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

Abstract

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...

Erosion of tungsten marker layers in W7-X

“…It was already concluded earlier that the erosion of tungsten at the divertor under attached plasma conditions is mainly caused by sputtering by light impurities, especially by B, C and O ions [38,39], while in ELMy H-mode the erosion during ELMs by accelerated fuel-and impurity ions [40] can exceed the erosion in between ELMs considerably [41]. As up to now no ELMs were observed in W7-X, the observed erosion of tungsten at the divertor is due to the attached plasma conditions with relatively high plasma temperatures at the strike line.…”

Summary of the 27th IAEA Fusion Energy Conference in the categories of EX/W, EX/D, and ICC

“…Figure 31 shows the erosion and deposition distribution along the outer target plate in the conditions at the OSP. The ERO code can reproduce the large gross erosion, the low net erosion, and thus the local high re-deposition on tile 5 with more than 94% in average and at 99% for the sputtering dominating intra-ELM phase [59].…”

Modelling of tungsten contamination and screening in WEST plasma discharges

“…H-mode discharges were not modelled in this work. Nevertheless, it has to be remarked that the presence of edge localised modes during H-mode can have a vital impact on machines equipped with W PFCs [10]. The paper is organized as follows: in section 2 a representative WEST plasma discharge and the corresponding plasma simulations are presented; in section 3 the set-up of the MC simulations tracking W impurities is described; in section 4 the role of the different PFCs in WEST W contamination from the simulations is discussed followed by an analysis of the influence of light impurities on the capability of heavy wall impurities to reach the separatrix in section 5; in section 6 the role of self-sputtering in plasma contamination in different plasma conditions is discussed; finally, in section 7 a direct comparison with experimental data of the WI emission line from visible spectroscopy is carried out.…”