JET ITER-like Wall Research Articles

First results and surface analysis strategy for plasma-facing components after JET operation with the ITER-like wall

During the carbon wall operations of JET since 2001, an extensive post-mortem analysis programme has been carried out under the JET Task Force Fusion Technology and a similar analysis programme is underway for the JET-ILW tiles removed during the 2012 shutdown. The first post-mortem results from the JET ITER-like wall tiles have shown that the overall amount of deposition on the divertor tiles and on remote divertor areas has been reduced by more than an order of magnitude with respect to JET-C. In addition, the obtained data indicate a possible interaction between Be and W such as the formation of mixed Be–W layers. This could be due to the surface roughness of the tiles, or could be caused by diffusion or even alloying. Ion-beam analyses and secondary ion mass spectrometry techniques give only elemental information, so other techniques such as x-ray diffraction, x-ray photoelectron spectroscopy, secondary electron microscopy/energy dispersive x-ray spectroscopy and nuclear microprobing are required. Since the nature of deposition and erosion has changed during the JET-ILW operations, a change in the post-mortem analysis programme is needed. For example, no cross-sectional samples from the sloping parts of tiles 4 and 6 are required. A strategy for post-mortem analyses of the marker-coated tiles will be presented in this paper.

Operational impact on the JET ITER-like wall in-vessel components

The JET ITER-like Wall (ILW) provides the same plasma facing component configuration as ITER during its active phase: beryllium in the main chamber and tungsten in the divertor. Moving from a carbon-based wall to an all metals wall requires some operational adjustment. The reduction in radiation at the plasma edge and in the divertor can lead to high power loads on the plasma facing components both in steady state and in transients and requires the development of radiative scenarios and the use of massive gas injection to mitigate disruptions. These tools are even more important now because an all metal wall is much less forgiving to thermal overloading the carbon based wall used to be. Here the impact of the first 11 months of operation on the ILW plasma facing components is discussed.

On the challenge of plasma heating with the JET metallic wall

The major aspects linked to the use of the JET auxiliary heating systems: NBI, ICRF and LHCD, in the new JET ITER-like wall are presented. We show that although there were issues related to the operation of each system, efficient and safe plasma heating was obtained with room for higher power. For the NBI up to 25.7 MW was safely injected; issues that had to be tackled were mainly the beam shine-through and beam re-ionization before its entrance into the plasma. For the ICRF system, 5 MW were coupled in L-mode and 4 MW in H-mode; the main areas of concern were RF sheaths related heat loads and impurities production. For the LH, 2.5 MW were delivered without problems; arcing and generation of fast electron beams in front of the launcher that can lead to high heat loads were the keys issues. For each system, an overview will be given of: the main modifications implemented for safe use, their compatibility with the new metallic wall, the differences in behaviour compared with the previous carbon wall, with emphasis on heat loads and impurity content in the plasma.

Development of an efficient real-time disruption predictor from scratch on JET and implications for ITER

Prediction of disruptions from scratch is an ITER-relevant topic. The first operations with the new ITER-like wall constitute a good opportunity to test the development of new predictors from scratch and the related methodologies. These methodologies have been based on the Advanced Predictor Of DISruptions (APODIS) architecture. APODIS is a real-time disruption predictor that is in operation in the JET real-time network. Balanced and unbalanced datasets are used to develop real-time predictors from scratch. The discharges are used in chronological order. Also, different criteria to decide when to re-train a predictor are discussed. The best results are obtained by applying a hybrid method (balanced/unbalanced datasets) for training and with the criterion of re-training after every missed alarm. The predictors are tested off-line with all the discharges (disruptive/non-disruptive) corresponding to the first three JET ITER-like wall campaigns. The results give a success rate of 93.8% and a false alarm rate of 2.8%. It should be considered that these results are obtained from models trained with no more than 42 disruptive discharges.

Results of the JET real-time disruption predictor in the ITER-like wall campaigns

The impact of disruptions in JET became even more important with the replacement of the previous Carbon Fiber Composite (CFC) wall with a more fragile full metal ITER-like wall (ILW). The development of robust disruption mitigation systems is crucial for JET (and also for ITER). Moreover, a reliable real-time (RT) disruption predictor is a pre-requisite to any mitigation method. The Advance Predictor Of DISruptions (APODIS) has been installed in the JET Real-Time Data Network (RTDN) for the RT recognition of disruptions. The predictor operates with the new ILW but it has been trained only with discharges belonging to campaigns with the CFC wall. 7 real-time signals are used to characterize the plasma status (disruptive or non-disruptive) at regular intervals of 32ms. After the first 3 JET ILW campaigns (991 discharges), the success rate of the predictor is 98.36% (alarms are triggered in average 426ms before the disruptions). The false alarm and missed alarm rates are 0.92% and 1.64%.

Design, manufacture and initial operation of the beryllium components of the JET ITER-like wall

The aim of the JET ITER-like wall project was to provide JET with the plasma facing material combination now selected for the DT phase of ITER (bulk beryllium main chamber limiters and a full tungsten divertor) and, in conjunction with the upgraded neutral beam heating system, to achieve ITER relevant conditions.The design of the bulk Be plasma facing components had to be compatible with increased heating power and pulse length, as well as to reuse the existing tile supports originally designed to cope with disruption loads from carbon based tiles and be installed by remote handling. Risk reduction measures (prototypes, jigs, etc.) were implemented to maximize efficiency during the shutdown. However, a large number of clashes with existing components not fully captured by the configuration model occurred.Restarting the plasma on the ITER-like Wall proved much easier than for the carbon wall and no deconditioning by disruptions was observed. Disruptions have been more threatening than expected due to the reduced radiative losses compared to carbon, leaving most of the plasma magnetic energy to be conducted to the wall and requiring routine disruption mitigation. The main chamber power handling has achieved and possibly exceeded the design targets.

The JET ITER-like wall experiment: First results and lessons for ITER

The JET programme is strongly focused on preparations for ITER construction and exploitation. To this end, a major programme of machine enhancements has recently been completed, including a new ITER-like wall, in which the plasma-facing armour in the main vacuum chamber is beryllium while that in the divertor is tungsten—the same combination of plasma-facing materials foreseen for ITER. The goal of the initial experimental campaigns is to fully characterise operation with the new wall, concentrating in particular on plasma-material interactions, and to make direct comparisons of plasma performance with the previous, carbon wall. This is being done in a progressive manner, with the input power and plasma performance being increased in combination with the commissioning of a comprehensive new real-time protection system. Progress achieved during the first set of experimental campaigns with the new wall, which took place from September 2011 to July 2012, is reported.

Modeling of the impact of runaway electrons on the ILW in JET

A major concern for ITER operation in H-mode with high fusion gain is the occurrence of disruptions which can damage plasma-facing components (PFCs) and therefore limit their lifetime. Moreover, runaway electrons (REs) can be generated and produce additional damage to the first wall. The closest experiments to study and verify numerical codes for ITER is JET ITER-like wall experiments with Be first wall and W divertor. Predictive numerical simulations of RE impact on the typical Be tiles are performed using the codes ENDEP and MEMOS. This work is focused on estimations of after effects of RE impact on the JET Be first wall (FW) and determination of critical parameters of RE beams resulting in melting of Be tiles. Simulations predict that Be PFC melting can occur at relatively low current density of RE beam ranged between 2 and 12kA/m2.

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

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, we demonstrate that a protection system based on CCD cameras can work and show examples of hot spot detections that stop the plasma pulse. The limits of such a design and the associated constraints on the operations are also presented.

Upgrade of the infrared camera diagnostics for the JET ITER-like wall divertor

For the new ITER-like wall at JET, two new infrared diagnostics (KL9B, KL3B) have been installed. These diagnostics can operate between 3.5 and 5 μm and up to sampling frequencies of ∼20 kHz. KL9B and KL3B image the horizontal and vertical tiles of the divertor. The divertor tiles are tungsten coated carbon fiber composite except the central tile which is bulk tungsten and consists of lamella segments. The thermal emission between lamellae affects the surface temperature measurement and therefore KL9A has been upgraded to achieve a higher spatial resolution (by a factor of 2). A technical description of KL9A, KL9B, and KL3B and cross correlation with a near infrared camera and a two-color pyrometer is presented.

Plasma Facing Materials for the JET ITER-Like Wall

The chosen materials for plasma facing components for the deuterium/tritium phase of ITER are beryllium and tungsten. These materials have already been widely investigated in various devices like ion beam or electron beam tests. However, the operation of this material combination in a large tokamak including plasma wall interaction, material degradation, erosion and material mixing has not been proven yet.The ITER-like Wall, which has been recently installed in JET, consists of a combination of bulk tungsten and tungsten coated CFC divertor tiles as well as bulk beryllium and beryllium coated INCONEL in the main chamber. The experiments in JET will provide the first fully representative test of the ITER material choice under relevant conditions.This paper concentrates on material research and developments for the materials of the JET ITER-like Wall with respect to mechanical and thermal properties. The impact of these materials and components on the JET operating limits with the ITER-like Wall and implications for the ongoing scientific program will be summarised.

JET ITER-like wall—overview and experimental programme**See the appendix of Romanelli F et al 2010 Proc. 23rd IAEA Fusion Energy Conf. (Daejeon, Korea).

This paper reports the successful installation of the JET ITER-like wall and the realization of its technical objectives. It also presents an overview of the planned experimental programme which has been optimized to exploit the new wall and other JET enhancements in 2011/12.

Integrated modelling of a JET type-I ELMy H-mode pulse and predictions for ITER-like wall scenarios

A baseline type-I ELMy H-mode JET tokamak discharge in low triangularity has been analysed using the JINTRAC integrated code suite to obtain a self-consistent description of the plasma edge dynamics and core plasma confinement. The inter- and intra-ELM transport model of JINTRAC has been adapted as such to match the experimental pre- and post-ELM plasma pedestal profiles and at the same time to recover the observed ELM dynamics in terms of ELM frequency, ELM energy loss, target ELM wetted area and target heat flows. The presented and validated modelling results for the JET all-carbon device reference case are utilized to predict a type-I ELMy H-mode for the JET ITER-like wall (ILW) assuming a full-tungsten divertor and beryllium main-chamber wall using JINTRAC. By keeping all relevant transport parameters fixed for the inter- and intra-ELM phase as in the all-carbon reference case it is observed that a moderate amount of seeded neon impurity is necessary to compensate for a similar level of radiation when carbon is absent in the system. The results of the ILW model setup are finally taken to estimate the total amount of tungsten particles eroded per ELM from the target plates. A rough estimate of the core radiative fraction due to W accumulation is given, predicting that no substantial impact on typical a JET ILW type-I ELMy H-mode discharge core plasma performance is expected.

X-ray microbeam transmission/fluorescence method for non-destructive characterization of tungsten coated carbon materials

Tungsten erosion, its subsequent transport and redeposition are of great interest, because a full tungsten divertor is foreseen to be used during the deuterium–tritium operational phase of ITER. The erosion of tungsten and carbon marker layers was extensively studied in the outer divertor of ASDEX Upgrade (AUG), and work is currently in progress to completely replace the existing JET CFC tiles with tungsten-coated tiles within the JET ITER-like wall project. The need for fast and non-destructive method which allows the quantitative determination of the thickness of a tungsten coating on a carbon material on large areas led us to evaluate a combined absorption/fluorescence X-ray (XRTF) technique. The method can provide fast analysis, high spatial resolution and a material selective detection of deposited layers and inclusions. It was applied on W coated fine grain graphite (FGG) tiles from AUG's divertor. It is proved that the method is able to provide information about the uniformity of the tungsten coating on a graphite or CFC substrate whilst the technique can be used to determine the thickness of the tungsten and other marker materials coatings. It represents a unique instrument for the post-mortem analysis of the coatings.

Coupling between JET pedestal ne–Te and outer target plate recycling

Systematic scans using D2 gas puffing and impurity-seeding (Ne, N2) have been carried out on ELMy-H and Advanced-Tokamak-like discharges to study power deposition amelioration at the divertor target plates in preparation for the JET ITER-Like Wall (ILW). Hereby it has been discovered that the recycling ion fluxes Idiv to the outer target plate exhibit a clear connection to pedestal ne and Te, whereby the Dα photon flux ΦDα from the outer divertor serves here as an approximate measure of Idiv. ΦDα can be expressed as (ne/Te)β with β=2.5/3 depending on the scenario (EH/AT). This coupling between SOL and pedestal across the edge transport barrier (ETB) is associated with a close connection between ne and Te in the ETB over the entire range of operational parameters, with the slope of logTe vs. logne being ∼0.9/1.24 for ET/AT scenarios, respectively.

Enhancement of JET’s mirror-link near-ultraviolet to near-infrared divertor spectroscopy system

Since 1994, JET has had a mirror-link spectroscopy system with a poloidal view of 150 mm of the outer divertor split into three ranges: near-ultraviolet (near-UV) (∼ 300–450 nm), visible (450–750 nm), and near-infrared (near-IR) (750–1200 nm). The system consists of three Czerny–Turner/charge coupled device (CCD) pairs: 1 m focal length for the near-UV, 0.75 m focal length for the visible, and 0.5 m focal length for the near-IR. All were aligned along the same optical path to the divertor. As part of the JET ITER-like wall enhancements, the diagnostic system will be upgraded in five areas: (1) frame rate, (2) quantum efficiency (QE), (3) radial coverage, (4) optical throughput, and (5) for the near-UV, spectral resolution and survey capability. New CCDs for the near-UV and visible will have increased QE and allow three times frame rate. The near-UV will benefit from a 0.75 m imaging spectrometer with three gratings. The optics have been redesigned to allow ∼ 360 mm view and greater than two times throughput. This paper will look at the design and implementation as well as the new diagnostic capabilities of the system.

Development of divertor tungsten coatings for the JET ITER-like wall

The main objectives of the JET ITER-like Wall Project are to provide a beryllium main wall and tungsten divertor with at least a 4 year lifetime to allow full evaluation of the materials and related plasma scenarios for ITER. Tungsten coatings will be used over most of the divertor area and this paper describes the latest developments in the coating technology and an analysis of the implications for the coating lifetime and machine operation. Both steady state and transient heat loads are assessed.

Testing of beryllium marker coatings in PISCES-B for the JET ITER-like wall

Beryllium has been chosen as the first wall material for ITER. In order to understand the issues of material migration and tritium retention associated with the use of beryllium, a largely beryllium first wall will be installed in JET. As part of the JET ITER-like wall, beryllium tiles with marker coatings are proposed as a diagnostic tool for studying the erosion and deposition of beryllium around the vessel. The nominal structure for these coatings is a ∼10 μm beryllium surface layer separated from the beryllium tile by a 2–3 μm metallic inter-layer. Two types of coatings are tested here; one with a nickel inter-layer and one with a copper/beryllium mixed inter-layer. The coating samples were deposited by DC magnetron sputtering at General Atomics and were exposed to deuterium plasma in PISCES-B. The results of this testing show that the beryllium/nickel marker coating would be suitable for installation in JET.

Power deposition modelling of the ITER-like wall beryllium tiles at JET

A precise geometric method is used to calculate the power deposition on the future JET ITER-Like Wall beryllium tiles with particular emphasis on the internal edge loads. If over-heated surfaces are identified, these can be modified before the machining or failing that actively monitored during operations. This paper presents the methodology applied to the assessment of the main chamber beryllium limiters. The detailed analysis of one limiter is described. The conclusion of this study is that operation will not be limited by edges exposed to plasma convective loads.

Analysis and design of the beryllium tiles for the JET ITER-like wall project

Work is in progress to completely replace, in 2008/9, the existing JET CFC tiles with a configuration of plasma facing materials consistent with the ITER design. The ITER-like wall (ILW) will be created with a combination of beryllium (Be), tungsten (W), W-coated CFC and Be-coated inconel tiles, with the material depending on the local anticipated heat flux and geometry. Over 4000 tiles will be replaced and the ILW will accommodate additional heating up to at least 50 MW for 10 s. One of the objectives is to maintain or improve the existing CFC tile power handling performance which has been achieved in most cases by hiding bolt holes, optimising tile size and profile and introducing castellations on plasma facing surfaces. This paper describes the generic problems associated with the Be tiles (power handling capacity and disruption induced eddy currents) and illustrates the solution selected for the inner wall guard limiter (IWGL) where the present CFC tiles will be replaced with Be.