Japanese tokamak physicist who led the JT-60 program and helped shape Japan’s path to ITER
Masaji Yoshikawa is a Japanese plasma physicist whose leadership of the JT-60 tokamak program at the Japan Atomic Energy Research Institute (JAERI) placed Japan among the world’s foremost contributors to magnetic confinement fusion research. Under his direction, JT-60 evolved from an ambitious national project into one of the most scientifically productive tokamaks ever built, generating results that directly informed the design basis for ITER.
JT-60 achieved first plasma in 1985 at the Naka Fusion Institute, north of Tokyo. The device was conceived during the 1970s as Japan’s answer to the American TFTR and the European JET—a large tokamak capable of exploring reactor-relevant plasma conditions. Yoshikawa oversaw the machine’s scientific program through a period of rapid progress, as the team demonstrated high-performance plasma regimes and pushed toward breakeven-equivalent conditions.1
In 1991, JAERI upgraded the device to JT-60U, equipping it with a divertor configuration and enhanced heating systems. The upgraded machine proved remarkably capable. In 1998, JT-60U achieved an equivalent fusion triple product that, had deuterium-tritium fuel been used instead of pure deuterium, would have corresponded to breakeven conditions. This “equivalent Q” milestone—sometimes cited as QDT,eq ≈ 1.25—remains one of the highest values ever recorded in a tokamak and demonstrated that Japan’s approach to plasma optimization was world-class.2
Yoshikawa was instrumental in positioning Japan as a full partner in the ITER project. The physics data generated by JT-60 and JT-60U—covering energy confinement scaling, disruption behavior, and divertor heat-flux management—became integral to the ITER physics basis. Japanese contributions to the international confinement scaling databases, many originating from JT-60U campaigns, helped establish the empirical scaling laws that underpin ITER’s projected performance.3
Beyond the machine itself, Yoshikawa contributed to shaping Japan’s broader fusion strategy, including the planning for JT-60SA—a superconducting successor built as a joint project between Japan and the European Union under the Broader Approach Agreement. JT-60SA achieved first plasma in 2023 and serves as a satellite facility for ITER, designed to explore advanced plasma scenarios and train the next generation of fusion scientists.4
Yoshikawa’s tenure at JAERI exemplified the value of sustained institutional commitment to a single machine through iterative upgrades. The JT-60 lineage—from the original device through JT-60U to JT-60SA—represents one of fusion’s most coherent long-term experimental programs. His emphasis on systematic optimization of plasma performance, rather than dramatic single-shot experiments, produced a body of work that remains foundational to tokamak physics. The operational techniques developed under his leadership, particularly in areas of current drive, bootstrap current optimization, and reversed-shear plasmas, continue to influence experimental programs worldwide.5