Following the first JET ITER-like wall operations a detailed in situ photographic survey of the main chamber and divertor was completed. In addition, a selection of tiles and passive diagnostics were removed from the vessel and made available for post mortem analysis. From the photographic survey and results from initial analysis, the first conclusions regarding erosion, deposition, fuel retention and material transport during divertor and limiter phases have been drawn. The rate of deposition on inner and outer base divertor tiles and remote divertor corners was more than an order of magnitude less than during the preceding carbon wall operations, as was the concomitant deuterium retention. There was however beryllium deposition at the top of the inner divertor. The net beryllium erosion rate from the mid-plane inner limiters was found to be higher than for the previous carbon wall campaign although further analysis is required to determine the overall material balance due to erosion and re-deposition.

https://iopscience.iop.org/article/10.1088/0031-8949/2014/T159/014010/pdf

Material migration patterns and overview of first surface analysis of the JET ITER-like wall

In order to preserve the integrity of large tokamaks such as ITER, the number of disruptions has to be limited. JET has operated previously with a low frequency of disruptions (i.e., disruption rate) of 3.4% [P. C. de Vries et al., Nucl. Fusion 51, 053018 (2011)]. The start of operations with the new full-metal ITER-like wall at JET showed a marked rise in the disruption rate to 10%. A full survey was carried out to identify the root causes, the chain-of-events and classifying each disruption, similar to a previous analysis for carbon-wall operations. It showed the improvements made to avoid various disruption classes, but also indicated those disruption types responsible for the enhanced disruption rate. The latter can be mainly attributed to disruptions due to too high core radiation but also due to density control issues and error field locked modes. Detailed technical and physics understanding of disruption causes is essential for devising optimized strategies to avoid or mitigate these events.

/pdf/the-influence-of-an-iter-like-wall-on-disruptions-at-jet-5fd9kvzph8.pdf

The influence of an ITER-like wall on disruptions at JET

ELM induced tungsten melting and its impact on tokamak operation

Global erosion and deposition patterns in JET with the ITER-like Wall

The impact of tungsten dust originating from divertor plates on the performance of edge plasma in ITER-like discharge is evaluated using computer modeling with the coupled dust-plasma transport code DUSTT-UEDGE. Different dust injection parameters, including dust size and mass injection rates, are surveyed. It is found that tungsten dust injection with rates as low as a few mg/s can lead to dangerously high tungsten impurity concentrations in the plasma core. Dust injections with rates of a few tens of mg/s are shown to have a significant effect on edge plasma parameters and dynamics in ITER scale tokamaks. The large impact of certain phenomena, such as dust shielding by an ablation cloud and the thermal force on tungsten ions, on dust/impurity transport in edge plasma and consequently on core tungsten contamination level is demonstrated. It is also found that high-Z impurities provided by dust can induce macroscopic self-sustained plasma oscillations in plasma edge leading to large temporal variations of edge plasma parameters and heat load to divertor target plates.

Tungsten dust impact on ITER-like plasma edge

Transient impurity events leading to an unexpected increase in radiated power have been studied in JET from the installation of the ITER-like wall. A total of 1800 events over 2800 plasma discharges have been detected. None have led to permanent changes in the plasma conditions. Of all the events 60% show traces of W and 25% of either Ni, Fe or Cr from either Inconel or steel structures. They occur mainly in diverted magnetic configuration, independently of strike-point position. The effect of disruptions on dust redistribution has been investigated using the Thomson scattering diagnostic and correlated with transient impurity event occurrence. The number of dust events detected increases with disruption force and, in comparison to the full-C wall, the amount of dust mobilized is found to be about an order of magnitude lower. Their time evolution correlates well with that of the transient impurity events.

/pdf/transient-impurity-events-in-jet-with-the-new-iter-like-wall-4u84pccvs8.pdf

J.C. Flannegan

Papers

Transient impurity events in JET with the new ITER-like wall