11 Mar 2019 to 18 Mar 2019
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Cryostat | Lower cylinder revealed
They were all there: those who designed it, those who forged it, those who assembled and welded it, and those who closely monitored the requirements and procedures connected with a 'safety important' component. Two years after an array of segments were delivered to ITER, the cryostat lower cylinder—one of the four sections that form the giant thermos that will enclose the machine — had been fully assembled. With scaffolding removed and just a thin translucent film to protect it, the massive structure was at last revealed, both delicate and mighty. 'This is the largest component that will go into the machine assembly pit,' said Patrick Petit, ITER In-Cryostat Assembly Section leader. 'It is also an example of broad and exemplary collaboration.' Like the other sections of the cryostat, the realization of the lower cylinder epitomizes the larger collaborative nature of ITER: designed by the ITER Organization, manufactured and pre-assembled by Larsen & Toubro Ltd in India, it was further assembled and welded by a German company under contract to India on international territory conceded by France. 'The realization of this component was not a single person's job,' said Anil Bhardwaj, ITER Cryostat Group leader. 'It has been quite a serious task for all of us, with a large variety of challenges, particularly regarding fitment and welding quality' added Vikas Dube, a mechanical engineer in his team, 'and although there were lots of lessons learned, we will face them again when we commence the assembly and welding of the upper cylinder in the coming months.' Read more about the fabrication of the ITER cryostat here.
Neutral beam injection | How ELISE is contributing to ITER
ITER's neutral beam injection system is based on a radio frequency source that has been the subject of decades of development in Europe. At Max Planck Institute for Plasma Physics (IPP) in Garching, Germany, an upgrade to the ELISE half-size source testbed will allow scientists to proceed further along the road to demonstrating ITER source requirements in complement to the full-size source under development at the ITER Neutral Beam Test Facility. The basic principle behind ITER's most powerful heating system is relatively simple: fast (>10,000 km/s) hydrogen or deuterium atoms are injected through the tokamak magnetic field cage into the plasma, where they transfer their energy to the plasma particles through collision. The physics and technology behind the production of fast atoms is much more complex, however, as beams of neutrals are always created by accelerating ions to high energy and then neutralizing them by adding or removing an electron on the fly. Most of the complexity is concentrated in the plasma sources where hydrogen ions are generated. They flow towards a first electrode and then venture out in a high electric field region were they will acquire their speed. The challenge for the ITER sources is that the required beam energy (1 MeV) imposes the use of negative ions, which have a better neutralization efficiency than the positive ions used the neutral beam injection systems of most other fusion devices. Inside the source, the negative ions are surrounded by plasma electrons that have the same charge. Therefore, the positive potential applied to extract and accelerate the ions causes the unwanted co-extraction of electrons which get scattered inside the accelerator, causing an enormous heat load to the system. The ion source is thus required to produce large and stable flux of negative ions while keeping the electron current low enough (a ratio of electron current to ion current of less than one is mandatory). The ITER source requirements in this sense are the most stringent ever, and it is only thanks to the continuous efforts of several laboratories around the world that the performances required the ITER neutral beam injection are finally within reach. The ELISE test bed Over the last two decades, the neutral beam injection group at the Max Planck Institute for Plasma Physics (IPP) has had a leading role in contributing to such efforts. The design of IPP's 1/8-scale (relative to ITER) prototype source BATMAN was a true game-changer back in the early 2000s. Compared to the alternative technology at the time for negative ions—a source based on the heating of a tungsten filament—BATMAN's technology introduced radiofrequency (RF) waves injected through an external antenna to generate the plasma. It proved to be maintenance-free and reliable, removing the issue of filament lifetime, and was readily chosen as the baseline design for the plasma production in the ITER negative-ion-based neutral beam injection source. Since then, research has continued at IPP on the ELISE test rig (for Extraction from a Large Ion Source Experiment), which features an ion source one-half as large as ITER's. The goal for ELISE has been to push forward the source performances in terms of extracted ion current and stability over time. The first goal was achieved for hydrogen last year after four years of regular improvements in the device. The full achievement of the second requirement has been limited so far by the power supply available to ELISE, but that is about to change. In ITER, a stable power flux (i.e., a stable ion current from the source) lasting up to 1 hour is required. Sustaining the plasma for this extremely long time is nowadays routinely achieved at IPP, but the beam duration has been limited up to now by the available hardware. The particles in the source could be continuously generated and made ready for the take-off but—due to power sources that could only generate the required electric fields in a pulsed fashion—they could only be accelerated for 10 s every 3 minutes. Using the current power sources IPP has been able to demonstrate the possibility of extracting reproducible, high-current beams of hydrogen. This satisfies the requirements of the heating neutral beam injectors for the first (non-nuclear) phase of the ITER research program. But in the nuclear phase, when a mixture of deuterium and tritium will be used to produce net energy, neutral beams of deuterium will be required. When operating the ELISE ion source with deuterium, the extracted negative ions show reasonably good stability but—due to pulsed extraction—the current of co-extracted electrons is everything but stable: a dramatic increase pulse after pulse and within the pulse is observed, thus limiting the source performance. In order to study the temporal dynamics of ITER relevant currents without the difficulty of a pulsed beam, the decision has been taken to install a steady state power supply on ELISE. The power supply has already been ordered and is expected to be commissioned early next year. The EUROfusion consortium is contributing to its cost in conjunction with IPP with the long-term view that the next phase of experiments at ELISE will also support the design of the future negative ion sources for DEMO, the fusion device that is planned after ITER. After this considerable upgrade the ELISE testbed will be able to proceed further toward meeting the remaining ITER source requirements, and complement the ITER test facility ion source development which is carried out at SPIDER. Click here to view a video of pulse #27316 filmed from three different angles (the camera looking at the plasma, the infrared camera looking at the copper calorimeter, and the camera looking at the tungsten-wire calorimeter).
Image of the week | Almost there
The Tokamak Building has reached its maximum height ... in terms of concrete that is. The 'jewel box' in reinforced concrete will grow no more; instead, it will be brought to the level of the adjacent Assembly Hall by the addition of steel-structure walls and a roof. The concrete portion of the building below will enclose the ITER machine. The airy gallery above—the Crane Hall—will provide the workspace needed as the heavy-lift assembly cranes travel back and forth between the buildings to deliver components into the Tokamak Pit. On the south side of the Tokamak Building, visible in this picture, a concrete slab will be poured to 'close off' the part of the structure that extends out wider than the Assembly Hall. (A similar slab will be poured on the opposite side.) On top of the slabs, heavy structural pieces called 'corbels' will support the steel pillars for the Crane Hall. The steel structure (pillars, walls, roof) is in production now and assembly operations are set to begin during the summer.
Powerful lasers | A mockup to demonstrate safety
During ITER operation, high-powered lasers will gather important diagnostic information on the properties and behaviour of the plasma, such as density, temperature and internal magnetic field. An integrated safety system will ensure that the lasers operate safely in all circumstances, including the unexpected. Lasers are common objects in our daily lives, found in CD/DVD players, office printers, the computer mouse, and laser pointers. But the lasers employed in ITER's diagnostic systems, according to diagnostic physicist Christopher Watts, are 'about 100,000 more powerful than a laser pointer.' High-powered laser beams generated in the Diagnostics Building will be relayed by a series of mirrors along beam tubes through the galleries and port cells into the vacuum vessel. There, they will focus on various locations inside the plasma to obtain the needed data and return to the Diagnostics Building with their precious information. Because unprotected laser beams—many radiating in the invisible spectrum—could be hazardous to the human eye, the beams are enclosed in metal pipes along the entire transmission line. But safety must be ensured even in the case of an accidental or unexpected breach in the laser enclosure. Watts has been working with occupational safety engineer Roger Victori from the Control System Division to develop a reactive safety system that can respond in all situations (normal or accidental) and during all phases (operation, servicing, alignment). 'We have developed a several-layer system that includes physical barriers as well as instrumented interruption functions,' Watts explains. In order to test the safety features of the system, they developed a tabletop mockup that employs a low-powered laser that is safe under all conditions. The mockup mimics the lasers' journey between the Diagnostic Building and the vacuum vessel, simulating the key elements of dormant safety measures that spring into action when a breach in the laser enclosure triggers an alert. The mockup's controls and sensors are coupled to an automated logic system controlling the safety aspects. These measures include automated laser beam 'blocks' at various locations along the transmission path that can engage to keep the laser confined to a certain region. If physical shielding is insufficient to mitigate the safety risk, electrical actuators interrupt the electrical power supply, shutting the laser down. Already the mockup has proven useful, as it has helped identify key safety and operational interdependencies. Once the logic and specifications of the safety functions are worked out in detail, the mockup safety system will be provided to the laser diagnostic suppliers as an example of a system that meets ITER's safety requirements.
Europe's DEMO | What it could be like
It looks like ITER, feels like ITER, but it's not ITER. In this depiction of what the site layout for the next-step fusion machine, DEMO, might look like in Europe, the familiar elements are all in place. But there is one extra building, and that makes all the difference ... DEMO is a concept for an electricity-producing, tritium-generating, long-pulse tokamak one step removed from an actual industrial and commercial fusion power plant. The ITER Members all have a different notion of what their DEMO could be. A central requirement for the European DEMO, as defined by the EUROfusion roadmap, would be to produce several hundred megawatts of net electricity to the grid (300 to 500 MW), which is approximately ten 3-4 times less than an average nuclear fission reactor. Europe's DEMO would be a "demonstration power plant" to be followed by the first-of-a-kind fusion power plant. And with careful observation, that is what we are seeing in this illustration, recently produced by EUROfusion and the European Domestic Agency for ITER, Fusion for Energy. Although larger than ITER, DEMO is organized along the same logic—central Tokamak and Assembly buildings, large electrical switchyard, magnet power conversion buildings, cryoplant with helium storage tanks, and heat removal through large cooling towers. The main departure from the ITER layout is the large blue building in the centre of the illustration. This building is where the thermal power generated by the DEMO tokamak will be converted to electrical power by way of turbines and alternators—just like in any electricity-producing plant, whether from coal, gas, fuel or nuclear fission. One other important difference between ITER and DEMO is hidden from view—the breeding system inside the DEMO vacuum vessel that ensures the continuous production of tritium to fuel the fusion reaction, a condition sine qua non for the future of fusion energy.
Fusion Summer School at IPP (Germany)
Physics and engineering students of European universities are invited to attend the 2019 Summer University at the Max Planck Institute for Plasma Physics (IPP) in Garching, Germany, from 16 to 20 September. The IPP Summer University for Plasma Physics and Fusion Research is designed for those students who have completed their bachelor's degree, but who have not yet decided on a PhD topic. Lectures are planned on plasma physics, plasma-wall interaction and materials research, ITER and the next steps toward fusion energy, and more. The course will include a tour of the ASDEX Upgrade tokamak experiment (pictured) and laboratories. Sign up by 31 May here.
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