Numbers, graphs and a wobbling purple haze on the monitoring screens—this is what a plasma shot looks like when seen from the control room of the WEST tokamak. Since its first plasma in December 2016, the former Tore Supra tokamak has logged some 2,500 shots. Upgraded, transformed, equipped with an actively cooled tungsten divertor, and graced with a new name—WEST (Tungsten (W) Environment in Steady-State Tokamak)—the machine is being groomed to act as a test bed for ITER, minimizing industrial and financial risks and obtaining experimental data to prepare for operation.   On 16 February, WEST shot the last plasmas of a campaign that had begun one month earlier with the coupling of the machine's two lower hybrid antennas. By the end of this year, the plasma heating system, including three ion cyclotron resonance heating antennas procured by China, should be fully operational.   WEST is now well advanced on the way to becoming an "ITER-like" machine. Out of the 456 actively cooled plasma-facing units in tungsten that make up the divertor, six (three procured by Japan and three by China) are already in place and six more (procured by Europe) will be installed in the coming months. The full actively cooled tungsten divertor configuration should be ready for operation at the end of 2019.   In the meantime, operators in the control room are "learning to drive." Although several features from the "old" Tore Supra have been preserved, WEST is definitely a new machine with a different magnetic configuration (extra coils have been installed under the divertor) that allows for the production of ITER-like D-shaped plasmas.   Jérôme Bucalossi (right) is confident that by the time the tungsten divertor is complete, WEST will have reached the high confinement mode that will be ITER's operational regime. Over the past few months, the WEST team has been busy fine-tuning the coils, adjusting the position and power of the first lower hybrid antenna, and monitoring the behaviour of the plasma-facing components. Jérôme Bucalossi, who heads the WEST project at CEA's Institut de Recherche sur la Fusion Magnétique (IRFM), is confident that by the time the tungsten divertor is complete, WEST will have reached the high confinement mode ("H mode") that will be ITER's operational regime.   Although almost routine by now (WEST produces an average of 30 pulses per operating day) the pulsating haze on the screens makes a fascinating sight—deuterium nuclei spinning madly for a few seconds inside a magnetic cage. Not quite fusion yet ... but a foretaste in anticipation of the real thing.   Click here to view a video of a plasma shot in WEST.

The first energy-generating devices of ITER's electron cyclotron resonance heating system will be finalized in 2018 after a multi-decade development program. Back in early 1990s, few were convinced that electron cyclotron resonance heating (ECRH) could fit the bill when it came to achieving the high-power, high-frequency parameters required for ITER plasma heating. In fact Mark Henderson specifically remembers his thesis advisor warning him to "go into any field but that one."   But technological breakthroughs on a number of key components of the gyrotron—the microwave-generating device at the core of the ECRH system—have led to steadily increasing performance and, today, in addition to being one of three external heating systems on ITER, ECRH is considered as one of the more viable candidates for the mix of external heating techniques under consideration for the larger, more powerful fusion reactors planned next.   "From performance measured in kilowatts in the early years of development, we now have devices routinely operating at 1 MW (one million watts) for periods of 1000 seconds," says Henderson, who leads the Electron Cyclotron Section at ITER. "This development has largely been driven by the needs of the fusion community, although gyrotrons also have applications as heating devices in industry or as microwave sources in spectroscopy and diagnostics."   ITER will rely on the ECRH system to initiate each plasma shot, contribute 20 MW of heating power to the plasma, and suppress certain types of plasma instabilities.   Within the long-pulse, continuous wave gyrotrons under development for ITER, a beam of electrons is generated and accelerated toward a large cavity. A strong field produced by a magnet outside of the cavity modifies the circular movement of the electrons, resulting in high-frequency waves that act like a laser beam and that can then be guided, using mirrors, to specific areas of the ITER plasma.   From the Radio Frequency Building where they are generated, the microwave beams will journey about 160 metres along waveguides to launchers at the equatorial and upper levels of the vacuum vessel. Electron cyclotron heating at ITER will require 12 sets of high-voltage power supplies, 24 gyrotrons, 24 transmission lines, and 5 launchers. This Russian-manufactured unit is installed at the gyrotron test bench of the Swiss Plasma Center. Equivalent to the gyrotrons manufactured in Russia for ITER, the production module was purchased by Europe to test upper launcher components and is available to other ITER partners for testing waveguide components. Note the superconducting magnet at the base of the gyrotron (manufactured by Jastec) and the large collector at the top (surrounded by lead to shield against X-rays). "The power and frequency requirements for ITER (1 MW at 170 GHz) were determined in the late 1990s," recalls Henderson, "however the difference of scale meant that we were dealing with all new technology. In the years since, the electron cyclotron community has established the physics basis, completely redesigned the launcher, developed high-precision waveguides for low-loss transmission, and pushed the performance of a number of key gyrotron components such as artificial diamond windows."   In this period of "finding what works and what doesn't," researchers in Europe, Japan, Russia, and the United States were all closely involved—the same Members (plus India) that are today procuring the electron cyclotron system for ITER*. Beginning in 2012 the first Procurement Arrangements were signed, followed by lengthy prototyping, review and test phases.   The first gyrotron units have now been completed in Japan and Russia and final testing is underway. By end-2018 factory acceptance tests will have concluded on two gyrotron units in Japan, two units in Russia, and power supplies in Europe. Installation activities are planned to start in 2020 for the eight gyrotron units needed for ITER's First Plasma; (16 others units will be installed at a later assembly phase).   "The electron cyclotron system will be coupled with the other heating systems in ITER—neutral beam injection and ion cyclotron—to provide a 'heating service' to the plasma," concludes Henderson. "Each has its functionalities and advantages that have been maximized through engineering to get the best performance we can. Soon, experimentation will show us what combination will be best for plasma performance in future devices."   * Five ITER Members are participating in the procurement of the electron cyclotron system at ITER: Europe (6 gyrotrons, 12 power supplies, 4 upper launchers), India (2 gyrotrons, 4 power supplies), Japan (8 gyrotrons, 1 equatorial launcher), Russia (8 gyrotrons), and the United States (all transmission lines).  

However complex the science or sophisticated the technology at ITER, there is one simple activity that conditions future success—the ability to lift and manoeuvre exceptionally heavy loads during the project assembly phase. With a nominal lifting capacity of 1,500 tonnes, the undisputed champion of ITER weightlifters is the double overhead crane in the Assembly Hall. Second on the podium is the gantry crane in the Poloidal Field Coils Winding Facility—a 30-metre-in-diameter steel structure supported by four hydraulic towers traveling on rails, capable of lifting the heaviest components of the ring-shaped coils throughout the last stages of fabrication.   Personnel from the European Domestic Agency and from crane manufacturer Ale Heavy Lift are overseeing the load tests. Europe is responsible for procuring 5 out of the 6 poloidal field coils required for ITER operation. Load tests for the gantry crane began on 16 February with static operations at 125 percent of its nominal lifting capacity of 400 tonnes. Dynamic operations followed at 110 percent in order to monitor the deformation of the beams. With a deflection of less than 119 millimetres, the tests were conclusive.   The bright red gantry crane was tested in three operational lifting configurations corresponding to poloidal field coils #2 and #5 (up to 342 tonnes, Ø 17 metres), poloidal field coils #3 and #4 (up to 385 tonnes, Ø 24 metres), and finally poloidal field coil #6 (396 tonnes, Ø 8.5 metres)¹.   ¹ Poloidal field coil #6 (PF6) is being manufactured in China under a European contract but will be cold tested in the on-site poloidal field coil facility.   For more on the poloidal field coils click here and here.

ITER's first 22-metre-tall vacuum vessel sector sub-assembly tool is going up quickly in the Assembly Hall. After a long period during which the anchoring elements—including base plates and rails—were set into place and precise measurements taken, the installation of ITER's first giant assembly tool is underway.   The inboard column already stands at 18 metres, with only 4 metres to go to reach its full height. To either side of the column, lateral wings are under construction and two other tall columns—the outboard columns—will be erected in the days and weeks to come. A testing phase will follow. The tool, once fully erected, will be capable of supporting loads of 1,200 tonnes. Vacuum vessel sectors shipped from Europe and Korea will first be upended from their horizontal shipping position, and then suspended vertically by one of two sector sub-assembly tools while a pair of toroidal field coils plus thermal shielding is attached. The final assembly will be transported by overhead crane to the Tokamak assembly arena.     In all, about half of the major components for the first tool have been installed. In parallel, the elements for the second tool have been manufactured and assembled at Taekyung Heavy Industries (THI) in Korea, where testing is underway.

The weather service had forecast a snowfall. What we got instead was gorgeous late afternoon light revealing every detail of the spectacular structure facing our windows. Most the scaffolding visible from ITER Headquarters has been removed and we can now clearly see two of the three cargo lift openings for the cask and plug remote handling system at the lower levels of the structure (centre left). To the right a large rectangular opening has been reserved for installation activities related to the neutral beam injection system. Further to the right, another smaller opening will serve the same purpose.   At the top of the concrete structure, a fourth cargo lift opening is being prepared (centre) and temporary structures, looking like oversized diving boards, have been installed to assist in the lifting operation for the temporary lid of the bioshield. In March, the lid will be raised from its current location to the top of the bioshield to create a protected work area below.

In a recent report in Nuclear Fusion, an international team of researchers outlines an approach for using electron cyclotron heating to control instabilities known as "neoclassical tearing modes" that can cause magnetic islands to grow in, and perturb, the plasma. Lead physicist Francesca Poli of the Princeton Plasma Physics Laboratory (PPPL) worked with two of her colleagues and researchers from the ITER Organization, the Max Planck Institute for Plasma Physics in Germany, and the Institute of Plasma Physics in Italy, to describe an approach that for the first time simulates the plasma, the magnetic islands and the feedback control from the electron cyclotron waves. The current from the electron cyclotron waves (see related article in Newsline) has to be matched with the magnetic island. The simulations performed help to determine the maximum misalignment that can be tolerated and under which conditions experiments should be run. Read the full article on the PPPL website.