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ITER NEWSLINE 245
JET's role as a test bed for ITER operation is crucial to the success of the giant tokamak being built in the south of France—so crucial that JET is now preparing to make a deliberate error, just to predict how ITER will behave in such a situation.
The situation in question is the melting of a tungsten tile: the picture at right shows the deliberately misaligned tile stack being prepared for installation in JET. It will be located in JET's exhaust system, the divertor, an area at the bottom of the vessel in contact with the plasma. During operation the plasma circulates around the vessel at high speed guided by the magnetic field lines, which meet the walls in the divertor at a very low angle.
The heat load is spread by the particles skimming the surface in this way, but any part sticking out—putting its head above the parapet—will be subject to up to thirty times more heat than its properly aligned neighbours.
The experiment is part of the exploration of material behaviour that JET is carrying out on ITER's behalf. Tungsten has an important role in tokamaks as the metal with the highest melting point, 3,422 degrees celsius—obviously a great advantage when containing hot plasma. However if tungsten does get vaporized and contaminate the plasma, its large number of electrons give rise to a lot of radiation, which can seriously affect energy confinement.
Since fusion plasmas usually operate in the range of hundreds of millions of degrees, reaching 3,422 degrees and melting tungsten does not sound hard—in fact it has been done before in experiments in other devices. What complicates this experiment is the requirement to melt the tile with transient heat generated by turbulent events known as Edge Localized Modes, or ELMs. While the base temperature of the tiles can be easily modelled and controlled to prevent melting, the sudden bursts of energy supplied by ELMs may be less predictable in ITER's larger, hotter plasma.
JET, as the largest operational tokamak in the world, is the only facility that has enough energy in its plasma to achieve a transient melt. Its ELMs are much studied—they can occur up to 30 times a second, but last less than a millisecond, so it will take care to get the peak temperatures above the melting point while keeping the steady temperature below. It is hard to know exactly how much damage these events could cause.
"After a melt, the best result would be if the next few pulses remove or erode the melted material and then we are back to operation that is as good as before," says Chief Engineer Valeria Riccardo. Just to be on the safe side the area chosen for the melt, Tile 5, Stack A, is at one end of the assembly. "It's an area not used much as the plasma contact point because it is difficult to achieve good density control in that configuration. That way even if the damage is not recoverable we can still use the better configurations for other experiments."
The first testing of plasma-facing components for ITER's outer divertor target full-scale prototype started at the Efremov Institute in St Petersburg, Russia in late October.
These crucially important and sophisticated heat-capturing elements will be in direct contact with the plasma—a first barrier that will withstand the main heat flux from plasma during operation. As the plasma temperature is to reach 100-150 million °C, and the expected heat load on the divertor surface up to 20 MW/m2, the components under test have challenging requirements to meet.
To conduct the tests in Russia, a special ITER Divertor Test Facility was assembled at the Efremov Institute as part of the Russian commitment for the ITER Project. Within the Facility, an 800 kW electron injector exposes the components to the same heat loads they will face inside the ITER vacuum vessel in the standard operational mode and allows the testing of their reliability.
The tests being carried out in Russia's northern capital are a vivid example of close international cooperation within the implementation of the ITER Project: the components were manufactured in Japan and shipped by our Japanese colleagues directly to St Petersburg for testing at the Russian facility. In compliance with the spirit of tight international collaboration, the works are being carried out in the presence of Russian and Japanese Domestic Agency specialists, as well as experts from the ITER Organization.
The first test results are expected in late November; several dozen test series will follow. The results will make it possible to adjust the manufacturing technology for these challenging plasma-facing components.
The latest advances in plasma physics were the focus of more than 1,000 scientists from around the world who gathered in Providence, R.I., from 29 October-2 November for the 54th Annual Meeting of the American Physical Society's Division of Plasma Physics (APS-DPP). Papers, posters and presentations ranged from fusion plasma discoveries applicable to ITER to research on 3D magnetic fields and antimatter. In all, more than 1,800 papers were discussed during the week-long event.
Researchers from the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) reported on experiments and computer simulations related to tokamak confinement and a variety of other research interests. These included specialized areas such as laboratory and astrophysical plasmas, where PPPL physicist Hantao Ji was prominent as a topic chair and speaker at a tutorial session.
Members of the Laboratory's National Spherical Torus Experiment Upgrade (NSTX-U) team gave a tutorial and three invited talks. Physicist Dennis Mueller presented the tutorial on "Physics of Tokamak Plasma Start-up."
The Laboratory sent 135 physicists, science educators and graduate students to the meeting and saw some of its research highlighted in news releases on the APS-DPP website. Of the 15 papers highlighted in this manner, seven came from PPPL.
The meeting focused considerable attention on boundary physics and plasma-material wall interactions, an area of growing emphasis at PPPL. Dennis Whyte, a professor of nuclear science and engineering at the Massachusetts Institute of Technology, presented a major review of the subject to a plenary session. Invited speakers on the topic of plasma-wall and impurity physics included PPPL scientists Filippo Scotti and Dick Majeski, principal investigator for the Laboratory's Lithium Tokamak Experiment (LTX).
PPPL physicists Michael Jaworski and Igor Kaganovich participated in a session on plasma-wall interactions, with Jaworski serving as chair and Kaganovich giving the first invited talk in the session.
The importance of boundary physics has been recognized in innovations like the so-called snowflake divertor, which limits the heat on tokamaks' inner walls. The divertor, developed by researchers at PPPL and the DOE's Lawrence Livermore and Oak Ridge national laboratories, won an R&D 100 Award in June from R&D Magazine.
The device "reduces both the power flux on plasma-facing components and the influx of impurities into the core plasma," said PPPL physicist Robert Kaita, the head of diagnostics and physics operations for the National Spherical Torus Experiment Upgrade (NSTX-U), and co-principal investigator for the LTX.
Considerable interest also was shown for inertial confinement fusion experiments at the National Ignition Facility (NIF) at the DOE's Lawrence Livermore National Laboratory. Speakers noted that producing fusion by heating a capsule producing energy with high-powered lasers was proving more difficult than expected. NIF scientists now seek to develop a more detailed understanding of the physics of this process in order to achieve ignition.
Last Wednesday, ITER Director-General Osamu Motojima called for an all-hands meeting in the Headquarters' brand-new amphitheatre in order to brief the ITER Organization staff on the outcome of the recent meetings of the project's scientific and managerial advisory committees. To this memorable event, Director-General Motojima had invited both the present and former chairmen of the Management Advisory Committee, Ranjay Sharan and Bob Iotti.
At the outset, the Director-General presented the conclusions of the 14th meeting of the project's Management Advisory Committee (MAC) that had taken place on 29-31 October. The MAC had acknowledged the intensive work done by the ITER Organization in collaboration with the seven Domestic Agencies since the special MAC meeting held in August. Required schedule recovery actions have been taken and the collaboration between the ITER Organization and the Domestic Agencies has been intensified through the establishment of the Unique ITER Team.
"However, the MAC recognized that further and intensive efforts are necessary," MAC Chair Ranjay Sharan explained. "The variances will have to be minimized by parallel working approaches and innovative methods. The MAC will closely monitor these approaches."
"Yes, there are issues," Iotti admitted, "but we are working closely together to resolve them." Of great concern: the delays related to six super-critical items—the buildings, the vacuum vessel, the poloidal field coils, the toroidal field coils, the central solenoid conductor and the cryostat.
Two other essential issues were the focus of this 14th MAC meeting: the rules for further distribution of credits amongst the ITER Members as proposed in the "MAC-10 Guidelines," and the proposal for a simplified assembly plan with the intention to recover some of the time slippages. "Based on the different feedback we received to this plan, the MAC suggests that the project remain focused on the normal step-by-step assembly strategy, but that it evaluate options to reduce risks and the time required for the assembly and the transport of components in order to provide more confidence in the dates for First Plasma and Deuterium-Tritium operation," Sharan said.
As for the technical assessment, the Science and Technology Advisory Committee (STAC) commended the ITER Organization and the ITER Domestic Agencies on significant progress made, especially in the manufacturing of ITER magnets. More than 350 tonnes (73,000 km) of niobium-tin (Nb3Sn) strand for the toroidal field conductor have been produced so far, corresponding to approximately 75 percent of total amount needed. Also, approximately 65 tonnes of poloidal field conductor strand (25 percent of supply) have been produced.
The STAC noted that—with the exception of the poloidal field coils—there are currently no new major delays in the critical path due to magnets. The STAC further complimented the ITER Organization's comprehensive report on remote handling and the good progress that has been made in developing a strategy for the installation, maintenance and potential repair of the first wall and the divertor.
"Take pride in what you have accomplished so far," and, "Work in cooperation with others as team," were the final comments from Bob Iotti and Ranjay Sharan respectively.
A workshop on fusion technology beyond ITER was successfully held between the Japanese and the Korean Domestic Agencies on 8-9 November at the National Fusion Research Institute in Daejeon, Korea. A first event of this kind, the workshop aimed at sharing the technology and experience of ITER procurement and also at discussing the development pathway for fusion engineering and technology beyond ITER in Japan and Korea.
More than 40 experts in fusion attended from both countries, including the head of the Korean Domestic Agency, Dr. Kijung Jung, and the head of the Japanese Domestic Agency, Dr. Eisuke Tada.
As both Domestic Agencies have entered into the full-fledged process of procurement for ITER, it was beneficial to share technical know-how, and to exchange ideas in regards to meeting the procurement schedule as well as securing core technology without any loss of productivity.
In addition, the workshop contributed to building close collaboration between the Japanese and the Korean Domestic Agencies, precisely in the spirit of the Unique ITER team for the successful implementation of all commitments for the ITER project.