A visionary plasma physicist at General Atomics, Tihiro Ohkawa invented the doublet plasma shape that evolved into the DIII-D tokamak and pioneered non-inductive current drive methods essential to steady-state fusion reactor design.
Tihiro Ohkawa was born on February 5, 1928, in Tokyo, Japan. He studied physics at the University of Tokyo, where he earned his bachelor's degree, and completed his Ph.D. in physics at the same institution in 1955. His doctoral work focused on particle accelerator physics, but he soon became captivated by the emerging field of controlled thermonuclear fusion. In 1959, he joined General Atomics (then General Atomic) in San Diego, California, where he would spend the entirety of his fusion career.1
Ohkawa's most distinctive contribution to fusion science was the invention of the doublet plasma configuration. In the mid-1960s, he proposed that shaping the plasma cross-section into a kidney-bean or figure-eight form—rather than the simple circular cross-section used in early tokamaks—could dramatically improve plasma stability and confinement. This idea was radical at the time, as most fusion researchers assumed that circular plasma cross-sections were optimal.2
General Atomics built a series of doublet experiments to test Ohkawa's ideas. Doublet I, a small experiment, operated in 1968 and confirmed that the doublet shape was achievable. Doublet II followed, and then Doublet III, a significantly larger device that operated from 1978 to 1986. When Doublet III was upgraded with an open divertor configuration in 1986, it was renamed DIII-D and became one of the world's most productive and influential tokamak experiments—a facility that continues to operate and produce critical results for ITER and future reactor designs.3
Ohkawa's second major contribution was his pioneering work on non-inductive current drive methods. In a conventional tokamak, the plasma current is driven inductively by a central solenoid acting as a transformer—but transformers are inherently pulsed devices, meaning that an inductive tokamak cannot operate in steady state. Ohkawa recognized this limitation early and proposed several methods for driving plasma current without induction.4
His most influential current drive concept was electron cyclotron current drive (ECCD), which uses focused beams of microwave radiation at the electron cyclotron frequency to selectively accelerate electrons and generate net plasma current. Ohkawa also contributed to the theory of bootstrap current and to the understanding of how various non-inductive current drive methods could be combined to sustain a steady-state tokamak plasma.5
Ohkawa remained at General Atomics throughout his career, eventually serving as vice president and director of the company's fusion program. He was elected to the National Academy of Engineering and received the American Physical Society's James Clerk Maxwell Prize for Plasma Physics in 1985, as well as the Fusion Power Associates Distinguished Career Award. His later work explored advanced fuel cycles and the potential for using fusion neutrons in hybrid fission-fusion systems.
Tihiro Ohkawa died on November 27, 2014, in La Jolla, California. His intellectual legacy is embedded in the physical infrastructure and theoretical foundations of modern fusion research. The DIII-D tokamak, a direct descendant of his doublet concept, remains the largest magnetic fusion experiment in the United States and continues to advance the physics basis for ITER and future power plants. His current drive work underpins every serious proposal for a steady-state fusion reactor.