Experts from the ITER Members met for two days at ITER Headquarters to share the latest technological developments in the design of the ITER disruption mitigation system. ITER will install a disruption mitigation system to protect machine components against excessive heat loads and electromagnetic forces resulting from plasma disruptions, in which there is a sudden loss of stored thermal and magnetic energy. These disruptions occur on all tokamaks, and ITER is no different, except that its size and high plasma current mean that the stored energies are at least an order of magnitude larger than on present devices and must be properly mitigated when disruptions occur. The disruption mitigation system will utilize shattered pellet injection to deliver massive amounts of protium and neon in the form of small ice fragments into the plasma to convert the energy into radiation. Despite having reached a high level of design maturity, as the above image demonstrates (showing part of the disruption mitigation system embedded into vacuum vessel sector #2), a number of technological challenges remain. These include a first-of-a-kind shattered pellet injection system (requiring an initial frozen pellet size seven times larger than on any other tokamak), material requirements (protium), environmental restrictions (radiation, limited space), and reliability and reproducibility requirements. The ITER Organization has launched an extensive technology program to provide the necessary design validation and the development of novel concepts and components. This has led to worldwide collaborations with institutes and companies from the ITER Members and, thanks to the efforts of these collaborators over the last three years, the know-how and understanding of the complex processes involved in forming, launching, delivering and shattering pellets has seen a steep rise. For the first time since the start of the disruption mitigation project, technology experts together with the design team came together at ITER from 8 to 9 February 2023 for a vibrant meeting in which the latest achievements, covering a broad range of technological challenges, were presented and discussed. February 2023 at ITER Headquarters: In addition to the in-person participants at the Technology Group Meeting of the ITER Disruption Mitigation System Task Force, approximately 40 experts were connected remotely. Some highlights of these activities are: Pellet formation: The low triple point and low heat conductivity of hydrogen (~15 times lower than the stainless steel formation cell) limits the heat removal capability during the desublimation process and makes the formation of large pellets (~3 cm x 6 cm) as required for ITER a challenge. Through different approaches and design solutions, the formation of such pellets was successfully demonstrated last year in three laboratories: Centre for Energy Research Budapest/Hungary, Département des Systèmes Basses Températures CEA-Grenoble/France, and Oak Ridge National Laboratory/US. An example of a cryostat showing the cold head where the pellet formation takes place, the pre-cooler and the thermal intercepts is shown below (figure 2a). Propellant gas suppression: The pellet is accelerated by a high-pressure gas puff. On its way to the plasma, the pellet will pass a component with internal barriers to allow the gas to expand, impeding it from further outflow into the flight tube. Computational fluid dynamic modelling is used to identify the best gas suppressor design for minimizing the amount of propellant gas entering the ITER vacuum vessel (see figure 2b, below). Otherwise, in case too much propellant gas arrives ahead of the pellet, the mitigation action with the shattered pellet would be jeopardized. Optical pellet diagnostic: It works like a speed camera except that it has only 50 microseconds to catch an image of the pellet passing the observation window at a speed of ~1,800 km/h (see 2c, below). Pellet flight accuracy: The pellets have to be launched with a precision of better than 1 cm over a distance of 6m for them to reach the shattering unit without colliding with the flight tube periphery. The trajectory precision is being assessed by firing pellets against a target plate (see 2d, below). Techniques (similar to muzzle brakes) to release the gas from the barrel will be employed to improve control over the pellet flight direction. Shattering: After just ~12 ms of flight time (it takes about 30 min to form a pellet!), the pellet trajectory is intersected by a structure where it shatters into a cloud of ice fragments which are then sprayed into the plasma. Different designs for this shattering structure are being investigated through simulations of this complex process (see 2e, below). The most promising configurations are then tested in the Support Laboratory at the Centre for Energy Research in Budapest, which contains a purpose-built shattering chamber with a large suite of dedicated diagnostics (see 2f, below). Alignment diagnostic: Ironically, the disruption mitigation system itself will suffer from the very plasma disruptions it is designed to mitigate, in the sense that potential movements of the vacuum vessel due to the electromagnetic forces during the disruption could lead to misalignment of the pellet flight lines. A 3D port plug alignment detector measuring any possible deviation from the default position has been developed and tested. Alternative concepts: As risk mitigation in the unlikely event that the baseline design fails to fulfil the key requirements the system must deliver, some R&D into novel and unusual alternative concepts are being developed within the technology program. As an example, a pellet formation device with a porous cold head is shown in figure 2g, below. Figure 2. Examples of the various activities to support the design of the ITER disruption mitigation system: (a) cryostat and cold head to perform fundamental studies (CEA-Grenoble, France); (b) gas flow modelling for propellant suppressor (CASPUS, U.K.); (c) cut away of the optical pellet diagnostic's front-end optics with modelled light rays for the different observation branches (Fusion Instruments, Hungary); (d) pellet impacting target for trajectory measurements (ORNL, US); (e) simulation of pellet fragmentation validated against experiments (EMI-Fraunhofer, Germany); (f) Support Laboratory for component testing (EK-CER, Hungary); (g) cryostat with porous cold head for fast pellet formation (PELIN, Russia). The ITER disruption mitigation system is an extremely complex, highly integrated, state-of-the-art plant system combining many different technologies.  Whilst disruption mitigation has always been envisaged on ITER, previous conceptual systems had been based on the rapid injection of massive quantities of gas. Only in recent years has it become clear, through R&D on tokamaks within the ITER Members, that a shattered pellet system would be required to meet the unique demands of disruption mitigation at the ITER scale. The disruption mitigation system technology program, despite being launched only in 2020, has made tremendous progress in such a short time, showing the power of bringing together experts across a broad range of disciplines into a highly coordinated and focused effort. The February meeting has shown that the technical realization of this challenging technology project is indeed feasible.  Activities will now push forward to culminate in a full design validation of the system in preparation for the disruption mitigation system final design review planned for 2024.

On 21 February 2023, Japan's National Institutes for Quantum Science and Technology (QST) and supplier Toshiba Energy Systems & Solutions celebrated the completion of the last regular toroidal field coil under Japanese procurement responsibility. The ceremony at Toshiba Energy Systems & Solutions Corporation was attended by representatives of the Japanese government, QST, and the ITER Organization. For Toshiba, it is the fourth toroidal field coil for ITER that the company has produced; for QST, this is the eighth it has procured overall. The Japanese Domestic Agency has worked with Mitsubishi Heavy Industries, Ltd., Mitsubishi Electric Corporation, Hyundai Heavy Industries, and Toshiba Energy Systems & Solutions Corporation for the supply 8 regular toroidal field coils (assembly of winding packs and coil structures) plus another 10 coil structures for the European Domestic Agency (responsible for procuring 10 coils). A ninth coil will be produced in Japan as a spare. Celebrating a ten-year industrial effort are representatives of the Japanese government, QST, Toshiba, and the ITER Organization (in-person and virtual). The D-shaped toroidal field coils, which completely enclose the vacuum vessel by matching its shape on both the inboard and outboard sides, are relied on for plasma confinement. In the huge ITER device, a set of 18 coils will produce a total magnetic energy of 41 gigajoules, generating an intense field all around the plasma (11.8 tesla at its maximum) to keep it confined in the centre of the vessel away from the walls. See press releases in Japanese from QST and Toshiba ESS.

Early in its new campaign, the Wendelstein 7-X stellarator has reached (and passed) a target, achieving an energy turnover of 1.3 gigajoules. What's more, the hot plasma was maintained for eight minutes.

Before they can be considered fit for service, the ring-shaped coils that circle the ITER vacuum vessel must be submitted to a series of tests designed to confirm their performance in the daunting environment of a burning plasma. The procedure begins with the delicate insertion of the coil inside a close-fitting, doughnut-shaped cryostat and extends more than one month. Whether performed at room temperature or at 80 K (minus 194 degrees Celsius), the tests aim to verify the helium cooling circuit's leak-tightness, the performance of the electrical insulation and the coil's mechanical behaviour under cryogenic conditions. Although cold tests have been performed before (on PF6 in December 2020, on PF5 in March 2021 and on PF2 in October 2021), the operation that began on Monday 20 February 2023 in the European poloidal field coil facility on site was exceptionally spectacular: PF4, the coil that was inserted into the open cryostat, is 24 metres in diameter and weighs in excess of 350 tonnes.   Moving such a delicate and massive component requires an equally delicate and massive lifting tool. The gantry crane in Europe's poloidal field coil facility—a 30-metre-in-diameter steel structure supported by four hydraulic towers traveling on rails—meets these requirements. Equipped with hydraulic slings and load sensors, it is capable of moving a large circular coil while ensuring at all times a uniform distribution of the load. (Click here to view a short time-lapse video of the coil lifting and insertion.)   Inside the cryostat, the coil will experience extreme cold, intense pressure, high voltage and considerable thermal stresses—a foretaste of its life to come in the daunting environment of a burning plasma. "As the gantry crane can only move along the rails in the longitudinal direction it is of the utmost importance that the cryostat be installed in the correct coordinates," explains Mónica Martínez, the European Domestic Agency's (F4E's) Technical Officer for poloidal field coils. Aligning the crane's laser pointer to a simple point marked on the building's floor does the trick: the coil can be gently lowered into the cryostat where it will spend more than one month, experiencing extreme cold, intense pressure, high voltage and considerable thermal stresses—a foretaste of its life to come. See the report on the Fusion for Energy website.

With some of the machine components that arrive at ITER as tall as a five-storey building or weighing as much as a jet liner, it's easy to overlook the smaller deliveries. But they too are key to the ITER program, because without the connectors, the distribution lines, and the millions of other pieces of small but essential hardware, the ITER machine would never start up. Last week, it was the turn of the ITER and European Domestic Agency vacuum teams to celebrate the delivery of cryo-jumpers—semi-flexible cryogenic pipes that distribute cryogens (gaseous and super critical helium) at 80K and 4.5K from the cold valve boxes to the torus and cryostat cryopumps. It represented the successful conclusion of a five-year contract between the European Domestic Agency, Fusion for Energy, and the German company Cryotherm GmbH & Co. KG. The delivery consisted of 32 sets of jumpers plus spares, which is all that is required for the ITER torus and cryostat cryopumps. The jumpers are a custom design and allow the pumps to be disconnected for maintenance activities. They also allow the cryogens to be distributed with minimal heat loss and pressure drop. For more on the ITER cryopumps, visit this page of the ITER website.

Visit the exhibition and sponsorship pages of the MT-28 website to find out more about participating as a company or an academic partner in this premier international forum for magnet-related technology and design.  The 28th International Conference on Magnet Technology (MT-28), planned in Aix-en-Provence, France, during the week of 10 to 15 September 2023, is a unique opportunity to reach the superconducting magnet community, industry, and students and researchers. Multiple exhibition and sponsorship opportunities are available. For all information, see the MT-28 website.

The European Domestic Agency for ITER, Fusion for Energy, seeks to reward the commercial use of fusion technologies in non-fusion markets. Its Technology Transfer Award 2023, open to European companies and organizations, comes with a prize of EUR 10,000. The Technology Transfer Award competition aims to encourage and promote projects where a fusion technology or know-how is used or is planned to be used outside of fusion applications.Applications will be evaluated according to the resources and efforts deployed by the candidate to achieve commercial use of the technology in a non-fusion market, as well as the socio-economic impact of the project on the market. Applications are open from 1 February 2023 to 19 May 2023 at this link.