Recent studies dedicated to the characterisation of in-vessel dust in JET with the new ITER-like wall (ILW) show that dust levels are orders of magnitude lower compared with the latter stages of the ...carbon-wall (CW) period and are decreasing with operational time. Less than 1 g of dust was recovered in a recent inspection, compared with more than 200 g of material recovered at the end of the JET-CW life. Recent inspection of the ILW shows low rates of re-deposition with only small areas of damage of a type likely to create particulate matter. Quantifiers from laser scattering techniques also indicate an order of magnitude reduction in dust relative to the JET-CW and show that the amount of dust mobilized after a disruption is proportional to the dynamic vessel forces. It is not possible to infer what fraction of dust (if any) might be created by disruptions. However, disruption mitigation is found to reduce the amount of dust seen after moderate disruptions by a factor of 4. Analysis of the transient impurity events (TIEs) associated with dust show that tungsten dominates. A significant contribution to TIEs is also seen from iron, nickel and chromium (probably from steel and Inconel components). The incidence of severe negative effects on operations from TIEs is found to be relatively rare, with <1% of ILW disruptions linked to TIEs. The evolution of the TIE rate closely follows changes in the laser scattering dust quantifiers; both trend downwards in time but peak during periods of higher disruption rate (thought to be primarily driven by the mobilization of existing dust).
•High-power type-I ELMy JET plasmas simulated using JINTRAC and ERO2.0.•Near-perfect screening predicted for highest gross W sources at divertor targets.•W influx to main plasma mostly from LFS ...divertor entrance due to erosion by CX atoms.•Predicted W I emission consistent with measurements in the LFS divertor.•W II emission underpredicted by a factor of 10; potential reasons identified.
Simulations of JET ITER-like wall high-confinement mode plasmas, including type-I edge-localised modes (ELMs), using JINTRAC for the background plasmas and ERO2.0 for tungsten erosion and transport, predict virtually perfect screening of the primary W erosion sources at the divertor targets during both the ELM and inter-ELM phases. The largest source of W influx to the main plasma is predicted to be the outer vertical divertor due to sputtering by energetic fuel (D, T) atoms from charge-exchange reactions. ERO2.0 predictions accurately reproduce the measured W I emission in the low-field side divertor, but underpredict the W II emission by a factor of 10. Potential reasons for the W II discrepancy include uncertainties in the atomic data, assumptions on the sheath properties and the sputtering angle distribution, and the impact of metastable states.
The occurrence of transient impurity events (TIE) leading to intense radiation spikes in JET plasma discharges has been studied since the installation of the ITER-like wall (ILW). To generate the ...observed average increase in radiated power of 1.5MW, a spherical particle of solid W of radius >90μm would be required. The drop in plasma energy caused by W-TIEs is fully recovered in 90% of all cases, only 1% inducing a longer term loss in plasma energy which sometimes leads to the shut-down of plasma operation. TIEs are correlated with disruptions and with measurements of the dust mobilized by disruptions using the high resolution Thomson scattering (HRTS) diagnostic. The dust characteristics giving rise to TIEs have been studied using the dust transport code DTOKS and the 1D impurity transport code STRAHL.
Tungsten sputtering rates and density profiles predicted using the edge plasma codes EDGE2D-EIRENE and DIVIMP are found to agree within a factor of 4 with measurements of neutral and singly-ionized W ...spectral line emission in the JET low-field side (LFS) divertor, and within a factor of 2 with SXR, VUV, and bolometric calculations of the W density in the main plasma. The edge plasma W predictions are extended to the core plasma using JINTRAC integrated core-edge modelling. Prompt redeposition of W is identified as the primary reason for the discrepancy between predicted and measured W emission in the divertor. The studied plasmas include attached divertor conditions in L-mode and type-I ELMy H-mode plasmas typical for JET.
To more accurately reproduce the spectroscopically inferred W sputtering rates in EDGE2D-EIRENE, imposing the experimentally observed Be concentration of order 0.5% in the divertor is necessary. However, the W density in the main plasma is predicted to be insensitive to whether or not W is sputtered by Be at the divertor targets. Instead, the majority of the predicted core W originated in L-mode from sputtering due to fast D charge-exchange atoms at the W-coated tiles above the LFS divertor, and in H-mode due to D and W ions at the targets during ELMs.
•EDGE2D-EIRENE and DIVIMP are validated against JET W diagnostics in L-mode and H-mode.•Discrepancy between predicted W I and W II emission due to prompt W redeposition.•Predicted core W density insensitive to L-mode and inter-ELM sputtering near targets.•The core W densities from experiment and integrated modelling agree to a factor of 2.•Accurate H-mode W predictions require ELM parameters precisely matched to experiment.
•Assessment of plasma conditions for electron temperature, density and pressure radially across the SOL at six distinct poloidal positions, including the low-field side (LFS) midplane and five ...Divertor Thomson Scattering (DTS) locations.•In low-recycling conditions, electron pressure is conserved (as anticipated), EDGE2D-EIRENE reproduces the plasma conditions within the uncertainties of the measurements, including the position of the separatrix at the LFS midplane.•In high-recycling conditions, the electron pressure is conserved from the LFS midplane to the second lowermost DTS chord; electron pressure loss occurred between the second lowermost and lowermost DTS chord.•In high-recycling conditions, EDGE2D-EIRENE predicts the SOL plasma from the LFS midplane downstream to the second lowermost DTS channel; predicts significantly broader electron temperature and density profiles, under-predicts electron density at the strike point by factor of 2.
Comparisons of profiles of the electron temperature (Te), density (ne) and pressure (pe) measured with Divertor Thomson Scattering in DIII-D low-confinement mode discharges to predictions from the edge fluid code EDGE2D-EIRENE 1 show that the models implemented in EDGE2D-EIRENE predict the measurements within their collective uncertainties if the Te at the separatrix (Te,sep) is 10 eV, or higher. The simulations do not predict, however, the peaked Te and ne profiles measured adjacent to the target plate when Te,sep, is below 10 eV, i.e., for the plasma downstream from the region of ionization of deuterium atoms. Inclusion of cross-field drifts and a fivefold reduction of radial transport cannot reconcile the discrepancy between the measurements and predictions.
•This paper presents characterisation of the plasma conditions in the low-field side divertor in JET ITER-like Wall hydrogen, deuterium, tritium and deuterium–tritium plasmas showing that:•The core ...plasma density required to detach the LFS divertor plasma is independent of the hydrogenic species protium, deuterium and tritium, and a 40%/60% deuterium–tritium mixture.•The detachment onset density is independent of the operational status of the JET cryogenic pump.•The onset of detachment coincides with electron temperatures in the low-field side divertor plasma between 2 eV and 3 eV.•The electron density peaks in the low-field side divertor at core density when the electron temperature at the low-field side divertor plate is 1 eV, which is higher than the divertor detachment onset density.•The density limit was found approximately 20% higher in protium than in tritium and deuterium–tritium plasmas.
Measurements of the ion currents to and plasma conditions at the low-field side (LFS) divertor target plate in low-confinement mode plasmas in the JET ITER-like wall materials configuration show that the core plasma density required to detach the LFS divertor plasma is independent of the hydrogenic species protium, deuterium and tritium, and a 40 %/60 % deuterium–tritium mixture. This observation applies to a divertor plasma configuration with the LFS strike line connected to the horizontal part of the LFS divertor chosen because of its superior diagnostic coverage. The finding is independent of the operational status of the JET cryogenic pump. The electron temperature (Te) at the LFS strike line was markedly reduced from 25 eV to 5 eV over a narrow range of increasing core plasma density, and observed to be between 2 eV and 3 eV at the onset of detachment. The electron density (ne) peaks across the LFS plasma when Te at the target plate is 1 eV, and spatially moves to the X-point for higher core densities. The density limit was found approximately 20 % higher in protium than in tritium and deuterium–tritium plasmas.
Heat flux deposition profiles, calculated from infrared imaging, are presented for JET and MAST cases in the presence of external magnetic perturbations. In L-mode, a clear splitting of the ...strike-point is observed and well reproduced by modelling based on field line tracing with no plasma magnetic response included (vacuum field approximation). In H-mode, in contrast, the splitting is not clearly observed in spite of being expected from vacuum field modelling. As illustrated by modelling results, screening effects could explain the absence of a clear splitting in H-mode.
The snowflake 1,2 divertor is a proposal for solving the heat and particle exhaust problem in fusion grade plasmas. Turning the X-point into a second order null gives the possibility of radially ...expanding the poloidal flux in the divertor region much more than in a SD, increasing the connection length, redistributing the power load on a larger area and enhancing radiative losses. Since the efforts associated to the design of reactor-relevant configurations, like the snowflake, are large, ENEA is studying this configuration using efficient and flexible numerical tools to design and optimise tokamak equilibrium configurations. Such studies are applied to the Divertor Test Tokamak FAST, a satellite tokamak proposed for the European roadmap towards fusion.
PDF
