A wealth of data harvested

Throughout 2023 and continuing this year, teams of engineers and scientists at the ITER Organization have been focusing intensely on a re-baselining exercise intended to mitigate some of the delays experienced by the project due to the Covid-19 pandemic and some hardware issues on the main vacuum vessel and associated thermal shields. One of the key components of the re-baselining is the replacement of beryllium by tungsten as plasma-facing armour on the main chamber walls, representing some 600 m2 of material surface.  

Beryllium is a low-atomic-number element, so that relatively high concentrations can be tolerated in the hot plasma core before fusion burn is compromised.  On the downside, the well-known issues associated with toxicity, high erosion under plasma bombardment and the tendency of eroded material to trap the tritium fusion fuel on the reactor walls mean that the benefits of a beryllium first wall for the plasma are offset by significant technological, operational and licensing issues for the plant as a whole. The particular issues of high erosion and fuel trapping, coupled with relatively low melting point, also mean that beryllium cannot be considered as a reactor-relevant wall material.  On the contrary, tungsten is widely accepted as the best candidate plasma-facing material for future fusion power plants. Its main drawback is that burning plasmas can tolerate only minuscule tungsten concentrations in the hot core.

While the importance for ITER of the recent JET success in producing record fusion plasmas with a beryllium first wall cannot be understated, the physics basis for "full tungsten" ITER operation, as proposed in the new re-baseline, is also far stronger than it was when the ITER Organization was established in 2007. At that time, the ASDEX Upgrade tokamak at the Max Planck Institute for Plasma Physics in Garching, Germany, had only just made the conversion to full tungsten coverage while, in the United States, the high-magnetic-field Alcator C-Mod device had been running for about a decade with metal walls, but using mostly molybdenum—also a high-atomic-number element, but a contaminant more tolerable to fusion-grade plasmas than tungsten. Since then several tokamaks have joined the race: such as the WEST tokamak near ITER at CEA Cadarache, the EAST tokamak at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) in Hefei and, more recently, the Korean Institute of Fusion Energy's superconducting KSTAR tokamak in Daejeon, which restarted operation in late 2023 after a year-long shutdown to install a new actively cooled, ITER-like tungsten divertor.   

Together, the results of these experiments at ITER partner facilities provide much greater confidence that a decision to start ITER operation with tungsten and reach the project's fusion goals is no longer the high-risk option it seemed in 2007. But there remain some physics uncertainties coming both from the magnitude of the extrapolation from the medium-sized tokamaks to ITER, and the need to have key results from more than a single device. The main issues of this extrapolation are known and are being addressed in the ITER Science Division, with the help of experts in the fusion community through the ITER Scientist Fellowship Network and the International Tokamak Physics Activity (ITPA). Their involvement has been actively sought by the ITER Organization, and met by an enthusiastic response, from the very beginning of the re-baselining activity. But some of the outstanding questions require new tokamak experiments and—with ASDEX Upgrade currently in major shutdown throughout 2023 until the summer of 2024, and WEST only just restarting operation after its last 2023 maintenance period but still without sustained high confinement (H-mode) performance—time is tight to gather new data in time to support the new baseline proposal to be submitted to the ITER Council in June 2024 for a decision in November.

During discussions at the 2023 IAEA Fusion Energy Conference, the ITER Organization therefore requested support from ASIPP and received, in November 2023, the extremely generous offer to place the EAST tokamak and its team at the disposal of the ITER Science Division for a mini-campaign from December 2023 up to the start of the Chinese New Year (10 February 2024) targeting ITER re-baselining activities. The offer was all the more generous taking into account that EAST has operated in the last decade with ultra-long-pulse, high-power-performance plasmas as main focus, for which the requirement for strong external current drive with a specific form of radio frequency plasma heating using so-called lower hybrid waves (not a heating scheme ITER will use) needs low density plasmas—precisely a regime in which high-atomic-number first wall materials are not ideal. To solve this, EAST has routinely used the injection of lithium powder into their discharges, providing a temporary low-atomic-number coating on plasma-facing surfaces and preventing the ingress of heavier impurities eroded from the walls. Operation to address specific ITER questions on the use of tungsten thus required significant advanced preparation for the EAST team and a clear focus for the experimental campaign.

The ITER Science Division settled on three main ITER priorities for the EAST experiments: the optimization and characterization of a wall conditioning process known as boronization, plasma start-up on tungsten surfaces, and the impact on H-mode operational space of running soon after, and far in plasma operation time from, a boronization coating. While these issues have to some extent been addressed on ASDEX Upgrade and WEST operating with tungsten, the new EAST experiments have made a significant addition to the database.

Targeted boronization experiments on EAST aimed to add further important data to the catalogue of experience on current devices, but also to go further and investigate the impact on the uniformity and quality of the coating as a function of the way in which the boronization is performed. In particular, comparing the efficacy of active radio frequency plasma generation, known as ion cyclotron wall conditioning (ICWC, which uses high power ion cyclotron antennas used normally for tokamak plasma heating) with the more standard technique of direct current glow discharges induced by specially designed passive anodes. ITER is being equipped to do both, but quantitative data on ICWC-induced boronization is almost non-existent.

Since the thin boron coatings are quickly eroded in areas of direct tokamak plasma contact (such as the zones on the wall where tokamak plasmas are usually initiated), a key question is how easy it will be on ITER to ramp up plasmas in such "limiter configurations" on areas where tungsten is fully exposed. In the early phases of tokamak pulses—lasting only a few seconds on ITER and usually less than one second on smaller devices—plasmas resting directly on tungsten surfaces are particularly prone to high radiation losses, when the eroded tungsten has a direct route into a still relatively fragile plasma. If that phase can be successfully navigated and the plasma is moved away from the walls to form a "diverted configuration"—when plasma heat and particles are predominantly exhausted to a region (the divertor) distant from the main reaction chamber—another important question is how the gradual removal of the boron coating impacts the high-performance H-mode plasma operation which is required to achieve Q=10 plasmas in ITER.

With these main themes in mind, experimental plans were drawn-up within the ITER Science Division through December 2023 and early January 2024, with the participation of external ITPA experts and EAST team members. While many were enjoying the Christmas festivities, the EAST team was operating and preparing for the ITER experiments which took place for the most part between 18 and 26 January, with three ITER Science Division members, Alberto Loarte, Richard Pitts and Tom Wauters, traveling to EAST to participate. 

By any standards, this was an intense few days of experiments, with EAST operating typically from 9:00 a.m. to 11 p.m. most days, including weekends. Fast-pulse repetition rates and a very efficient operating team combined to make for an eye-watering experimental pace, all conducted from the futuristic and quite magnificent EAST tokamak control room, which must surely rank as the finest of its kind. Remote participation is also very efficient at EAST, allowing Joerg Hobirk from the ASDEX Upgrade team and other ITER Science Division members to participate in the experiments.

In all, some 6 boronization processes and over 400 tokamak pulses were executed during the campaign, providing a wealth of data which will require quite some effort and time for the EAST team, in collaboration with ITER Organization staff and external experts, to properly analyze, understand and present. Valuable new information has been obtained on boronization uniformity and process and on routine plasma start-up and stationary operation on a pure-tungsten, water-cooled outer limiter (including a record 17 sec limiter discharge). A clear demonstration has also been obtained that good H-mode confinement can be obtained in both boronized and unboronized conditions in proximity to tungsten main chamber surfaces, provided edge localized magnetohydrodynamic (ELM) activity is kept to low levels. Since this is already a condition for adequate lifetime of the ITER tungsten divertor targets, the EAST experiments provide yet more evidence for the importance of ELM control on a full-tungsten ITER.

In a short break from experiments, the ITER scientists were treated to an eye-opening visit to the new CRAFT site (Comprehensive Research Facility for Fusion Technology), a short drive from the ASIPP campus. The facility was completed in early 2022 and is now prototyping components for the Chinese fusion reactor program and manufacturing coils for the next big project to follow EAST, the BEST tokamak, which will use deuterium and tritium fuel and is due to begin operation later this decade. No one spending time in the CRAFT visitors centre, even seasoned fusion campaigners, can fail to be impressed by the way in which the amazing field of fusion science and technology is communicated there to the wider public.

The ITER Organization thanks ASIPP Director General Yuntao Song for opening the EAST facility for these experiments and for the generous hospitality extended to ITER scientists. Though intense, this visit to EAST was made a rewarding and efficient experience by the excellent and seamless organization provided by Xianzu Gong, Head of the Division of Tokamak Experiments, and his deputy Rui Ding. The entire EAST team (especially Jinping Qian, Manni Jia, Youwen Sun, Qingquan Yang, Ling Zhang, Xinjun Zhang and Guizhong Zuo) deserves enormous thanks and credit for working so hard before and during the experiments, and in advance for all the work to come in analyzing the data obtained.

A happy new Year of the Dragon to all!