinematic mounts standing vertically to each accommodate a mirror of 500 mm in diameter.

Mechanical and microstructural analysis of tungsten exposed in JET deuterium plasmas

A tungsten Langmuir probe exposed in the JET divertor during the ITER-like wall campaigns (ILW) has been studied to evaluate changes in mechanical properties and microstructure. The tip of the probe that was exposed to plasma was cross-sectioned and polished for post mortem analysis. Analysis involved a comparison with a non-exposed probe to determine the effect of plasma exposure on material microstructure and mechanical properties. Visually the probe appeared to have melted and re-solidified during its time in the vessel. Secondary electron (SE) images of cross sections showed the formation of bubbles near the exposed surface that ranged from 50 µm to sub-micron sized. Electron backscatter diffraction (EBSD) revealed that the average grain size had increased from 33 µm to 570 µm. The investigation also showed that hardness had increased from 5.2 to 6.1 GPa and pop-in behaviour was supressed after exposure. This was initially attributed to the uptake of deuterium (D) but nuclear reaction analysis (NRA) indicated that no deuterium remained in the sample and hinted that some other type of defect was modifying the mechanical properties.

Deuterium and beryllium depth profiles into the W-coated JET divertor tiles after ITER-like wall campaigns

A number of 35 samples cored from JET divertor tiles exposed to JET plasmas in ILW1 (ITER Like Wall – first campaign), ILW2, ILW1 + ILW2 and ILW3 campaigns were analyzed using the Glow Discharge Optical Emission Spectrometry (GDOES) technique. The depth profiles of the elements contained in the surface layer of the tiles (W, Mo, Be, C, O and D) have been quantitatively determined up to a depth of 30–60 μm.The upper zone of Tile 3 (the lower vertical zone of the inner divertor) seems to be an erosion zone as it was demonstrated with a Mo marker layer in the ILW1 campaign. By increasing the input energy in the following campaigns this erosion did not increase, but it almost disappeared, probably due to significant amounts of Be that were transported from the main chamber and deposited on that area. The Be concentration increased in the upper zone of Tile3 in ILW2 and ILW3 by a factor of about 5 in comparison with the ILW1 campaign. It looks like the Be layer protects the tile surface from erosion.Depending on the amount of Be deposited on specific areas of the W-coated CFC (Carbon Fiber Composite) tiles, two mechanisms of D retention were identified. If the peak concentration of Be is less than ∼4.5 at.% D is mainly trapped into the defects of the W/Mo structure including W/Mo and Mo/C interfaces. When the peak concentration of Be exceeds ∼4.5 at.%, D is mainly retained in the Be deposits. The GDOES results were correlated with SEM analyses performed on the GDOES crater flanks and with the TDS (Thermal Desorption Spectroscopy), results obtained with the same samples after GDOES analyses. They are in good agreement with the corresponding results obtained by other authors using NRA (Nuclear Reaction Analysis), SIMS (Secondary Ion Mass Spectrometry) and TDS.

An overview of tritium retention in dust particles from the JET-ILW divertor

Tritium (T) retention characteristics in dust collected from the divertor in JET with ITER-like wall (JET-ILW) after the third campaign in 2015–2016 (ILW-3) have been examined in individual dust particles by combining radiography (tritium imaging plate technique) and electron probe micro-analysis. The results are summarized and compared with the data obtained after the first campaign in 2011–2012 (ILW-1). The dominant component in ILW-1 dust was carbon (C) originating from tungsten-coated carbon fibre composite (CFC) tiles in JET-ILW divertor and/or legacy of C dust after the JET operation with carbon wall. Around 85% of the total tritium retention in ILW-1 dust was attributed to the C dust. The retention in tungsten (W) and beryllium (Be) dominated particles was 100 times smaller than the highest T retention in carbon-based particles. After ILW-3 the main component contributing to the T retention was W. The number of small W particles with T increased, in comparison to ILW-1, most probably by the exfoliation and fragmentation of W coatings on CFC tiles though T retention in individual W particles was smaller than in C particles. The detection of only very few Be-dominated dust particles found after ILW-1 and ILW-3 could imply stable Be deposits on the divertor tiles.

Assessment of filtered cameras for quantitative 2D analysis of divertor conditions during detachment in JET L-mode plasmas

Estimates for 2D distributions of electron temperature, , electron density, , and atomic deuterium density, , in the JET divertor volume have been inferred from deuterium Balmer line intensity ratios obtained from tomographic reconstructions of divertor camera measurements. This enables also investigation of ionization, , and recombination, , rates. The analysis shows a decrease of to 0.5–1.0 throughout the outer divertor during detachment in low-confinement (L-mode) plasmas. Simultaneously, the high- region and the distribution in the outer divertor are observed to elongate and shift from the outer strike point towards the X-point. The observations are in qualitative agreement and follow the same sequence with modelling predictions of EDGE2D-EIRENE simulations of a density scan. While the method was found to provide good representation of the evolution of volumetric recombination during detachment, in agreement with the simulations, the movement of the ionization front upstream could not be followed due to lack of spatial overlap between the ionization region and the necessary emission distributions. Consequently, the representation of the ionization conditions and the particle balance in the detached outer divertor are compromised.

Accounting for Edge-Plasma Parameter Dynamics at JET Divertor Sputtering Estimation during Edge-Localized Modes

The sputtering of tungsten divertor tiles at the JET ITER-like wall (ILW) during edge-localized modes (ELMs) is one of the main sources of tungsten impurity in the core plasma. The analytical approach for interpretation of the Langmuir probe (LP) measurements of ion flux at the divertor tile is elaborated to model the ELM ion parallel transport and to estimate the flux of sputtered tungsten atoms from the divertor under inter-ELM and intra-ELM conditions. Accounting for the pedestal temperature and density decrease at the pedestal crash during ELM allows obtaining the profiles of the particle and heat fluxes at the divertor tile corresponding to the measurements at the physically correct magnetic connection length. Estimates of the tungsten sputtered flux under intra-ELM and inter-ELM conditions show that first type ELMs contribute significantly (~85%) to the overall tungsten sputtering, which is in a good agreement with the divertor optical emission spectroscopy of the 400.9 nm atomic neutral tungsten line in JET discharges.

The role of drifts on the isotope effect on divertor plasma detachment in JET Ohmic discharges

• The magnitude of the isotope effect is determined by the pressure in the sub-divertor. • Pumping of gas within the JET divertor is effective near the outer target only. • The re-distribution of the neutrals due to drifts decreases the throughput. • The total decrease of the throughput reduces the isotope effect. Experiments in JET-ILW Ohmic confinement mode plasmas show that the line-averaged detachment onset density in deuterium discharges is approx. 10% lower than in hydrogen discharges. The magnitude of the isotope effect on the detachment onset density depends on the divertor geometry, the magnetic configuration and the throughput of the sub-divertor/divertor cryopump system. Simulations with the edge fluid code EDGE2D-EIRENE revealed that the pumping of neutral gas within the JET divertor is effective near the outer divertor target only. The studies show that the magnitude of the isotope effect is determined by the molecular pressure in the sub-divertor pumping plenum. According to the simulations, operating in vertical configurations or closer proximity of the strike point increases the molecular pressure (and thus throughput) in front of the outer pumping plenum by up to 15% compared to the horizontal configuration, thus producing a stronger isotope effect on the detachment onset density. Similarly, EDGE2D-EIRENE predicts that plasma, hence neutral, redistribution due to E × B drifts in favourable B T configurations (ion B × grad(B) towards the divertor) decreases the throughput. The total decrease of the throughput reduces the isotope effect on the detachment onset density, and decreases the detachment onset density for both isotopes.

Estimation of 2D distributions of electron density and temperature in the JET divertor from tomographic reconstructions of deuterium Balmer line emission

• 2D n e and T e estimates obtained from deuterium Balmer line ratios in JET divertor. • High n e observed to shift from strike point to above X-point in L-mode detachment. • Divertor T e observed to fall from 3—4 eV to 0.5—1.0 eV in L-mode detachment. • 2D estimates within 25% agreement with other spectroscopic n e and T e measurements. Estimates for 2D distributions of electron density, n e , and temperature, T e , have been obtained using reflection-corrected tomographic reconstructions of deuterium Balmer line emission in the JET divertor. The solutions are obtained by a two-dimensional Monte Carlo optimization model, based on intensity ratios of the Balmer D α , D γ and D ε emission and the ADAS photon emission coefficients with additional constraints provided by line-integrated spectroscopic measurements of n e and T e . Synthetic benchmarks utilizing EDGE2D-EIRENE simulation outputs yield reproduction of reference n e and T e distributions within 25% with statistical error estimates of 5–15%. Experimental analysis of an L-mode n e ramp shows extension and movement of the outer divertor high-n e region with n e,div up to 1.5 × 10 20 m −3 from the outer strike point to above the X-point and decrease of the divertor T e to the range of 0.5–1.0 eV within 25% agreement with the line-integrated measurements. Comparison between the local estimates and their corresponding line integrals suggests underestimation of n e and T e by up to 75% and 50%, respectively, due to the geometric effects of line integration.

Estimation of ELM effects on Be and W erosion at JET-ILW

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.

Comparison of erosion and deposition in JET divertor during the first three ITER-like wall campaigns

The manuscript presents an overview of the erosion and deposition data in the inner and outer JET divertor observed during the first three ITER-like wall campaigns (JET-ILW1, JET-ILW2, JET-ILW3). Erosion and deposition were studied using core samples cut out from divertor tiles. For the studied samples a similar general deposition pattern was observed in all three campaigns: More than 60% of the total deposition occurred in the upper region of the inner divertor on tiles 0 and 1, where Be was transported and deposited from the scrape-off layer. High erosion was observed only on tile 5. In JET-ILW2 and 3, erosion together with high power fluxes was observed in the outer divertor at the bottom of tile 7. Additionally, deposition peaks were observed on the sloping parts of tiles 4 and 6, which were more pronounced in JET-ILW2 and 3 due to placing the strike point more often on these tiles. The deposits consisted primarily of Be, with some additional D and C. Deposition rates were observed to decrease from campaign to campaign, with the C deposition rate decreasing the most, more than 2 times from JET-ILW1 to JET-ILW3. D retention up to levels of ∼1 at% was observed up to large depths in the W protective coatings in all campaigns.

Sub-divertor fuel isotopic content detection limit for JET and its impact on ICRF core heating and DTE2 operationaaThis manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable,

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.

Erosion, screening, and migration of tungsten in the JET divertor

The erosion of tungsten (W), induced by the bombardment of plasma and impurity particles, determines the lifetime of plasma-facing components as well as impacting on plasma performance by the influx of W into the confined region. The screening of W by the divertor and the transport of W in the plasma determines largely the W content in the plasma core, but the W source strength itself has a vital impact on this process. The JET tokamak experiment provides access to a large set of W erosion-determining parameters and permits a detailed description of the W source in the divertor closest to the ITER one: (i) 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 and predictions for the tritium (T) isotope are made. This includes the quantification of intra- and inter-edge localised mode (ELM) contributions to the total W source in H-mode plasmas which vary owing to the complex flux compositions and energy distributions in the corresponding phases. The sputtering threshold behaviour and the spectroscopic composition analysis provides an insight in the dominating species and plasma phases causing W erosion. (ii) The interplay between the net and gross W erosion source is discussed considering (prompt) re-deposition, thus, the immediate return of W ions back to the surface due to their large Larmor radius, and surface roughness, thus, the difference between smooth bulk-W and rough W-coating components used in the JET divertor. Both effects impact on the balance equation of local W erosion and deposition. (iii) Post-mortem analysis reveals the net erosion/deposition pattern and the W migration paths over long periods of plasma operation identifying the net W transport to remote areas. This W transport is related to the divertor plasma regime, e.g. attached operation with high impact energies of impinging particles or detached operation, as well as to the applied magnetic configuration in the divertor, e.g. close divertor with good geometrical screening of W or open divertor configuration with poor screening.JET equipped with the ITER-like wall (ILW) provided unique access to the net W erosion rate within a series of 151 subsequent H-mode discharges (magnetic field: T, plasma current: MA, auxiliary power MW) in one magnetic configuration accumulating 900 s of plasma with particle fluences in the range of 5– in the semi-detached inner and attached outer divertor leg. The comparison of W spectroscopy in the intra-ELM and inter-ELM phases with post-mortem analysis of W marker tiles provides a set of gross and net W erosion values at the outer target plate. ERO code simulations are applied to both divertor leg conditions and reproduce the erosion/deposition pattern as well as confirm the high experimentally observed prompt W re-deposition factors of more than 95% in the intra- and inter-ELM phase of the unseeded deuterium H-mode plasma. Conclusions to the expected divertor conditions in ITER as well as to the JET operation in the DT plasma mixture are drawn on basis of this unique benchmark experiment.

Physically principled reflection models applied to filtered camera imaging inversions in metal walled fusion machines.

Ray-tracing techniques are applied to filtered divertor imaging, a diagnostic that has long suffered from artifacts due to the polluting effect of reflected light in metal walled fusion machines. Physically realistic surface reflections were modeled using a Cook-Torrance micro-facet bi-directional reflection distribution function applied to a high resolution mesh of the vessel geometry. In the absence of gonioreflectometer measurements, a technique was developed to fit the free parameters of the Cook-Torrance model against images of the JET in-vessel light sources. By coupling this model with high fidelity plasma fluid simulations, photo-realistic renderings of a number of tokamak plasma emission scenarios were generated. Finally, a sensitivity matrix describing the optical coupling of a JET divertor camera and the emission profile of the plasma was obtained, including full reflection effects. These matrices are used to perform inversions on measured data and shown to reduce the level of artifacts in inverted emission profiles.

Modelling of the effect of ELMs on fuel retention at the bulk W divertor of JET

Effect of ELMs on fuel retention at the bulk W target of JET ITER-Like Wall was studied with multi-scale calculations. Plasma input parameters were taken from ELMy H-mode plasma experiment. The ene ...

Investigation of deuterium trapping and release in the JET divertor during the third ILW campaign using TDS

Selected set of samples from JET ITER-Like Wall (JET-ILW) divertor tiles exposed in 2015-2016 has been analysed using Thermal Desorption Spectrometry (TDS). The deuterium (D) amounts obtained with ...

Analysis of deposited layers with deuterium and impurity elements on samples from the divertor of JET with ITER-like wall

Inconel-600 blocks and stainless steel covers for quartz microbalance crystals from remote corners in the JET-ILW divertor were studied with time-of-flight elastic recoil detection analysis and nuclear reaction analysis to obtain information about the areal densities and depth profiles of elements present in deposited material layers. Surface morphology and the composition of dust particles were examined with scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analysed components were present in JET during three ITER-like wall campaigns between 2010 and 2017. Deposited layers had a stratified structure, primarily made up of beryllium, carbon and oxygen with varying atomic fractions of deuterium, up to more than 20%. The range of carbon transport from the ribs of the divertor carrier was limited to a few centimeters, and carbon/deuterium co-deposition was indicated on the Inconel blocks. High atomic fractions of deuterium were also found in almost carbon-free layers on the quartz microbalance covers. Layer thicknesses up to more than 1 μm were indicated, but typical values were on the order of a few hundred nm. Chromium, iron and nickel fractions were less than or around 1% at layer surfaces while increasing close to the layer-substrate interface. The tungsten fraction depended on the proximity of the plasma strike point to the divertor corners. Particles of tungsten, molybdenum and copper with sizes less than or around 1 μm were found. Nitrogen, argon and neon were present after plasma edge cooling and disruption mitigation. Oxygen-18 was found on component surfaces after injection, indicating in-vessel oxidation. Compensation of elastic recoil detection data for detection efficiency and ion-induced release of deuterium during the measurement gave quantitative agreement with nuclear reaction analysis, which strengthens the validity of the results.

An improved model for the accurate calculation of parallel heat fluxes at the JET bulk tungsten outer divertor

Parallel heat flux calculations at the JET divertor have been based on the assumption that all incoming heat is due to the projection of the heat flux parallel to the magnetic line, , plus a constant background. This simplification led to inconsistencies during the analysis of a series of dedicated tungsten melting experiments performed in 2013, for which infrared (IR) thermography surface measurements could not be recreated through simulations unless the parallel heat flux was reduced by 80% for L-mode and 60% for H-mode. We give an explanation for these differences using a new IR inverse analysis code, a set of geometrical corrections, and most importantly an additional term for the divertor heat flux accounting for non-parallel effects such as cross-field transport, recycled neutrals or charge exchange. This component has been evaluated comparing four different geometries with impinging angles varying from 2 to 90°. Its magnitude corresponds to 1.2%–1.9% of , but because it is not affected by the magnetic projection, it accounts for up to 20%–30% of the tile surface heat flux. The geometrical corrections imply a further reduction of 24% of the measured heat flux. In addition, the application of the new inverse code increases the accuracy of the tile heat flux calculation, eliminating any previous discrepancy. The parallel heat flux computed with this new model is actually much lower than previously deduced by inverse analysis of IR temperatures—40% for L-mode and 50% for H-mode—while being independent of the geometry on which it is measured. This main result confirms the validity of the optical projection as long as a non-constant and non-parallel component is considered. For a given total heating power, the model predicts over 10% reduction of the maximum tile surface heat flux compared to strict optical modelling, as well as a 30% reduced sensitivity to manufacturing and assembling tolerances. These conclusions, along with the improvement in the predictability of the divertor thermal behaviour, are critical for JET future DT operations, and are also directly applicable to the design of the ITER divertor monoblocks.

Erosion and deposition in the JET divertor during the second ITER-like wall campaign

Erosion of plasma-facing materials and successive transport and redeposition of eroded material are crucial processes determining the lifetime of plasma-facing components and the trapped tritium inventory in redeposited material layers. Erosion and deposition in the JET divertor were studied during the second JET ITER-like wall campaign ILW-2 in 2013–2014 by using a poloidal row of specially prepared divertor marker tiles including the tungsten bulk tile 5. The marker tiles were analyzed using elastic backscattering with 3–4.5 MeV incident protons and nuclear reaction analysis using 0.8–4.5 MeV 3He ions before and after the campaign. The erosion/deposition pattern observed during ILW-2 is qualitatively comparable to the first campaign ILW-1 in 2011–2012: deposits consist mainly of beryllium with 5–20 at.% of carbon and oxygen and small amounts of Ni and W. The highest deposition with deposited layer thicknesses up to 30 μm per campaign is still observed on the upper and horizontal parts of the inner divertor. Outer divertor tiles 5, 6, 7 and 8 are net W erosion areas. The observed D inventory is roughly comparable to the inventory observed during ILW-1. The results obtained during ILW-2 therefore confirm the positive results observed in ILW-1 with respect to reduced material deposition and hydrogen isotopes retention in the divertor.