Posted on: 10th March 2017
Fusion devices may go into shutdown, but the scientists, engineers and technicians certainly do not. EUROfusion’s flagship device JET is currently in the middle of an engineering shutdown, but preparations are already on for the next experimental campaigns. Our colleagues from the Culham Centre for Fusion Energy (CCFE), where JET is located, had a look at what is going on during this preparatory phase:
Already the Remote Handling team has performed two key tasks that will benefit European fusion researchers using the machine. Using the MASCOT system, which allows them to work remotely inside JET with tools fitted to a manipulator, they have calibrated the detector that measures neutrons – the all-important indicator of power from fusion reactions. They have also carried out a photographic survey of the vacuum vessel to inspect the condition of the wall and assess how its materials are being affected by the hot plasma that fuels JET.
With JET’s next full fusion power experiments using deuterium and tritium fuel planned for 2019, it is important that the neutron detector gives highly accurate measurements of the more energetic (14 MeV) neutrons that will be produced during these tests.
A special neutron calibration was therefore needed, with a particular source able to produce neutrons with the same energy range as those produced in deuterium-tritium plasmas. A neutron generator, which is a small accelerator able to generate neutrons of 14 MeV, has been deployed inside the torus by the JET remote handling system – the first time a particle accelerator has been put inside a tokamak!
In the neutron generator, a mixed deuterium/tritium beam is accelerated onto a fixed target containing deuterium and tritium; thus producing neutrons with a similar energy by replicating the processes that will occur in a JET plasma.
A team of scientists from the JET Neutron and Gamma group and other European laboratories (ENEA-Frascati, CNR-Milan, IPPLM-Warsaw, KIT – Karlsruhe, JSI-Ljubljana and the University of Uppsala) came up with a design that allowed accurate calibration of the neutron generator during two experimental campaigns carried out at the National Physical Laboratory in London.
EUROfusion Project Leader Paola Batistoni commented: “Designing the electrical wiring and fitting all equipment to MASCOT has represented another major part of the project for which the JET Remote Handling team has played a key role. The whole set of instrumentation has to work perfectly when inside the JET vessel as, in case of a failure, it would be impossible to intervene without bringing the whole assembly back to the laboratory. From this point of view it is like having instruments working on a satellite.”
The methodology developed and the experience gained in this project will provide unique tests and guidance for the calibration procedure to be adopted in the next-generation ITER fusion machine in France.
High Resolution Photographic Survey
The next step for the Remote Handling team was to undertake a high resolution photographic survey of the interior of JET.This process involves taking pictures of the vacuum vessel’s inner wall using special photogrammetry and stereo cameras mounted on the remote handling boom. Photogrammetry consists of a single lens camera collecting images of multiple targets from a variety of locations to derive accurate coordinates. Stereo photography uses a matching pair of cameras mounted in a frame, pointing to a focus point.
The images obtained with this system are used to determine the condition of the wall following the 2016 JET experiments. They are compared to the images taken during the final stages of the previous JET shutdown, thus determining deterioration of the tile coatings and melt damage to the beryllium tiles.
The positioning of the camera has to be extremely precise and therefore requires excellent remote handling navigation skills. The operator uses the camera images in conjunction with a virtual reality model of the vessel and the MASCOT to allow accurate identification of targets. The system has to be positioned at 90 different locations inside the vessel, enabling local measurements accurate to ± 0.4mm. This delicate technique requires time – approximately two days of remote operations – and specialised equipment.
This operation was successfully completed last month and the images are now being reviewed to aid future JET research – and, crucially, that of ITER, which will use the same wall materials.