No abstract

The influence of non-axisymmetric instabilities, specifically the n=1 toroidal mode, was studied on the magnetic flux contours that are numerically reconstructed by use of magnetic field profiles measured in Proto-TS-4 device. The device has a cylindrical vacuum vessel with a couple of coaxial magnetized plasma guns at both ends and a magnetic flux conserver with a side axial length of 600mm and a diameter of 940mm. The main measurement system consists of a 2-dimensional magnetic probe array on the poloidal plane(r-z plane) with two components, B z and B t (total 84 channel pickup coils). The B z field data are used to reconstruct the poloidal magnetic surfaces(Ψ) on the assumption that plasma configuration has axisymmetry. In this study, deformation of the reconstructed flux contours has been studied particularly when the plasma configurations were subjected to a macroscopic instability, specifically n=1 toroidal mode instability. The influence of this component on the reconstructed contours was analytically calculated and the results were qualitatively compared with the contours experimentally observed. Types of n=1 error, namely, shift or tilt, on the reconstructed magnetic surface were also qualitatively discussed.

SUMMARYFor the past 15 years, the TS-3 merging project has explored several new types of CT/ST (Compact Tori and Spherical Tokamak): merging formation of FRCs (Field Reversed Configuration), magnetic reconnection experiment, and ultrahigh beta ST formation. The successful results of these experiments have enabled us to construct an upscaled merging device, namely, the TS-4. The new TS-4 has two flux cores (major radius = 0.5 m) for inductive plasma formation, a center coil (external toroidal field coil and OH coil), for q value control and plasma sustaining, 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 a high-beta (≈ 0.8) FRC. The produced FRC has a significant field deformation, suggesting a contribution of the 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.

The co-helicity merging operations of compact toroid(CT) and spherical tokamak(ST) have been performed with external toroidal field in the CT/ST merging device TS-4. The low-q(safety factor) CT mergings as the compact RFP merging and the spheromak merging show the flux conversion from toroidal to poloidal in the course of the reconstruction of the Taylor force free state. The relaxation to the Taylor state proceeds through the following three states; (1)axisymmetric merging with increasing toroidal flux, (2)increase in the poloidal flux Ψ and, (3)relaxation to the Taylor state. The high-q ST merging shows different relaxation process from those of the compact RFP and the spheromak mergings. Increases in Ψ were not clearly observed in the ST merging. The measured eigen values λ show that ST's, especially high-q ST's, approach a unique intrinsic equilibrium state that has a λ proportional to Ψ with a longer lifetime than that of CT's. When external toroidal field is set in a certain range between the low-q operation and the high-q operation for ST's, an abnormal phenomenon was found in the ST formation, namely, a drastic decrease in the plasma lifetime. This phenomenon is characterized by very week poloidal flux generations during the initial plasma production phase and the subsequent plasma separation phase when the plasma starts detaching from the flux-core.

SUMMARYThe co-helicity merging operations of compact toroid (CT) and spherical tokamak (ST) have been performed with external toroidal fields in the CT/ST merging device TS-4. The low-q (safety factor) CT merging as the compact RFP merging and the spheromak merging show the flux conversion from toroidal to poloidal in the course of the reconstruction of the Taylor force free state. The relaxation to the Taylor state proceeds through the following three states: (1) axisymmetric merging with increasing toroidal flux; (2) increase in the poloidal flux Ψ; and (3) relaxation to the Taylor state. The high-q ST merging shows different relaxation process from those of the compact RFP and the spheromak mergings. Increases in Ψ were not clearly observed in the ST merging. The measured eigenvalues λ show that ST's, especially high-q ST's, approach a unique intrinsic equilibrium state that has a λ proportional to Ψ with a longer lifetime than that of CT's. When external toroidal field is set in a certain range between the low-q operation and the high-q operation for ST's, an abnormal phenomenon was found in the ST formation, namely, a drastic decrease in the plasma lifetime. This phenomenon is characterized by very weak poloidal flux generations during the initial plasma production phase and the subsequent plasma separation phase when the plasma starts detaching from the flux core.