Abstract
The Large Helical Device (LHD) is a heliotron device with two helical coils, each of which has a structure of three current layers. It is designed so that the current in each layer can be controlled independently by changing the combination of the coil current in the layers, it is possible to vary the effective minor radius of the helical coils, which enlarges the flexibility of the configuration. The properties of the plasmas for several combinations of the layers are investigated numerically. In the vacuum configuration, it is found that the combination of layers corresponding to a large effective coil radius has a large outermost surface. In this case, the rotational transform decreases and the magnetic hill is reduced compared with the configuration with all three layers. The large Shafranov shift, which is due to the small rotational transform, enhances the magnetic well and the magnetic shear to stabilize the Mercier mode; however, it degrades the equilibrium beta limit. In the case of the combination of layers for a small effective coil radius, the Mercier mode is destabilized, because the magnetic hill is enhanced. The effect on the bootstrap current is also studied
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