Joint Study on Confinement Improvement in Helical Systems

目前的合作项目已在1994年和1995年两年内成功地完成。我们取得了丰硕的成果，主要有四个方面；(1) MHD稳定性，(2)轨道-非经典输运-加热，(3)磁表面的岛屿形成和破裂，(4)异常输运和约束改善的新模型。对基于仿星器展开法和平均法的全三维稳定码“CAS3D”和二维稳定码“RESORM”进行了对比和基准测试，两者对LHD等离子体的稳定性有较好的一致性。对高n气胀模的三维分析表明，在高n气胀模在托卡马克中稳定的heliotron螺旋系统中，正磁切变模可能是不稳定的。发现低n气胀模式在螺旋系统中的表现与在托卡马克中的不同。新古典理论已经发展到一般的环面系统。结果发现，在托卡马克中不存在的与径向电场成正比的自举电流在螺旋系统中存在。因此，可以减小自举电流。如果沿该方向流动，减少转动变换，则可以稳定交换模式，从而改善约束。利用平衡码“HINT”研究了磁表面的岛屿形成和相关的断裂。发现了“自我修复”现象，从而改善了约束。提出了螺旋系统和托卡马克中异常输运的新模型，并解释了一些尚未解决的实验事实。

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.