K. Ichiguchi

M. Okamoto

N. Nakajima

O. Motojima

National Institute for Fusion Science (NIFS-Repository)

§1. Vacuum Configurations of LHD with Multi Layer Helical Coils lchiguchi, K., Nakajima, N., Okamoto, M., Motojima, 0. Each helical coil in the LHD is composed of three layers which are called the outer (0) , the mid (M) and the inner (I) layers. The choice of the combination of the layers deter-mins the effective minor radius of the helical coils. Here we use the coil parameter defined by (1) where m and f denote the number of period and the pole number of the helical coils, respec-tively. ac and Rare the minor and the major radii of the helical coils, respectively. There are five kinds of the combination of the layers in the LHD device. The standard configura-tion corresponds to /c = 1.25, where all three layers are used for generating the confinement magnetic field. If only the 0-layer is used, the radius of the helical coil becomes large and /c increases to 1.38, while /c decreases to 1.12 in the case of only the !-layer. At first, we calcu-late the vacuum configuration·s corresponding to the combinations of the layers. In the calcu-lation for each combination, the same ratios of the currents flowing between the poloidal field coils as those of the standard configuration are used, while the total amount of the currents of all the poloidal field coil is adjusted so that the position of the magnetic axis should be located at 3. 75m. Figure 1 shows the poincare plots of the vacu urn magnetic field lines in the cases of /c = 1.12, 1.25 and 1.38. From this figure, the average radius of the outermost surface in-creases as /c becomes large. We also obtained that increasing /c decreses the rotational trans-form, because the distance between the mag-netic surfaces and the helical coils changes. In the standard configra.tion, the magnetic hill spreads in the v.rhole region inside the out-ermost surface. The magnetic hill is reduced 130 as /c increases, and very shallow magnetic well appears in the vicinity of the magnetic axis in the case of /c = 1.38. On the other hand, the magnetic hill is enhanced when /c becomes small. This can be explained by the ellipticity of the magnetic surfaces. The confinment of the deeply trapped par-ticles can be approximately estimated by plot-ting the contour of the modiBI. It is ob-tained that the dependence of the fraction of the confinment region of the deeply trapped particle on lc is weak. The deeply trapped particle in about 80% region inside the outer-most surface will be confined in the range of 1.12 ~ /c ~ 1.38. (a. ) (b) ( c ) Fig.1 Magnetic Surfaces of /c = (a)1.38, (b)1.25 and (c)1.12. 

Vacuum Configurations of LHD with Multi Layer Helical Coils

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