Joint Study on Confinement Improvement in Helical Systems

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The present collaboration project has been done successfully for two years of 1994 and 1995. We have obtained fruitful results which consist of main four categories ; (1) MHD stability, (2) Orbit-neclassical transport-heating, (3) Island formation and breaking of magnetic surfaces, and (4) New model for anomalous transport and confinement improvement.The comparison and the benchmark test of the full three dimensional stability code "CAS3D" and the two dimensional stability code "RESORM" based on the stellarator expansion or the average method have yielded a good agreement between the two for the stability of the LHD plasma. Three dimensional analysis of the high-n balloning mode revealed that the mode can be unstable for positive magnetic shear in the heliotron helical system for which the mode is stable in the tokamak. The low-n balloning mode has been found to behave differently in the helical system from in the tokamak.The neoclassical theory has been developed to the general toroidal system. As a result, it has been found that the bootstrap current proportional to the radial electric field, which does not exist in the tokamak, exists in the helical system. Thus it is possible to reduce the bootstrap current. If it flows in the dirrection to reduce the rotational transform, the interchange mode can be stabilized, resulting in improvement of the confinement.Island formation and related breaking of magnetic surfaces have been studied by using the equilibrium code "HINT". The "Self-Healing" phenomenon has been discovered which leads to the confinement improvement.A New model for the anomalous transports in helical systems as well as tokamaks has been proposed and explained some unresolved experimental facts.