Installing a divertor cassette in the ITER vacuum vessel will be like inserting a model ship into a bottle. Both operations require careful planning, dexterity and millimetric precision within severe space constraints. But where a model ship is meant to remain in the bottle forever, the 54 divertor cassettes need to be replaced at least once during ITER's lifetime.   Like the old mariner's bottle for the model ship, the vacuum vessel structure will provide the support for the divertor. As a consequence, the nine sectors of the vacuum vessel have to come first in the assembly sequence and be welded together before the installation of the divertor can begin. That leaves only three relatively narrow passageways into the vacuum vessel (the lower remote-handling ports) for divertor installation.   In 1994, when ITER was still in the early stages of its Engineering Design Activities, development began on the procedures, software and machinery capable of handling the delicate task of conveying the 54 segments of the divertor, called "cassette assemblies," through the port openings and into position at the bottom of the vessel as part of a perfectly circular arrangement.   At the time, the ITER machine, still a paper project, was twice the size of what it is today and each divertor cassette weighed 25 tonnes.   The ITER divertor sits in a circular arrangement at the bottom of the vacuum vessel. Its function is to keep the plasma clean by collecting (thanks to a specific magnetic topology) the impurities of the plasma and the helium "ash" of the fusion reaction. This massive, actively-cooled structure is exposed to an extremely high heat load — ten times higher than that of a spacecraft re-entering Earth's atmosphere. The first Divertor Test Platform was established in the ENEA centre of Brasimone, Italy, under the auspices of the European Fusion Development Agreement (EFDA). As ITER was downsized to its present parameters (and divertor cassette weight had dropped to about 10 tonnes), the challenge was passed on to a team of international experts and a second Divertor Test Platform (DTP2) was established at the VTT Technical Research Centre in Tampere, Finland, some 180 kilometres northwest of Helsinki.   Three weeks ago, just as Finland was coming out of its long subarctic winter, twenty years of effort, ingenuity and technological innovation culminated in a final demonstration: the insertion of a central cassette mockup into (and removal from) its dockings inside of a 1:1 scale section of the ITER vacuum vessel.   Four years after having demonstrated the faultless exchange of two other types of divertor cassettes (standard cassettes and second cassettes), the loop had come to a close. "This was the last operation that needed to be demonstrated and also the most challenging as the three central cassettes they must close the circular arrangement of the divertor assembly," said Mario Merola, ITER Internal Components Division head, as clapping resounded in the vast hall that hosts the test stand.   Once ITER enters deuterium-tritium operation, the neutron flux from the fusion reactions will progressively activate the inner components of the machine and all operations (maintenance, inspection, repair, exchange, etc.) will have to be performed by remote handling. The event also carried a strong significance for the representatives of the European Domestic Agency for ITER, also present in the DTP2 hall. After years of European-financed R&D on the ITER remote handling systems (design, mockup fabrication and demonstrations), the time had come to pass the challenge on to industry — in June 2014, Europe signed a EUR 40 million contract with a partnership of laboratories and companies led by Assystem(2) for the design, manufacturing, delivery, on-site integration, commissioning and final acceptance tests for the ITER Divertor Remote Handling system.   In the DTP2 control room, a few steps away from the 20-metre-long cassette multifunction mover, VTT senior researcher Hannu Saarinen sits, eyes riveted to an array of screens. To his left, a large virtual image shows the progression of the cassette inside the narrow tunnel of the port; on smaller screens, below, numbers and figures scroll endlessly. Without a way to fit a camera into the tunnel, all information is based on sensors and virtual reality; without it, operators would be blind. "More than 80 percent of the operation is pre-programmed," explains Saarinen. "We use the joystick only for small adjustments."   Saarinen and his colleague Vesa Hämäläinen are sitting only a few metres from activity on the mockup. But they could as well be separated by millions of kilometres of space or thousands of leagues of ocean depth. "This has been one of the biggest challenges of the operation: working with pure models without any visual connexion," says VTT Executive Vice-President Jouko Suokas. "But it has been an excellent platform to increase our competency in virtual reality and control software. This expertise is now being transferred to industry, which was one of the key reasons for our involvement in this project."   In the "laboratory conditions" provided by DTP2 in Tampere, the operation was a model of perfection. Years before the 54 divertor cassettes will be inserted into the ITER vacuum vessel, work is beginning now so that—in the industrial environment of ITER assembly—the same level of perfection is achieved.   (1) Assystem leads a team of well-known experts in the remote handling field, comprising the Culham Centre for Fusion Energy, CCFE (UK); Soil Machine Dynamics Ltd, SMD (UK); VTT Technical Research Centre (Finland); and Tampere University of Technology, TUT (Finland).  

For Carlo Damiani—the European Domestic Agency's Project Manager for remote handling systems—and his team this is a big day.   They have just arrived in Tampere, Finland to witness the final demonstration ITER divertor remote handling (see article in this issue). There is an unusual buzz in the facility and every protocol needs to be respected. The remote handling operators take their positions in front of the big screens. All eyes are glued on the monitors as the 10-ton mockup of a divertor cassette starts moving gracefully along the rails. Parameters scroll by indicating speed, angle and the time left to complete the task. On the screens the cassette emerges slowly, subtly lifted and finally locking into place. They did it!   For Salvador Esqué, following the project on behalf of the European Domestic Agency, it's a feeling of relief and excitement. "It's almost like a camel going through the eye of a needle. Can you imagine the millimetric precision that is required and the weight that we are lifting and transporting? It's really impressive."   The test has been successfully concluded and Damiani is already thinking of the next steps. "What [has been demonstrated] is the beginning of a brand-new technology chapter written thanks to ITER. We need to design and manufacture remote handling systems that are resistant, agile and precise. It's an opportunity for industry, SMEs and laboratories to think out of the box, innovate in engineering, and shape the future fusion reactors."   The European Domestic Agency team on site in Tampere, Finland. Europe's contribution to ITER remote handling systems is in the range of EUR 250 million. The European Domestic Agency, Fusion for Energy, and its suppliers will have to deliver the divertor and neutral beam remote handling systems, the cask transfer system and the in-vessel viewing and metrology system.   Jouko Suokas, the Executive Vice President for Smart Industry and Energy Systems at VTT Tampere, host to the divertor test platform DTP2, was also present at the demonstration. After thanking his team, he commented: "Playing a role in this big-science project has helped us to generate new know-how. To give you an example, we have developed new expertise in areas like mechanical engineering, manipulator arms, special tooling, control system software, virtual reality and so on...The potential spin-offs and expertise are some of the key reasons of our involvement. The possible industrial applications are widespread in the field of industry, such as in off-shore movable machine manufacturers, power plants or manufacturing."   The European Domestic Agency has produced a video of the milestone event here. Read the original story on the European Domestic Agency website.

Researchers from General Atomics and the US Department of Energy's Princeton Plasma Physics Laboratory (PPPL) have made a major breakthrough in understanding how potentially damaging heat bursts inside a fusion reactor can be controlled. Scientists performed the experiments on the DIII-D National Fusion Facility, a tokamak operated by General Atomics in San Diego. The findings represent a key step in predicting how to control heat bursts in future fusion facilities including ITER. The studies build upon previous work pioneered on DIII-D showing that these intense heat bursts—called "ELMs" for short—could be suppressed with tiny magnetic fields. These tiny fields cause the edge of the plasma to smoothly release heat, thereby avoiding the damaging heat bursts. But until now, scientists did not understand how these fields worked. "Many mysteries surrounded how the plasma distorts to suppress these heat bursts," said Carlos Paz-Soldan, a General Atomics scientist and lead author of the first of the two papers that report the seminal findings back-to-back in the same issue of Physical Review Letters this week. Paz-Soldan and a multi-institutional team of researchers found that tiny magnetic fields applied to the device can create two distinct kinds of response, rather than just one response as previously thought. The new response produces a ripple in the magnetic field near the plasma edge, allowing more heat to leak out at just the right rate to avert the intense heat bursts. Researchers applied the magnetic fields by running electrical current through coils around the plasma. Pickup coils then detected the plasma response, much as the microphone on a guitar picks up string vibrations. The second result, led by PPPL scientist Raffi Nazikian, who heads the PPPL research team at DIII-D, identified the changes in the plasma that lead to the suppression of the large edge heat bursts or ELMs. The team found clear evidence that the plasma was deforming in just the way needed to allow the heat to slowly leak out. The measured magnetic distortions of the plasma edge indicated that the magnetic field was gently tearing in a narrow layer, a key prediction for how heat bursts can be prevented. "The configuration changes suddenly when the plasma is tapped in a certain way," Nazikian said, "and it is this response that suppresses the ELMs." The work involved a multi-institutional team of researchers, who for years have been working toward an understanding of this process. These researchers included people from General Atomics, PPPL, Oak Ridge National Laboratory, Columbia University, Australian National University, the University of California-San Diego, the University of Wisconsin-Madison, and several others. The new results suggest further possibilities for tuning the magnetic fields to make ELM-control easier. These findings point the way to overcoming a persistent barrier to sustained fusion reactions. "The identification of the physical processes that lead to ELM suppression when applying a small 3D magnetic field to the inherently 2D tokamak field provides new confidence that such a technique can be optimized in eliminating ELMs in ITER and future fusion devices," said Mickey Wade, the DIII-D program director. The results further highlight the value of the long-term multi-institutional collaboration between General Atomics, PPPL and other institutions in DIII-D research. This collaboration, said Wade, "was instrumental in developing the best experiment possible, realizing the significance of the results, and carrying out the analysis that led to publication of these important findings." First page caption: Computer simulation of a cross-section of a DIII-D plasma responding to tiny magnetic fields. The left image models the response that suppressed the ELMs while the right image shows a response that was ineffective. (Photo by General Atomics) Read the original article on the PPPL website.

This specular 360° image of the Asdex Upgrade vacuum vessel from photographer Volker Steger won the second prize of the German Science Photo Award (Deutscher Preis für Wissenschaftsfotografie) in March 2013.

The ELISE test rig at the Max Planck Institute for Plasma Physics (IPP) in Garching, Germany has now achieved world-record values after two years of research work. In first-time, one-hour operation, a pulsed particle beam of hitherto unattained quality was produced—thick as a tree trunk, homogeneous, stable over time and nine amperes strong.   ELISE (for Extraction from a Large Ion Source Experiment), as the largest test rig of its kind, is developing the heating system that is to bring the plasma of the ITER reactor to a temperature of many million degrees. The core element is a novel high-frequency ion source developed at IPP, which produces the high-energy particle beam.   In order to heat the ITER plasma to many millions of degrees Celsius, two high-energy particle beams with heating powers of 16.5 megawatts each are to be pumped into the 800-cubic-metre plasma volume. The cross-section of these particle beams will be about door size, greatly exceeding the size of the beams used up to now (roughly plate-size cross-sections, with much lower powers).   ITER's neutral beam heating system will take on about half of the overall plasma heating. The size of ITER imposes enhanced requirements: particle beams have to be much thicker, individual particles have to be much faster in order to penetrate the plasma deeply enough, and negative ion source technology must be used (instead of the positive-source technology used up to now). The high-frequency ion source developed at IPP for this purpose was adopted as a prototype in the ITER design. The contract for adapting the source to ITER requirements also went to IPP at the end of 2012.   The ELISE test rig at IPP Garching. © IPP, Robert Haas ELISE permits the investigation of an ion source half the size of the one required for ITER. It produces a particle beam with a cross-sectional area of about one square metre—an increase in size over previous sources that made the revision of technical solutions for heating necessary. ELISE has thus been advancing step by step to new orders of magnitude.   Recently, the ion source achieved operation pulses lasting one hour, during which a stable and homogeneous ion beam of nine amperes lasting 20 seconds could be produced every three minutes. The gas pressure in the source and the quantity of the electrons retained conformed to ITER specifications. In short, a world record.   To enable hydrogen atoms to be accelerated, they have to be made tangible to electric forces as charged particles (positively or negatively charged ions). This is done in the ion source: high-frequency waves injected into hydrogen gas ionize and disintegrate part of the hydrogen molecules. The plasma created—a mixture of neutral particles, negative electrons and largely positively charged ions—flows to a first lattice-shaped electrode.   Through the several hundred apertures of this lattice, an equally large number of individual ion beams are extracted. On being accelerated through another two lattices, the finger-thick individual beams finally merge as a wide single beam, whose cross-section in ELISE is about a square metre. If the surfaces of the ion source are coated with appropriate material, e.g., caesium, the hydrogen atoms there can then take up electrons.   This provides the negatively charged hydrogen ions needed for ITER. To get rid of the unwanted, simultaneously extracted electrons, their flight to the first lattice is obstructed by a transversal magnetic field in the plasma. Small permanent magnets incorporated in the second lattice then guide the electrons out of the beam for good.   The much heavier ions, on the other hand, keep flying almost unhampered. It is not only this magnetic inner life that makes the ELISE lattices technical masterpieces; there is also an elaborate water-cooling system that, despite the high wall load during the heating pulse, keeps every individual aperture in place within hundredths of a millimetre in relation to its partner in the following aperture.   To make all this function properly, there are numerous individual parameters that have to be precisely tuned to one another, for example high-frequency power, caesium concentration, wall temperature, lattice voltages and the magnetic field for deflecting the electrons. Only then does one get the desired result—a stable and homogeneous beam of fast, negatively charged hydrogen atoms.   To enable the fast ions later in ITER to traverse the magnetic field unhampered into the plasma, they first have to be neutralized again. Finally, as fast hydrogen atoms they are injected into the plasma and surrender their energy to the plasma particles.   Once the source has been cleaned, operation will be resumed at full power to attain the full target values. A full-size negative ion source will then be investigated in PRIMA (the ITER neutral beam test facility under construction in Padua, Italy). In preparation, the Italian team will train at IPP for the next two years as development on ELISE is ongoing. Read the original story on the IPP website.

​Think science and art are poles apart? Think again. Three artists who have been inspired by nuclear fusion will display their work at the  "Making a Sun on Earth"' exhibition, which runs at the Cornerstone Arts Centre in Didcot, UK from 10 March to 26 April. And they hope their collaboration with Culham Centre for Fusion Energy will challenge people's ideas about science. Find out more here.

​The next Fusion Summer University at the Max-Planck-Institute for Plasma Physics (IPP)will be held in Garching (near Munich) from 14 to 18 September 2015. The course covers the main aspects of plasma physics with emphasis on nuclear fusion. The application deadline 31 May 2015. For more information please click here.