Studies on the Production and Upgrading of Confinement Characteristics of Spherical Tokamaks by Means of Merging Effect of Compact Torus Plasmas

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For the past fifteen years, the TS-3 device at the University of Tokyo has been utilized to study a great variety of Compact Tori and Spherical Tokamaks (CT/ST's) particularly in association with their merging formations and heating. The studies have included the magnetic reconnection experiments, the merging formations of the Field Reversed Configurations (FRC), and the merging formations of ultra high beta spherical tokamaks with rapid application of the external toroidal field to the FRC. The successful operations of these experiments enabled us to construct an up-scaled merging device, namely, the TS-4 device. The new device TS-4 contains a pair of internal poloidal flux cores of the Princeton S-1 type (major radius 0.5m) for the inductive plasma formation, a center coil assembly, containing both of the external toroidal field coils and the OH coils, for the q value control and for the plasma sustain. The initial operations of the TS-4 (since 2001) have successfully demonstrated th … More e cohelicity merging and the counter-helicity merging of two spheromaks as have been observed in the TS-3 machine in the past. Constrasting to the TS-3 results, the counter helicity merging in TS-4 without the external toroidal field revealed rather slow annihilations of the internal toroidal magnetic field, clarifying different natures of the formation of a high beta FRC from those observed in the previous TS-3 machine. The counter helicity merging in a weak external toroidal field, namely, the merging of the low q ST and the RFP, makes merged compact toroids relax transiently to low q spherical tokamaks with a very high beta value. On the other hand in a strong external toroidal field, only the cohelicity merging of ST's has been successfully operated. It has been observed that when the external toroidal field is increased above the certain critical value, the stabilization of ST even with a complete free boundary becomes very clear in association with the disappearance of the low n toroidal mode instabilities. These natures of the merging operations of CT/ST's encourage us to proceed with the novel proposal of compact fusion reactors that are assisted by the merging effect for the initial plasma production and steady state maintenance of the configuration. Less