News from Around the World

September 19, 2017


Just a short distance from the ITER site, the French Institute for Magnetic Fusion Research (IRFM) is modifying the Tore Supra plasma facility which, once transformed, will become a test platform open to all ITER partners — the WEST project (acronym derived from W Environment in Steady-state Tokamak). This summer, WEST's high frequency antenna spent one month in Titan's punishing environment - the last and most severe of its trials before being integrated into the refurbished CEA-Euratom tokamak Tore Supra. The component is one of three identical antennas that will deposit an ITER-relevant heat load of 10 MW per square metre on WEST's tungsten divertor. Titan (Testbed for ITer ANtenna) is a 17.5-cubic-metre vacuum vessel that can be heated to temperatures up to 250°C. It is equipped with a water loop capable of providing pressurized water (44 bars, also at 250°C) and can be connected to a high power radiofrequency generator for electrical tests. In the high-vacuum atmosphere of Titan, the 4.5-metre-long, 3-tonne component was submitted to three temperature cycles in order to induce dilatations and reveal possible leaks in its high pressure cooling circuit and vacuum volumes. The antenna's capacity to withstand high voltage was also tested and the parameters for its optimal performance determined. All tests were completed successfully and on 7 September, the spectacular and delicate operation of inserting the component into its port was performed. Carefully balanced with lead counterweights, the antenna—looking strangely like a giant squid—was lifted by crane, pulled by ropes and guided by hand all the way into its dedicated port in WEST. (The port measures approximately 60 x 80 centimetres and is about four times smaller than ITER's average port). By October, when all the ancillary systems are installed (cabling, hydraulic positioning devices, heat insulators, diagnostics ...) the antenna will be ready to face another punishing environment—the actual plasmas of an ITER-like tokamak.


Experiments in the Korean tokamak KSTAR in 2017 achieved record-length periods of ELM suppression by the application of three-dimensional magnetic fields with internal coils, which is the same approach for ELM control in ITER. Edge localized modes (ELMs), which can occurduring high-performance operation mode (H-mode),expel bursts of energy and particles from the plasma. The energy released can cause erosion in surrounding material, with potential impact on the lifetime of plasma-facing materials. The new KSTAR results demonstrate the robustness of the ELM control scheme adopted for ITER. They have also provided interesting information regarding the influence of the effects of the plasma shape on the robustness of this scheme for its practical application in ITER. In addition, robust ELM suppression has been obtained in KSTAR with 3D magnetic fields with one and two symmetry periods in the toroidal direction (n = 1, 2) over a range of plasma currents and toroidal fields, whose ratios corresponds to the expected range for long-pulse operation in ITER (burns of 1,000 to 3,000 seconds). This indicates that there might be more flexibility regarding the shape of the 3D magnetic field that needs to be applied for ELM control in the ITERlong-pulse scenarios than previously considered.


The cryostat vessel body of the JT-60SA tokamak has been successfully manufactured and pre-assembled at a factory in Spain, and will soon be transferred to the JT-60SA site in Naka, Japan, for final installation. This large containment vessel, formed from 12 sectors, will provide thermal insulation and a vacuum environment around the machine's magnet components. The JT-60SAtokamak is part of the Broader Approach agreement signed between Japan and Euratom, and implemented by QST Japan and the European Domestic Agency for ITER. It represents an upgrade of a previous tokamak at the Naka facility,designed to support the operation of ITER and to investigate how best to optimize the design and operation of fusion power plants built after ITER. First Plasma is planned for 2020, at the end of a six-year assembly and commissioning period. The cryostat vessel body has been procured as a voluntary contribution to the Broader Approach Agreement by Spain (through its national fusion research centre CIEMAT) and manufactured by Asturfeito, S.A. Once assembled in Japan, it will be capped by a top lid procured by QST. Pre-assembly at the Spanish facility allows stakeholders to be sure that all tolerances have been respected and that re-assembly at the Naka site will proceed smoothly. The 12 sectors are expected to arrive in Japan at the end of the year, between December 2017 and January 2018. See the full report (and video) published on the European Domestic Agency website.


A key issue for next-generation fusion reactors is the possible impact of many unstable Alfvén eigenmodes, wave-like disturbances produced by the fusion reactions that ripple through the tokamak plasma. These Alfvén eigenmodes allow alpha particles to escape from the reaction chamber before they can heat the plasma. Understanding these waves and how they help alpha particles escape is a key research topic in fusion science.

If only one or two of these waves are excited in the reaction chamber, the effect on the alpha particles and their ability to heat the fuel is limited. However, theorists have predicted for some time that if many of these waves are excited, they can collectively throw out a lot of alpha particles, endangering the reactor chamber walls and the efficient heating of the fuel. Recent experiments conducted on the DIII-D National Fusion Facility, which General Atomics operates for the U.S. Department of Energy (DOE) in San Diego, have revealed evidence that confirms these theoretical predictions. Losses of up to 40 percent of high-energy particles are observed in experiments when many Alfvén waves are excited by deuterium beam ions used to simulate alpha particles and higher-energy beam ions in a fusion reactor such as ITER.

In the wake of this research, physicists at the DOE’s Princeton Plasma Physics Laboratory (PPPL) produced a quantitatively accurate model of the impact of these Alfvén waves on high-energy deuterium beams in the DIII-D tokamak. They used simulation codes called NOVA and ORBIT to predict which Alfvén waves would be excited and their effect on the confinement of the high-energy particles. The researchers confirmed the NOVA modeling prediction that over 10 unstable Alfvén waves can be excited by the deuterium beams in the DIII-D experiment. Furthermore, in quantitative agreement with the experimental results, the modeling predicted that up to 40 percent of the energetic particles would be lost. The modeling demonstrated for the first time, in this type of high-performance plasma, that quantitatively accurate predictions can be made for the effect of multiple Alfvén waves on the confinement of energetic particles in the DIII-D tokamak.

"Our team confirmed that we can quantitatively predict the conditions where the fusion alpha particles can be lost from the plasma based on the results obtained from the modeling of the DIII-D experiments" said Gerrit Kramer, a PPPL research physicist and lead author of a paper that describes the modeling results in the May issue of the journal Nuclear Fusion. The joint findings marked a potentially large advance in comprehension of the process. "These results show that we now have a strong understanding of the individual waves excited by the energetic particles and how these waves work together to expel energetic particles from the plasma," said physicist Raffi Nazikian, head of the ITER and Tokamaks Department at PPPL and leader of the laboratory’s collaboration with DIII-D.

The NOVA+ORBIT model further indicated that certain plasma conditions could dramatically reduce the number of Alfvén waves and hence lower the energetic-particle losses. Such waves and the losses they produce could be minimized if the electric current profile in the center of the plasma could be broadened, according to the analysis presented in the scientific article. Experiments to test these ideas for reducing energetic particle losses will be conducted in a following research campaign on DIII-D. "New upgrades to the DIII-D facility will allow for the exploration of improved plasma conditions," Nazikian said. "New experiments are proposed to access conditions predicted by the theory to reduce energetic particle losses, with important implications for the optimal design of future reactors."

Members of the research team contributing to the published article included scientists from PPPL, General Atomics, Lawrence Livermore National Laboratory and the University of California, Irvine.


ITER construction continues on a broad front across all subsystems. For a complete story visit http://www.iter.org