Basic study for innovative concepts of stellarator configurations

Basic roles of several essential plasma boundary modulations on magnetohydrodynamic (MHD) equilibria are investigated. The appropriate combination of principle helical modulations to eliminate bumpy field is explained to realize quasi-axisymmetric (QAS) and quasi-helically symmetric (QHS) configurations. The triangular modulation is effectively utilized to form the magnetic well. The bumpy modulations of plasma boundary is essential to reduce the toroidicity in the magnetic field, which can lead to QHS configurations. Based on these roles of plasma boundary modulations, the classification of QAS and QHS configurations is considered. The possibility of quasi-bumpy symmetric (QBS) configuration is also mentioned. (author)