JT-60SA is now the largest operating tokamak: What does that mean for ITER and JET?

November 6, 2023, 12:00PMNuclear News

Operators of JT-60SA—a joint international fusion experiment being built by Japan and Europe in Naka, Japan—achieved the first tokamak plasma in the machine in late October, making it the world’s largest operational tokamak.

“The result will be carefully examined and the team will continue to perform more tests during the next weeks,” the JT-60SA team stated in an October 24 announcement. “A ceremony to officially inaugurate JT-60SA will be held in Naka on December 1, with representatives and guests from both Japan and Europe.”

Super advanced: According to the JT-60SA website, SA stands for “super advanced,” since the experiment will use superconducting coils and study advanced modes of plasma operation. JT-60SA uses infrastructure from a previous JT-60 Upgrade experiment as well as new hardware, and it will feature up to 41 megawatts of heating power provided by two different heat sources: electron cyclotron resonance heating and neutral beam heating.

JT-60SA will not use the deuterium and tritium fuel that ITER and most fusion power plants now on the drawing board would use, but instead will initially use hydrogen, and later deuterium. Those fuel choices mean the machine will have minimal radioactivity over its operating life, maximizing flexibility and potential reconfigurations.

According to the JT-60SA website, the machine would be capable of reaching or surpassing break-even—with as much power released by the fusion reactions as is provided through heating—if deuterium and tritium were used. But instead of going for break-even, JT-60SA is designed to “provide critical information for ITER operation, allowing ITER to go on to produce much more power than is injected.”

Research by design: ITER—now under construction in France—will be bigger than JT-60SA in terms of both size and the group of contributors but will have some assistance from the smaller JT-60SA, which was designed with a complementary research and development program. JT-60SA can also help researchers optimize the operation of fusion power plants that follow ITER, including DEMO, an international fusion project set to follow ITER.

Researchers will be able to study power and particle handling for 100 second high-power discharges, with water-cooled divertors compatible with high heat fluxes. In addition to studying how best to get to and beyond breakeven conditions, JT-60SA is designed to explore the steady-state operation envisioned for tokamak-based power plants and high beta plasma regimes.

With JT-60SA, researchers can explore ITER-relevant high density plasma regimes and will have the capability to operate in different divertor configurations (both single and double null) to test different scenarios. A JT-60SA research plan details the project’s immediate experimental goals.

Following JET: JT-60SA has surpassed the Joint European Torus (JET) as the world’s largest operating tokamak as JET winds down its final series of experiments by the end of the year ahead of decommissioning, which is scheduled to begin in 2024.

JET began operating in 1983 and has performed 40 years of experiments, but some would like to see its work continue. A group of researchers organized a petition to extend JET beyond 2023: “The petition was launched on October 16th, 2023, after the ITER talk at IAEA FEC [International Atomic Energy Agency Fusion Energy Conference] describing ITER’s new plans: a tungsten wall and dominant electron cyclotron heating. Such a dramatic ITER move changes the timeline of magnetic fusion significantly. The closure of JET in 2023 now seems premature as it would result in a skills shortage, and technological and scientific gaps: no equivalent fusion DT [deuterium-tritium] plant will be available for many years to come. Rather than closing JET at the end of 2023 and decommissioning it from 2024 (as presently planned by UKAEA), we propose the installation of a tungsten wall in JET and an ECRH [electron cyclotron resonance heating] system in JET, as an international collaboration.” By November 2, the petition had garnered over 800 signatures.

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