Plasma-surface interaction in the Be/W environment: Conclusions drawn from the JET-ILW for ITER

Plasma operation with an all metal first-wall: Comparison of an ITER-like wall with a carbon wall in JET

Tungsten, the primary material under consideration as the divertor material in magnetic-confinement nuclear fusion reactors, has been known for the last decade to form ‘fuzz’—a layer of microscopic, high-void-fraction features on the surface—after only a few hours of exposure to helium plasma. Fuzz has also been observed in molybdenum, tantalum, and several other metals. Helium bubbles in tungsten and other metals are also known to change the hardness of the surface, accumulate at grain boundaries and dislocations, and increase hydrogen isotope retention. This article reviews helium- and hydrogen-induced surface evolution, including fuzz formation, in tungsten and other plasma-facing materials, as well as modeling and experimental efforts that have been undertaken to understand the mechanisms of fuzz formation, helium and hydrogen transport in plasma-facing materials, and relevant atomic-scale and electronic effects relevant to plasma-facing materials.

https://iopscience.iop.org/article/10.1088/2053-1591/aa8c22/pdf

Helium, hydrogen, and fuzz in plasma-facing materials

The JET ITER-Like Wall experiment (JET-ILW) provides an ideal test bed to investigate plasma-surface interaction (PSI) and plasma operation with the ITER plasma-facing material selection employing beryllium in the main chamber and tungsten in the divertor. The main PSI processes: material erosion and migration, (b) fuel recycling and retention, (c) impurity concentration and radiation have be1en studied and compared between JET-C and JET-ILW. The current physics understanding of these key processes in the JET-ILW revealed that both interpretation of previously obtained carbon results (JET-C) and predictions to ITER need to be revisited. The impact of the first-wall material on the plasma was underestimated. Main observations are: (a) low primary erosion source in H-mode plasmas and reduction of the material migration from the main chamber to the divertor ( factor 7 ) as well as within the divertor from plasma-facing to remote areas ( factor 30 - 50 ). The energetic threshold for beryllium sputtering minimises the primary erosion source and inhibits multi-step re-erosion in the divertor. The physical sputtering yield of tungsten is low as 10 - 5 and determined by beryllium ions. (b) Reduction of the long-term fuel retention ( factor 10 - 20 ) in JET-ILW with respect to JET-C. The remaining retention is caused by implantation and co-deposition with beryllium and residual impurities. Outgassing has gained importance and impacts on the recycling properties of beryllium and tungsten. (c) The low effective plasma charge ( Z eff = 1.2 ) and low radiation capability of beryllium reveal the bare deuterium plasma physics. Moderate nitrogen seeding, reaching Z eff = 1.6 , restores in particular the confinement and the L-H threshold behaviour. ITER-compatible divertor conditions with stable semi-detachment were obtained owing to a higher density limit with ILW. Overall JET demonstrated successful plasma operation in the Be/W material combination and confirms its advantageous PSI behaviour and gives strong support to the ITER material selection.

Natural and manufactured materials rely on complex hierarchical microstructures to deliver a suite of interesting properties. To predict and tailor their performance requires a joined-up knowledge of their multiphase microstructure, interfaces, chemistry and crystallography from the nanoscale to the macroscale. This Perspective reflects on how recent developments in correlative characterization can bring together multiple image modalities and maps of the local chemistry, structure and functionality to form rich multimodal and multiscale correlated datasets. The automated collection and digitization of multidimensional data is an essential part of the picture for developing multiscale modelling and ‘big data’-driven machine learning approaches. These are needed to both improve our understanding of existing materials and exploit high-throughput combinatorial synthesis, processing and testing methods to develop materials with bespoke properties. This Perspective explores correlative characterization for bringing together multiple imaging modalities with maps of local chemistry, structure and functional performance to improve our understanding and manufacturing of existing materials and facilitate data-centric materials innovation.

Completing the picture through correlative characterization

The new JET ITER-like wall (made of beryllium and tungsten) is more fragile than the former carbon fiber composite wall and requires active protection to prevent excessive heat loads on the plasma facing components (PFC). Analog CCD cameras operating in the near infrared wavelength are used to measure surface temperature of the PFCs. Region of interest (ROI) analysis is performed in real time and the maximum temperature measured in each ROI is sent to the vessel thermal map. The protection of the ITER-like wall system started in October 2011 and has already successfully led to a safe landing of the plasma when hot spots were observed on the Be main chamber PFCs. Divertor protection is more of a challenge due to dust deposits that often generate false hot spots. In this contribution we describe the camera, data capture and real time processing systems. We discuss the calibration strategy for the temperature measurements with cross validation with thermal IR cameras and bi-color pyrometers. Most importantly...

/pdf/a-protection-system-for-the-jet-iter-like-wall-based-on-56px5lkj2v.pdf

A protection system for the JET ITER-like wall based on imaging diagnostics

The ITER-like wall (ILW) at JET provides a unique opportunity to study the combination of material (beryllium and tungsten) that will be used for the plasma facing components (PFCs) in ITER. Both the limiters (Be) and divertor (CFC W coated and bulk W) have been designed to maximize their power handling capability. During the last experimental campaign (October 2010–July 2011) this capability has been assessed and even challenged in the case of the Be wall. The Be limiters' power handling capability (19 MW m−2 s−1/2), predicted with a simple model, has been proven to be robust by the experiments despite an unexpected power load pattern. This capability has been pushed to its limit leading to Be melt events, which revealed that the power load is toroidally asymmetric. The protection system of the ILW did not prevent melt events mainly because the protection strategy relies on the assumption that the power load is toroidally symmetric. The bulk W divertor target performed as predicted. Operations were constrained by: (i) an energy load limit (60 MJ m−2); (ii) the limited number of cycles of the surface temperature above 1200 °C in order to prevent thermal fatigue. This latter limit has been exceeded about 300 times and no signs of damage or thermal fatigue have been observed by the photogrammetric survey.

https://iopscience.iop.org/article/10.1088/0031-8949/2014/T159/014009/pdf

Power handling of the JET ITER-like wall

The JET ITER-like wall project (ILW) replaces the first wall carbon fibre composite tiles with beryllium and tungsten tiles which should have improved fuel retention characteristics but are less thermally robust. An enhanced protection system using new control and diagnostic systems has been designed which can modify the pre-planned experimental control to protect the new wall. Key design challenges were to extend the Level-1 supervisory control system to allow configurable responses to thermal problems to be defined without introducing excessive complexity, and to integratethe new functionalitywith existing control and protection systems efficiently and reliably. Alarms are generated by the vessel thermal map (VTM) system if infra-red camera measurements of tile temperatures are too high and by the plasma wall load system (WALLS) if component power limits are exceeded. The design introduces two new concepts: local protection, which inhibits individual heating components but allows the discharge to proceed, and stop responses, which allow highly configurable early termination of the pulse in the safest way for the plasma conditions and type of alarm. These are implemented via the new real-time protection system (RTPS), a centralised controller which responds to the VTM and WALLS alarms by providing override commands to the plasma shape, current, density and heating controllers. This paper describes the design and implementation of the RTPS system which is built with the Multithreaded Application Real-Time executor (MARTe) and will present results from initial operations.

Centralised coordinated control to protect the jet iter-like wall ∗

Thermal response up to melting of the exposed edge of aWlamella in the JET divertor

ELM divertor heat load in JET-ILW and full-W ASDEX Upgrade

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