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Posted April 10th 2017
All of EUROfusion’s experimental campaigns are dedicated towards making ITER’s plasmas a success. But fusion plasmas are not yet fully understood. Hence, Fusion in Europe has put a magnifier inside TCV, ASDEX-Upgrade, MAST-U and WEST. Those tokamaks might be considerably smaller than their big brother JET, but they help to address important problems like the control of ELMs, how to flexibly operate different plasma regimes, and testing materials for ITER.
The high-confinement mode and its ELMs
The holy grail of fusion research is to find an operational regime which produces high performance plasmas without the observed and unwanted bursts of energy and particles. These bursts are called ELMs (Edge Localised Modes) and can result in heat loads which damage the plasma facing components. For this reason, they should, at least, be controlled.
ELMs appear in the most commonly used regime of tokamak operation, the so-called high confinement mode (H-mode). This mode develops when plasmas are heated above a certain power threshold, which increases with density, magnetic field and machine size. The H-mode regime has the advantage of creating plasma which maintains (confines) the energy for a longer time compared to other modes.
This year, EUROfusion will take advantage of the flexibility of TCV and ASDEX-Upgrade in order to further investigate methods for ELM control.
In search of “small ELM or ELM-free” regimes
Controlling ELMs is one way of solving the problem. Another one is the investigation of other plasma regimes that feature high confinement while simultaneously creating small “not damaging” ELMs or no ELMs at all.
One of these regimes is the so-called ‘Quiescent H-Mode’ (QH-mode). It has been investigated on several tokamaks before, amongst them ASDEX and JET, but has only been tested on the systems when they had a vessel wall made of carbon. However, fusion power plants are expected to have tungsten walls. Since ASDEX-Upgrade and JET have now been upgraded to an inner wall made of tungsten, EUROfusion has decided to operate them again in QH-mode. As the general plasma properties change while going from a carbon to a metallic wall, finding out the impact of the wall on the QH-mode will be interesting.
Another promising ELM-free regime is the “I-mode”, which has a thermal but no particle barrier so that it can reach high energy confinement but does not confine impurities. It has been studied mainly on Alcator C-Mod and ASDEX Upgrade before. Further experiments are planned this year on AUG and TCV, and in 2018 on MAST-U and WEST.
“Naturally ELM-free regimes would be a huge advantage!”, states Amanda Hubbard from the Plasma Science and Fusion Center (Massachusetts Institute of Technology) who has devoted her research to the study of I-Mode regimes.
Overall, these approaches seem to offer the best solutions, but are still very much a work in progress. Nevertheless, Amanda Hubbard is excited to see the outcome:
“Great progress has been made in understanding and increasing the operating conditions, particularly density”.
Keeping it together
Disruptions are just additional challenges in fusion experiments. Plasma is an ionised gas. Magnets in the tokamak keep it in shape and thus prevent the metal wall from being melted by the strong heat and particle flux. But in the event of disruptions, the magnetic confinement gets lost. Consequently, the ionised gas hits the vessel wall. It is highly energised and damages the metallic surface. Occasionally, electron runaway beams follow a disruption. These also often result in strong heat loads and forces exerted on to the wall.
The fusion research community has already developed methods designed to avoid or as the scientists say “to mitigate” those disruptions. Testing the effectivity of those methods at TCV and ASDEX-Upgrade will be one of the tasks of EUROfusion’s programme this year.
“We are increasingly creating the synergies between all these machines. We have to, in particular, greatly facilitate the exchange of information amongst our scientists. This year is going to be very important as about 400 scientists will travel around Europe. We should be able to get big results out of our ‘smaller’ tokamaks“, says Dr Marie -Line Mayoral, EUROfusion’s Experimental programme group leader.
Testing ITER’s divertor tiles
EUROfusion also welcomes the new French tokamak WEST. After sealing the vacuum vessel on the 24th of February, WEST is ready to start the first campaign on the 4th of April. It will last until the 30th of June including a three-week interruption to install the first Ion Cyclotron Resonance Heating Antenna.
The French Research Unit CEA which hosts the tokamak aims to ensure that ITER’s divertor will be produced efficiently and that it will perform to its best in the harsh conditions of a fusion plasma. Divertors are the ashtray of fusion experiments. It is a specially armoured structure containing the so called divertor plates which must handle the heat and particle loads thrown at.
Firstly, WEST’s researchers will have to make sure that procurements for ITER’s future ashtray are well established. They are designed to optimise the industrial scale production. This means not only the establishment of the supplier chain but also the proper training of ITER’s staff.
Secondly, the French Research Unit CEA will minimise the risks for ITER divertor operations. Not an easy task since the plasma-facing components will be exposed to a heat load that is ten times higher than that of a spacecraft re-entering Earth’s atmosphere (10-20 MWm²). EUROfusion supports the study of tile shape’s and their properties when exposed to a strong heat and particle flux.
WEST still has a long way to go. So far, the tokamak has only just started operation. The next step for the French Research Unit CEA is to develop its regime of operation and to prepare all the specific diagnostics.
WEST is not the only EUROfusion device which is going to test materials for ITER. Among them are, for instance, the Magnum-PSI linear plasma generator at the Dutch Institute for Fundamental Energy Research (DIFFER) and the PSI-2 experiment at Forschungszentrum Jülich.
Framing divertor concepts
There are several divertor concepts and they must all be tested thoroughly in order to define the best choice for DEMO, the first experimental fusion power plant. Experiments at TCV and the newly upgraded spherical tokamak MAST will enable EUROfusion’s scientists to further explore some of the already existing schemes.
The 2017 European fusion programme will, for the first time, involve experiments at three so-called Medium Size Tokamaks (MSTs). The Upgraded Mega Ampere Spherical Tokamak (MAST-Upgrade) in the UK is about to join its friends ASDEX (Axially Symmetric Divertor Experiment)-Upgrade in Germany and the Tokamak à Configuration Variable (TCV) in Switzerland. Additional support is provided by from France with its material testing tokamak, WEST. The Tungsten (or Wolfram in German) Environment in Steady-state Tokamak already saw its first plasma in December 2016.
EUROfusion has put the feasibility of ITER’s operation into the heart of its research. Of course, the main design for ITER is fixed but its plasma can still be modified. All of the data gathered by the medium-sized tokamaks will be used in computer models which should help predict the behaviour of the plasma including challenges such as ELMs, runaway electrons and disruptions. After testing the simulations again on the medium-sized machines and JET, it will then be time to scale them up.
ASDEX-Upgrade started operating beginning of March with 24 days dedicated to the EUROfusion programme. The first EUROfusion TCV experiments are expected to commence in May with 27 dedicated days. Finally, MAST-U hopefully join its European colleagues at the end of year with ten days of operations.
Experiments at smaller devices in Germany, Switzerland and the UK complement the work being carried out at the big unit JET. These medium sized tokamaks are a vital part of the stepwise approach to extrapolations to ITER and DEMO while featuring unique experimental capabilities and flexibility. The lessons learnt from them pass directly to their big brother JET, where they can be demonstrated in deuterium-tritium in the closest conditions to ITER.
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