you're currently reading the news digest published from 31 May 2021 to 07 Jun 2021



Portfolio | It's happening in the pit

There has always been something breathtaking about the vast cylindrical volume that will accommodate the ITER Tokamak. When it was empty, it felt like a high-tech rendition of a Roman amphitheatre, complete with stands and terraces, and boxes for the happy few. As the 14,000-cubic-metre volume progressively fills with components and equipment, it now eludes comparison: nothing in the world remotely resembles what we call the 'assembly pit.' One year after the installation of the cryostat base, on 26 May 2020, there is nothing to remind us of the Roman amphitheatre. Stainless steel now lines the arena floor and lower half of the cylinder, the first-row seats are crowded with a circular arrangement of sturdy pedestal-like structures, and centre stage is almost fully occupied by components arranged in concentric circles. For the occasional visitor, the feeling is one of strangeness, of a failure to grasp the function of all these elements and how they relate to one another. For the denizens of the ITER world, it all makes sense. What they see are the first steps of a prodigious undertaking: the assembly, from the bottom up, of the most complex machine ever designed. First came the base section of the cryostat. The shape of a deep soup plate, it forms the bottom of the giant thermos that encloses the machine and provides thermal insulation to the superconducting magnets cooled to cryogenic temperature. With the subsequent installation of the 12-metre-tall lower cylinder, the cryostat now reaches halfway up the walls of the assembly pit. The thermos will be complete when the upper cylinder, presently in storage, and the top lid are installed at later stages of the assembly sequence. The vacuum inside the cryostat is not sufficient to protect the machine's magnetic system, which operates at the ultra-cold temperature of 4 K (minus 269 °C), from the warmth of the outside environment. An extra barrier, a 'thermal shield,' is needed to prevent heat transmission through the electromagnetic waves that any warm source generates, a physical phenomenon called 'radiation.' The first section of the cryostat thermal shield was installed in mid-January 2021. The thin, actively cooled silver-plated component closely fits inside the depression in the cryostat base. Other sections will be installed as machine assembly progresses, eventually lining the entire interior surface of the cryostat wall. Early this year, smaller but no less strategic structures began appearing in the pit: the first of the 18 toroidal field gravity supports were bolted to the cryostat base rim, and temporary supports were installed to accommodate poloidal field coil #6 (PF6), the first ring magnet of the machine assembly sequence. Along with machine components, the pit also accommodates temporary structures and tools that are required for the assembly process but that will be dismantled later. One of the most spectacular of these in-pit assembly tools is the central column, a 600-tonne tower-like structure that will stand more than 20 metres high and support the radial beams holding the nine sub-elements of the vacuum chamber as they are welded together. The first element of the central column was installed in March 2021; it now occupies the very centre of the pit. The installation of PF6, less than one month later on 21 April, was both a spectacular and highly symbolic moment in ITER history. A tokamak is essentially a vacuum chamber within a magnetic cage, and the 330-tonne ring-shaped PF6 was the first magnet to be positioned in the assembly pit. With little room to move and a lot of ongoing co-activity, workers are now busy installing support thermal shield panels to raise yet another thermal barrier, this time around the 18 toroidal field gravity supports. In preparation for the installation of the next ring-shaped coil, poloidal field coil #5, workers are completing work on six temporary supports. Coil insertion in the pit is scheduled in late July/early August. The amphitheatre is gone and the once majestic stage is now a cramped environment. But the ballet of workers, tools and machines, as pictured in the portfolio below, still makes for a splendid show.

Fusion world | ORNL researchers excited to be part of history at JET

As part of a long-running collaboration, the US Oak Ridge National Laboratory is supporting landmark experiments aiming for a fusion milestone at JET. This summer, a quarter century after a chance introduction to a landmark fusion experiment as a teenager, Oak Ridge National Laboratory (ORNL) researcher Ephrem Delabie will take part in one of the most significant fusion experiments of the past few decades. In November 1997, Delabie was a Belgian high school student into things like insects, plants, and rock climbing. One day he picked up a popular science magazine and read about a recent breakthrough: researchers at a facility called the Joint European Torus (JET) had generated 16 million watts of fusion power by heating the hydrogen isotopes deuterium and tritium to well over 100 million degrees Celsius. The 16 million watts of fusion power amounted to 67 percent of the power applied to heat the plasma, or a 'Q' of 0.67 as the ratio is known. It was a new world record by a large margin. Delabie was impressed to read how the JET tokamak created the plasma pressure, density and temperature necessary for deuterium and tritium ions to fuse, converting mass to energy. He thought, 'I would like to work there later.' In 2011, he finally did, landing a postdoc position at the Culham Centre for Fusion Energy in the United Kingdom where JET, operated for EUROfusion, is located. Originally affiliated with the Dutch DIFFER Institute, Delabie now works at JET for ORNL—just as the facility is poised to once again study deuterium-tritium (DT) plasmas producing millions of watts of fusion power, and to extend high fusion power production to longer durations. Delabie's role continues a decades-old collaboration between ORNL and JET to develop the facility's diagnostic systems for measurement of plasma performance and related instrumentation.                             JET Programme Leader Lorne Horton, himself a former ORNL researcher, said the long-standing partnership has worked quite well. 'We are looking forward to another DT experiment with greatly improved diagnostic capability, including an expanded and improved range of systems provided by ORNL,' he said. These collaborations also serve ITER. One of JET's main goals is to maximize fusion performance in the regimes of operation foreseen for ITER. Other goals include demonstrating integrated radiative scenarios to match the power exhaust, or divertor, detachment required in ITER. JET also aims to show clear alpha-particle effects and clarify isotope effects, address key plasma-wall interaction issues, and demonstrate radio-frequency heating schemes relevant to ITER's full power operation. 'These JET experiments are historic,' said Ted Biewer, group leader for diagnostics and control in ORNL's Fusion Energy Division. 'It's a rover-touching-down-on-Mars type thing for fusion.' Several types of hydrogen isotopes can fuse to produce energy. A 50/50 DT mix is very well suited for creating a so-called 'burning plasma,' in which fusion can sustain itself—a requirement for a future power plant. Experiments with such a mix have been rare, however, in part because tritium is in short supply and challenging to handle.   Biewer is excited to be involved in this historic event. His connection to JET dates to the mid-2000s when, as a young ORNL researcher, he spent several years developing a diagnostic technique called charge exchange spectroscopy (CXS). Pioneered in the 1970s by ORNL's Ralph Isler, this technique measures the temperature and velocity of the ions in the plasma. By looking at interactions between different particles, including impurity ions from the wall of the tokamak, scientists could make a crucial distinction: how much energy in the plasma was attributable to fusion heating—and how much came from the external heating. ORNL and JET researchers have adapted CXS over the years as the tokamak evolved, such as when the interior wall was changed from carbon to beryllium and tungsten to mimic ITER. 'It's a continual evolution and improvement of techniques,' Biewer explained, 'thereby also revealing new science.' More recently, in partnership with EUROfusion, ORNL upgraded JET's CXS instrumentation yet again to so-called 'main ion' CXS, which does not use atoms from the tokamak walls at all. Instead, measurements are derived from interactions between energetic neutral beams and the fuel (deuterium or tritium) ions. With Biewer overseeing the effort from ORNL and Delabie doing the hands-on work at JET, the team designed and installed new hardware. They have already collected a lot of data on deuterium experiments and, since December, on tritium, as they prepare for the summer's DT work. In addition to operating these diagnostics for JET, Delabie is using them to run experiments on how energy travels in the plasma. 'We mainly look at how the core heat transport changes when the isotope mass of the plasma is changed,' he explained. 'We can compare hydrogen plasmas and deuterium plasmas, and now we're going to have the tritium plasmas. So, we'll have the full data. JET is the only machine now capable of doing this.' The CXS instrumentation has been performing well, Delabie reported, although COVID has forced him to monitor experiments remotely. Both he and Biewer hope that, come June, they can observe DT fusion for the first time in their lives—with any luck from the JET control room. 'We study nuclear fusion, but most of the time we see very little fusion actually happening, because we usually work with deuterium plasmas,' which by nature produce far less fusion that DT, explained Delabie. 'And this is one of the few cases that we will actually have large amounts of fusion power to measure: If everything goes well, we may have about 15 MW of fusion power. That changes how the plasma behaves, potentially. You can do a lot with modelling; but to actually see that, to actually measure that, is really a once-in-a-lifetime opportunity. It's one of the reasons I wanted to work at JET.' With all eyes on JET's upcoming DT experiments, scientists like Biewer and Delabie are feeling a mix of excitement and trepidation — a case of deuterium tremens, if you will. 'It's exciting,' said Delabie, 'but it's also making me a little bit nervous because I know it's important.' See the original story on the ORNL website. 

Image of the week | First weld for the top lid

Of the four sections that make up the ITER cryostat, the top lid is the thickest, second heaviest (more than 700 tonnes) and most structurally complex. On Monday 7 June, following a traditional coconut ceremony in the Cryostat Workshop that was broadcast live to India, two welders working on opposite sides of the component's skin, produced the 'first arc' of a welding campaign that is scheduled to last four to five months. The collective knowledge and experience that the ITER Organization, ITER India, Larsen & Toubro Ltd and MAN Energy Solutions have accumulated over the past four and a half years since welding operations began on the cryostat base will be a precious asset in the current operation. 'There are some very specific challenges to the welding of this massive component,' explains Anil Bhardwaj, the leader of the Cryostat & Auxiliaries Group. 'It has a rather thick skin, a large circular opening at its centre and a complex arrangement of ribs underneath. Distortions due to the welding process will be a constant preoccupation and will require constant monitoring.' The cryostat top lid is made of twelve 30-degree sections that will require approximately 300 metres of deep welds (50 millimetres deep on average, but up to 280 millimetres at the flange levels) and more than 4 tonnes of filler metal. After the cryostat base, lower cylinder and upper cylinder, the top lid will be the last section of the ITER cryostat assembled and welded in India's on-site Cryostat Workshop.


Now available: virtual visits of ITER

The ITER Organization has added a permanent virtual visit option to its visit program. Developed in reply to the COVID-19 pandemic and the necessary restrictions to in-person visiting, the virtual visit now co-exists alongside other options, including individual and group visits, on the ITER webpage dedicated to visitors. The virtual visit option will remain available to the public even after COVID-related restrictions end, allowing fusion aficionados from around the globe a chance to better appreciate the progress on the ITER construction site. What can you expect from a virtual visit? In approximately 90 minutes, the ITER visit team will introduce you to the world of fusion, before explaining the ITER Project in detail. Highlights include a video shot by drone and a tour of all the main buildings on site through immersive 360° images.   If you are interested in booking a virtual visit of ITER, please see the Visits page of the ITER website and click on 'Virtual Visits.' The calendar indicates the open slots for group or individual virtual visits in either English or French.   Come and join us. We look forward to welcoming you (virtually) to ITER! --The ITER Visits team


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