Comparative experiments of co- and counter-helicity merging in the compact toroid merging device TS-4

Abstract

For the past fifteen years, the TS-3 merging project has explored several new type of CT/ST(Compact tori and Spherical tokamak): merging formation of FRCs(Field Reversed Configuration), magnetic reconnection experiment, and ultra-high beta ST formation. The successful results of these experiments have enabled us to construct an up-scaled merging device; namely the TS-4. The new device TS-4 has two flux cores(major radius=0.5m) for inductive plasma formation, a center coil(external toroidal field coil and OH coil) for q value control and plasma sustainment, and DC external field coils. The initial operation of the TS-4 (since 2001) demonstrated co-helicity and counter-helicity merging of two spheromaks. The counter-helicity merging revealed annihilation of toroidal magnetic field with a significant overshoot motion of reconnected field lines. It resulted in the formation of an high-beta (≈0.8) FRC. The produced FRC has a significant field deformation, suggesting contribution of toroidal plasma flow to its equilibrium. The co-helicity merging doubled the toroidal fluxes of spheromak, causing its flux conversion from toroidal to poloidal. The merging spheromaks relaxed into another spheromak with beta ≈0, suggesting the large thermal energy loss due to its non-aximetric dynamo activity.