Plasma Confinement Studies in Multipole by Tihiro Ohkawa's Group of General Atomic in the 1960's

Abstract

In 1960, Tihiro Ohkawa and D. W. Kerst at General Atomic (GA) proposed an inertial ring device (so-called multipole) program for plasma confinement. After Kerst left for the University of Wisconsin, Ohkawa and his group made a linear octopole device to show that injection from coaxial plasma gun was stable. Against GA theoretical group's opposition to launching the multipole program, Ohkawa's unit demonstrated that the multipole device could produce quiescent plasma. In 1965, Ohkawa reported the result of their study using the "Toroidal Octopole" at the Second International Conference on Plasma Physics and Controlled Nuclear Fusion Research. L. Spitzer called it "a major milestone of the controlled fusion programme" in his summary of the conference. Thereafter, Ohkawa's group started a comparative study between octopole and quadrupole systems. "Toroidal Octopole" was converted to "Toroidal Quadrupole" configuration to see whether the conversion affected the degree of instability. They consequently found that the plasma confinement of quadrupole was less stable than that of octopole due to a shallow magnetic well. Ohkawa thought that the purpose of the multipole program was to find its fundamental difficulty and to see if it could be removed. He came to the conclusion that multipole devices would not become a practical fusion reactor, and therefore proposed to change multipole internal rings into toroidal plasma current. The purpose of Ohkawa's experiment was the construction of "Holy Grail (stable state of confinement plasma)" for MHD (magneto-hydro-dynamics) instability. Consequently, Ohkawa's group on toroidal multipole research demonstrated that "Holy Grail" was available for the structure of average minimum B. In addition, plasma physics on micro-instability made progress on the basis of their experiments.