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ITER-Like Wall at JET

Forschungszentrum Jülich designed and constructed a divertor, which is part of the new first wall in the leading fusion experiment, JET - the "ITER-like wall". This wall consists of beryllium and tungsten, while the divertor is made entirely of tungsten, the material with the highest melting point. This metal, which only melts at a temperature of 3,455 °C, will also be used at a later stage in ITER. Jülich scientists are currently testing their promising design at JET.

The Joint European Torus (JET), currently the world’s largest tokamak, is blazing a trail in fusion research – particularly as regards the construction of its successors, ITER and DEMO. Originally put into operation in 1984 as a metal limiter machine, JET has experienced major alterations and been adapted to reflect current knowledge several times over the last 30 years. After a graphite wall was introduced and a divertor installed, for example, it was possible at JET in 1997 to achieve fusion plasmas with a fusion power gain of 0.64 using a 1:1 mixture of deuterium and tritium - thus coming very close to break-even point. This record still stands today. With this achievement, JET successfully demonstrated the physical feasibility of fusion.

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An element of the external divertor collector plate made of solid tungsten elements in a lamella structure, developed for JET by Forschungszentrum Jülich.

However, these experiments in the carbon era also brought to light several problems surrounding the use of graphite as a wall material. These included severe erosion and high fuel retention – i.e. the storage of the deuterium and tritium fuel gases in loosely bound co-deposits located below the graphite surface – even in areas at some distance from the plasma, such as underneath the divertor or in slots between the graphite tiles. The erosion of the wall material, fuel retention, and methods of cleaning the first wall to release the fuel gas again have been at the centre of research on plasma-wall interaction ever since. Consequently, the focus of research worldwide was adjusted to understand and control material migration, develop cleaning methods, and finally to find and categorize materials for the first wall as alternatives to graphite. Forschungszentrum Jülich is one of the leading institutions involved in this area of plasma-wall interaction as well as in many others.

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JET vacuum vessel. Left: Graphite wall (CFC) in 2009; right: the “ITER-like wall” in 2011 – vacuum vessel wall made of beryllium and, at the bottom of the image, the divertor with tungsten lamellae developed and constructed by Forschungszentrum Jülich.

In 2006, the decision was taken in Europe to carry out a further modification to JET by replacing the entire first wall made of graphite – around 10,000 tiles – with alternative materials using remote-controlled robotic arms. This primarily involved installing plasma-facing components made of beryllium in the main chamber, as would later be done at ITER. Beryllium is an element that is not susceptible to chemical erosion and has a low atomic number. The lower sputtering leads to lower fuel retention in remote areas and an easier release of fuel gas via cleaning methods, while its low atomic number results in a tolerable impurity concentration in the fusion plasma. However, beryllium has a considerably lower damage threshold than graphite with respect to thermal loads and is therefore unsuitable as a material for plasma-facing divertor components, which are responsible for energy and particle removal. In both JET and ITER, this task was entrusted to the element with the highest melting point (3,422 °C) – tungsten. Although this element is the most reliable wall material known to researchers, it unfortunately has a very high atomic number. This is why only minute traces of the element are permitted as an impurity in the vacuum vessel, before the fusion plasma is cooled down and finally extinguished due to excessive radiation of electromagnetic energy.

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Developed for JET by Forschungszentrum Jülich: an endoscope – a special optical observation system for measuring tungsten impurity concentration in the divertor without physical contact.

Crucial factors here are both the release of tungsten, which is caused by material erosion upon contact with the plasma due to impurities such as oxygen or carbon, as well as the transport properties of the element in the central plasma following its release. Scenarios must therefore be developed that ensure compatibility of the wall materials with the plasma and that prevent excessive tungsten erosion, and thus too high a concentration of impurities. Erosion is minimized in particular using radiative cooling with gases injected from outside, which makes it possible to stay below the physical sputtering threshold and thus leave the material undamaged. The radiative cooling in the divertor also prevents the tungsten tiles from being exposed to excessive thermal loads and destroyed by melting.

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Important for walls made from tungsten: a fast gas injection system – the disruption mitigation valve (DMV), which simply blows out emerging instabilities in the plasma boundary layer. Forschungszentrum Jülich developed such a system for JET.

The potential influence of transient - i.e. short-term - phenomena such as edge localized modes (ELMs) or disruptions is particularly critical for tungsten. These can cause irreparable damage to the tungsten tiles. Controlling this phenomenon is indispensable for ensuring the continuous and stable operation of a fusion reactor with tungsten walls. Current research priorities at Jülich include determining the sources of impurities in tungsten, tungsten transport, developing scenarios for reactor operation with tungsten using radiative cooling, controlling transient events (disruptions, ELMs) through targeted massive gas injection, and investigating the effects of uncontrolled phenomena on wall material and plasma operation.

The ITER-like wall experiment at JET, which acts as a reference case for the active phase of ITER, was successfully put into operation in September 2011. Preparatory work was carried out in experiments in 2008 and 2009 to develop suitable configurations and scenarios that would be compatible with the new boundary conditions. In addition, reference scenarios were developed for the next JET plasmas in order to document the expected decrease in carbon impurity concentration and the associated reduction in fuel retention and material transport.

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The disruption mitigation valve system in operation at JET: the increasing radiation of light energy as a result of gas injection can be seen above, and below is a list of the most important data for the relevant plasma experiment at JET.

On the basis of its extensive expertise, Forschungszentrum Jülich was commissioned to design and construct the divertor of the ITER-like wall at JET and one of its researchers was appointed to the position of Project Scientist. Forschungszentrum Jülich was responsible for designing the plasma-facing components and their maximum load limits, developing suitable solutions such as the massive tungsten divertor for controlling the highest thermal loads, testing these components in plasma-, ion-, and electron-beam facilities, and drawing on its know-how in the modelling and simulation of erosion and operation at peak load. In order to ensure scientific operation, Forschungszentrum Jülich also plays a leading role in the development and construction of diagnostics systems that determine the erosion sources and impurities in the main chamber and the divertor. Jülich also coordinated the measurement of electromagnetic radiation caused by impurities, the construction and installation of a fast gas injection system for preventing and controlling disruptions, and the design of perturbation coils for suppressing edge layer modes.

Forschungszentrum Jülich is currently represented by a task force leader at JET in order to address these very issues. The first year of operation demonstrated the expected reduction in sputtering yield, reduced transport to remote areas, and considerable overall reduction in fuel retention by one order of magnitude, ensuring the safer operation of ITER with respect to the accepted fuel inventory. Dedicated experiments were carried out to qualify the design of the full tungsten divertor and demonstrated the feasibility of using tungsten in the most critical areas. The JET tokamak has been used as a test-bed for ITER and Jülich scientists have played a key role in its exploitation.


Contact

Dr. Sebastijan Brezinsek

Tel. +49 2461 61-6611
Fax +49 2461 61-2660
s.brezinsek@fz-juelich.de


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