On 9 January 2003, Japan lost one of its most prominent theoretical plasma physicists when Masahiro Wakatani died unexpectedly from a cerebral hemorrhage at his home in Nara, Japan.
Wakatani was born on 15 May 1945 in Osaka, Japan. He graduated from Kyoto University in 1968 with a BSc in nuclear engineering and received a doctor of engineering degree in electrical engineering from the same university in 1973. His doctoral research, under Ryohei Itatani, was on the magnetohydrodynamic (MHD) equilibrium and stability of a toroidal pinch. In his thesis, he calculated the collision diffusion coefficients for that device, using the now-famous neoclassical transport theory that had just been developed by Roald Sagdeev and Alec Galeev. Their theory indicated that trapped particles significantly increase diffusivity in toroidal confinement devices.
Following the completion of his thesis, Wakatani joined the Japan Atomic Energy Research Institute, in Naka, as a member of the thermonuclear research division. In 1976, he moved to the Institute of Plasma Physics at Nagoya University. At both places, he continued to work on MHD stability and neoclassical transport, although his interest had shifted toward the tokamak, whose confinement by then had outperformed that of other fusion devices.
Wakatani joined the faculty of Kyoto University’s Plasma Physics Laboratory in 1978 as an associate professor. As leader of a theory group, he began collaborating with experimentalists who worked on the Heliotron, a plasma confinement device with a helical magnetic field characterized by high shear. That same year, he published the reduced MHD equations for helical systems—a simplified version of the full MHD equations that still retained the essential physics for a helical plasma confinement device and also were amenable to nonlinear numerical simulations using the computational power available at that time. Wakatani’s reduced MHD equations, later generalized by Henry Strauss for helical systems with high plasma pressure, turned out to be extremely useful for analyzing the complex MHD properties of those devices. The equations are still the popular, basic model for helical systems.
In the early 1980s, Wakatani began to study anomalous transport due to turbulence arising from microinstabilities, a major obstacle to improved plasma confinement. In 1983, Wakatani and one of us (Hasegawa) derived a set of nonlinear evolution equations (now known as the Hasegawa-Wakatani equations) that describe strong turbulence caused by the resistive drift-wave instability at the edge of a plasma. With these equations, we showed that an inverse cascade of energy from short to long wavelengths occurs in turbulence. We also found a prototype mechanism for generating large-scale poloidal flow structures, for instance, zonal flows and streamers in a tokamak; such structures are currently the subject of extensive investigation.
Wakatani was appointed to a full professorship in the Plasma Physics Laboratory in 1985. Ten years later, because of a major reorganization within Kyoto University, he became a professor in the newly established Graduate School of Energy Science and in the department of nuclear engineering.
In addition to his scientific accomplishments, Wakatani was a talented administrator at Kyoto University and in the international fusion community. At the time of his death, he was a key member of the University Council; he provided valuable direction in preparation for the imminent reorganization of the entire national university system in Japan. He played a leading role in the worldwide program to design ITER, an international thermonuclear experimental reactor. From 1980 to the time of his death, he was a steering-committee member of the Joint Institute for Fusion Theory (JIFT), which coordinated US–Japan collaboration activities. He headed up the Japanese delegation to the first JIFT workshop and organized more workshops in Japan. He was the JIFT visiting professor to the US in 1988.
Wakatani was a dedicated teacher who supervised dozens of thesis students. Graduates of the “Wakatani school” now populate theory groups around Japan. Despite his busy schedule, he found time to write four fine plasma physics textbooks. He was devoted to his family and especially enjoyed listening to his wife and three daughters perform classical chamber music.
As a scientist, collaborator, administrator, teacher, and friend, Wakatani was a gentleman—courteous, dignified, calm, and considerate. He looked for praiseworthy characteristics in people. Even during heated discussions, he was willing to listen, be fair, and provide information. When asked for help, he did not say no, and he would commit himself to even those tasks that were an imposition on his work schedule. His creativity, integrity, and leadership were deeply respected by the international scientific community.
