M. Wakatani

M. Yokoyama

Y. Nakamura

National Institute for Fusion Science (NIFS-Repository)

§ 15. Helias-Heliac Hybrid Stellarator for Cur-rentless Plasma Confinement Wakatani, M., Yokoyama, M., Nakamura, Y. (Plasma Physics Laboratory, Kyoto Univ.) Fusion Technology 17(1990)123. 4) Yokoyama,M., Nakamura,Y., Wakatani,M., Nucl. Fusion, 34(1994)288. It is recognized that one of the well-optimized stellarators is Helias for Wen-delstein 7-X [1] from MHD equilibrium, stability, particle confinement, neoclassi-cal transport, bootstrap current and di-vertor. Recently we find a new helical axis stellarator with characteristics of magnetic surfaces similar to Helias by developing a coil system similar to Heliac [2][3], which is called Helias-Heliac hybrid stellarator (HHHS) [4]. A highly modulated f = 1 helical coil with four field period (M=4) is placed in an axisymmetric field produced by the standard toroidal and vertical field coils as shown in Fig.l. Here the outer modulated f = 1 helical coil is placed on an axisymmetric chamber with a circu-lar cross-section in order to produce mag-netic well in the whole confinement region. Magnetic surfaces at three cross-sections are shown in Fig.2. This type of HHHS is also possible for M=3 and M=5. There are three parameters to control the magnetic configuration shown in Fig.2 ; ')'1 is ratio of toroidal magnetic field by the toroidal coils to that by the inner f = 1 coil, 1'2 is ratio of the outer f = 1 coil current to the inner f = 1 coil current and 1'3 is r a-tio of the vertical coil current to the inner f = 1 coil current. It is notable that the average radius becomes aav = 0.46m and Rav/aav = 5 for 1'2 = 0. However, weak magnetic hill with maximum value of 3% appears in this case. Fig.l. Coil configuration of M=4 HHHS. References 1) Grieger, G., et al., Phys. Fluids B4(1992)2081. 2) Alejaldre, C., et al., Fusion Technology 17(1990)131. 3) Hamberger, S. M., et al., Fig.2. Toroidal coils are not shown here for simplicity. The inner f = 1 heli-cal coil has winding law of e - 7r = 4(¢-0.175 sin 2¢) and the outer f = 1 helical coil has e = 4( ¢+0.175 sin 2¢ ), where B and ¢> are poloidal and toroidal angle, respectively. 0 0 1.0 :.o l.O Cross-sections of magnetic surfaces of M=4 HHHS with 1'1 = 0.5, 1'2 = -0.028/2.44 and 1'3 = -0.333/2.44 at ¢> = 0, ¢> = 2 7r /16 and ¢> = 27r /8. Here average major radius is Rav = 2.3m and average minor radius is aav = 0.31m. IBI C?ntours of the inner f = 1 helical coil with 2.44MA are also shown. IBI at the magnetic axis is 0.546T at <P = 0 and 1.405T at ¢> = 271"/8. 127 

Helias-Heliac Hybrid Stellarator for Currentless Plasma Confinement

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