Development to new stage of helical plasma confinement

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1) Progress has been made on physics and engineering design of CHS-qa, which is supposed to be a post-CHS device. Stability β up to 4% and good orbit confinement of high energy ions have been achieved. High performance results in high elongation of plasma cross-section. 2) Ion thermal diffusivity of high ion temperature mode of CHS plasma has been estimated by using the effective helical ripple of the neoclassical theory. The estimated values are close to the experimental one. The ion diffusivity for quasi-omnigenous configuration realized by a strong inward shift of the magnetic axis, where the bottom of helical ripple has the same magnetic field strength, has been estimated. The expected ion temperature for the quasi-omnigenous configuration is 1.5keV, which is much higher than the value obtained so far in high Ti mode of 1keV. 3) Confinement improvement of electron results from the potential bifurcation in CHS, which leads to the high electron temperature mode where high central electron temperature of 2keV is realized. The same phenomenon of the internal transport barrier has been observed in T-10 tokamak. The transport barrier is formed at the position where the radial electric field shear is strong for both plasmas (CHS and T-10). 4) Qualitative explanation has been made on the fact that the equilibrium βvalue increases as the elongation of plasma cross-section increases. The elongation should play a role in getting a high performance plasma. It is shown that magnetic measurements can not determine uniquely the current and pressure profiles of toroidal plasmas.