you're currently reading the news digest published from 04 Jul 2022 to 11 Jul 2022



Management | Walking through issues and achievements

The ITER worksite covers 42 hectares, comprises 39 buildings, and accommodates a myriad of different interconnected and interdependent activities. As the project progresses and its complexity increases, the need for senior management to have a comprehensive perception of activities becomes more acute every day. Of course, there are procedures: managers receive reports and hold meetings, and they are alerted about issues and risks. Four times a week, the Domain Heads and the Chief Strategist meet with the Director-General and share the day's concerns. And whenever managers can free a slot in their overbooked agendas, they don their blue helmet and yellow vest to go out on site and check out issues for themselves. Organized monthly, the management site walkthrough brings all these individual pieces together—like listening to the whole orchestra in order to fine-tune every single instrument.  'Issue' is the word most heard during a management walkthrough. But 'issues' are a normal part of a one-of-kind project that brings together 35 nations, with infrastructure spreading over the equivalent of 56 soccer fields and that mobilizes close to 2,500 workers from hundreds of subcontracting companies. 'With so many companies, so much co-activity, so many interfaces, there are issues every minute ... it couldn't be otherwise,' says ITER Director-General (interim) Eisuke Tada. Presently, most of the issues discussed have to do with interfaces. As more and more equipment is installed and as the different networks extend out and ramify, the original 3D drawings and meticulously planned equipment sequences face a daily confrontation with reality. When piping or massive steel structures extend for hundreds of metres, a minute deviation at one end translates into an interfacing difficulty at the other. When factories cannot deliver in time because of tension in the market for steel or disruptions to international transport, installation sequences need to be reorganized and adjusted. Most often, when someone on the work site says 'issue,' the collective echo responds 'under control.' 'It is of prime importance to be here with the teams and acknowledge the difficulties they encounter,' says Alain Bécoulet, Head of the Engineering Domain. 'Although we are kept informed through regular meetings and established procedures like those regarding non-conformities, being here in the field is different—to me, it feels like I'm updating my data files. This walkthrough is a precious moment of communication with the people most directly concerned.' Walking through the ITER installation, its massive structures and complex networks, alien-looking components and dizzying vistas, one could forget that this formidable research installation is but a means to an end: fusing sub-atomic particles in order to demonstrate the feasibility of hydrogen fusion. And the scientific challenge begins here, among the cranes and the scaffolding. Even at this stage, with operation scheduled in the second half of the decade, 'it is very important to have a clear view of the work being performed on site,' says Tim Luce, Head of the Science & Operation Domain. 'It is important to properly plan and execute the commissioning of systems, of course. But also, the ability to achieve the project's objectives depends vitally on the quality of the workmanship and the decisions made in the course of the present phase.' A walkthrough of the ITER worksite does more than provide the comprehensive overview that is indispensable to senior management. It is also about experiencing, in person, the day-to-day challenges, frustrations, and also the determination of the teams. 'What I saw today is how hard people work and how dedicated they are,' said Eisuke Tada as the group was breaking up after the two-hour outing. 'What I saw is an illustration of the 'one-team spirit' that permeates all those involved, whether ITER staff or subcontractors.'

Science | Understanding current flow during plasma disruptions

Scientists from the COMPASS tokamak at the Czech Academy of Sciences in Prague and the ITER Organization have performed a set of experiments to improve the understanding of plasma disruptions. This joint effort has provided the first experimental evidence of a physical limit to the flow of electric currents between the plasma and tokamak components during these events. This finding will help scientists to refine models and provide improved predictions of plasma dynamics and the associated forces on in-vessel components during ITER disruptions. In tokamaks, disruptions are sudden losses of the thermal and magnetic energy stored within the plasma, which occur when operating near plasma stability limits or when systems malfunction and plasma control is lost. Disruptions, particularly on large devices like ITER, can lead to high power fluxes and mechanical forces on in-vessel components, which can impact their lifetime. A major contributor to these loads are the electric currents that circulate between the plasma and the components during the 'current quench' phase of the disruption when the plasma current collapses. The amplitude and distribution of these electric currents (the so-called halo currents), together with the strong magnetic fields always present in tokamaks, determine the local mechanical stresses on the in-vessel components and thus need to be understood in detail to refine expectations for ITER. Physicists from the Institute of Plasma Physics in Prague working on the COMPASS tokamak and from the ITER Organization have collaborated to perform a series of careful experiments designed to measure the halo currents circulating between the plasma and surrounding components during disruption current quenches. The result is a unique database of high spatial resolution measurements of these currents. The experiments, reported in this recent publication, have demonstrated (Figure 1) that the local value of the halo current cannot exceed the local value of the plasma particle flux to the in-vessel components. This physics limit, already known for decades to apply under standard plasma conditions in the cool plasma edge where magnetic field lines intersect solid surfaces, has been found on COMPASS during these extremely transient disruptive phases using arrays of dozens of tiny, finely spaced electric sensors known as Langmuir probes embedded in the divertor target surfaces (see LPA and LPB in Figure 2). This probe system captured the halo and the plasma flux simultaneously for the first time during purposely triggered disruptions. The experiments also confirmed previous findings elsewhere that the global value of the halo current is proportional to that of the current flowing in the plasma before the disruption. This, together with the new limit identified in these experiments, means that the area of in-vessel components over which the halo current flows grows with increasing current when the limit is at work. This spreads the halo current across the in-vessel components and lowers the local stresses compared to what would be predicted if such a limit were not applied.  To improve predictions of plasma behaviour during disruptions, complex simulations are performed for ITER. These COMPASS results have already shown that inclusion of the newly identified halo current limitation in the simulation of disruptions is essential to reproduce plasma behaviour observed on COMPASS itself, as reported in a related publication. To consolidate predictions for ITER disruptions, it is important to experimentally determine the role that the newly identified limit plays on the dynamics of disruptions across other tokamaks operating within the ITER Members' R&D institutions and to reproduce the observations using the simulation codes applied to model ITER.  This will allow to both better understand and predict the loads expected during disruptions in ITER and to optimize their mitigation by the sophisticated disruption mitigation system being prepared for ITER.

Image of the week | A different shade of grey

With the exception of the poloidal field coil winding facility, operated by the European Domestic Agency Fusion for Energy, and of the Cryostat Workshop, where India assembled and welded the 54 segments of the ITER cryostat, all buildings on the ITER platform come in the same livery: an alternating cladding of mirror-like stainless steel and grey-lacquered metal. For ENIA, the architecture firm that was chosen in 2009 to work on the exterior of the buildings, this choice allows the scientific installation to blend into its natural environment and also expresses, "the precision of the research work being performed inside of the buildings." One building out of the 39 that the installation comprises, however, will be treated differently. Instead of an alternating cladding, the Control Building presently under construction will be dressed in dark grey metal "cassettes' at its base and in undulating stainless steel at its 'crowning' upper levels. ENIA explains its choice by the 'singularity' of the building, which hosts the control rooms, computer systems and servers that act as the very brain of the installation. 


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