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Posted September 11th 2009
Nine years ago the Fusion Technology Task Force was established at JET. About 50 scientists and engineers are now working in this field. Under the umbrella of Fusion Technology you find scientific research, engineering development and modelling knowledge. Along with many others in the fusion community the group is looking forward to the ITER-Like Wall, which will be working at the cutting edge of the current technology.
Among the various possible reactions for controlled fusion, the Deuterium-Tritium (DT) reaction is certainly the best choice for future fusion machines, since it has by far the best energy yield. The Joint European Torus (JET) was built from the very beginning to run with DT and it is currently the only fusion machine with this capability until its successor ITER will start its operation.
During its 25 years of operation, JET has used tritium fuelling safely in three campaigns including an extended DT experimental programme in 1997-98. In addition, it has operated with beryllium components in the machine for more than 20 years, pioneering work towards fusion and opening up new areas of investigation. As a consequence JET gained tremendous experience in terms of Active Gas Handling, Remote Handling, Waste Management, Health Physics and Safety as well as a better understanding of transport, erosion and deposition of materials. These last processes control the interaction of the plasma with the first wall materials of the vacuum vessel and are of paramount importance for ITER. So JET set up the Fusion Technology (FT) Task Force Work Programme focused on six main areas of research.
The two faces of tritium
The DT fusion reaction produces helium, which is a completely harmless gas. However, part of the DT fuel does not react but is trapped and subsequently released by the plasma-facing materials or also exhausted along with the helium. Under the various FT Work Programmes, numerous tasks have been carried out investigating different methods of recovering the lost tritium (detritiation). For instance, in the field of solid, liquid and gas waste management, the Task Force launched a series of tasks aimed at tritium removal from any kind of waste that can be produced by a fusion facility. As an example, molecular sieves, widely used in all tokamaks, make a large contribution to the waste stream in any fusion plant. In order to reduce waste and disposal costs, a detritiation facility treating molecular sieves has been developed and successfully tested. Similar tasks have led to the production of a final design for water detritiation and gas purification systems.
During plasma operation ― everything is on the move
A major objective of Fusion Technology at JET is to carry out Scientific and Technical tasks related to Plasma-Wall Interactions and more specifically to the erosion of the first wall materials. This includes material transport inside the vacuum vessel, its interaction with the deuterium and tritium fuel and finally its co-deposition in remote areas of the machine producing so-called co-deposited layers. Various methods have been developed and tested for removing co-deposited layers and the recovery of tritium. To achieve these aims a close collaboration with several European Associations has been established, e. g. Institut de Fizica Atomica, Romania (Ion Gap Plasma Generator), Commissariat a l’Energie Atomique, France (Laser Induced Breakdown Spectroscopy; LIBS), Forschungszentrum Karlsruhe (Radio-Frequency) and last but not least UKAEA (photo-cleaning).
Laser light as a by-product remover
A particularly successful task consisted of using Laser Induced Breakdown Spectroscopy (LIBS) for analysis and removal of co-deposited layers on tiles directly in the JET vacuum vessel, which is known as in-situ analysis. The technique uses a highly energetic laser pulse which, when focused on a tile, excites and atomises the sample to form a plasma. The light emitted by it is characteristic of the elemental composition of the ablated material. LIBS can therefore not only detect co-deposited layers, but also remove them. The main advantage of this in-situ method is saving the time normally spent in removing the tiles remotely, cleaning and reinstalling them.
After preliminary experiments it was possible to focus the laser beam onto the surface of a divertor tile towards the bottom of the vessel. The divertor is the lower part of machine where the larger amounts of the co-deposited material can be found. The intensity of the laser was sufficient to ablate the deposited layer and create a bright plasma. This world-class series of experiments clearly demonstrated that LIBS is applicable for in-situ real-time analysis and clean-up of the JET divertor.
Over the past eight years, the Fusion Technology Work Programme has launched more than 132 tasks with a budget exceeding 21 M Euros and involving Fusion Associations across Europe. Throughout this period the Work Programme has helped to identify major issues related not only to the Plasma Wall Interaction and Tritium Retention but also to safety aspects. Moreover, Fusion Technology has investigated new materials and coatings, tried numerous detritiation techniques and demonstrated that in hi-tech experiments, progress is achieved by careful, patient work rather than in huge strides. With this perspective the team is looking forward to the new ITER-Like Wall project, which will require imput from many technologies, and where new mixed materials have to be tested under extraordinary conditions. However, armed with the vast experience gained over the past years Fusion Technology is ready to face the challenge.
Work programme of Task Force Fusion Energy
Tritium in the tokamak
After the DT campaign some of the tritium fuel remains trapped by the first wall materials of the vacuum vessel. This tritium amount must be precisely known.
Tritium process and waste management
The tritium trapped by various materials has to be recovered. For this purpose several processes must be developed and tested.
Plasma Facing Components
The interaction of the plasma with the first wall materials of the vacuum vessel produces a wide variety of by-products. Their transport and accumulation inside the fusion machine, as well as their removal, has to be investigated and well understood.
Determining the temperature of the plasma facing surfaces or the thickness and composition of the co-deposited layers is vitally important. For this purpose various techniques need to be developed and optimised for their use in the JET environment.
Neutronics and Safety
The neutrons produced during the normal fusion reaction react with all elements found in their path, generating some radionuclides. Determining these new elements and their precise contribution to the increase of the background dose rate is essential.
A test bed is a platform for experiments on large components that are foreseen to be used in ITER. Testing such components and optimising their operability is the final step before their manufacture.
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