H. Takeno

I. Katanuma

J. Miyazawa

Y. Nakashima

Y. Yasaka

National Institute for Fusion Science (NIFS-Repository)

Cusp-type direct energy converter (CuspDEC) hasbeen developed to use as a direct energy conversion de-vice for advanced fusion1). It has a function to sepa-rate charged particles according to their charge polarityand energy. This function is applicable to reduce theenergy of divertor plasma: ions and electrons of the di-vertor plasma could be separated by a CuspDEC, andthey could ﬂow into individual divertor plates. If theplates are biased appropriately according to particle en-ergy, those particles are decelerated, and thus thermalload of the plates could be reduced2). In order to realizethis scenario, the CuspDEC should be settled at the nar-row divertor space. In such a situation, composition ofthe CuspDEC by permanent magnets (PM-CuspDEC)may be suitable. This report presents new structure ofCuspDEC using permanent magnets.In the new structure of magnetic ﬁeld, not onlypermanent magnets, but also current ﬁeld by solenoidcoils are used which simulates a guiding ﬁeld of divertorplasma. In the present study, cylindrical hollow magnetsmagnetized axially are used. The magnets are alignedco-axially with solenoid coils at the downstream in thedirection of the plasma ﬂow.Samples of the ﬁeld line structure are illustrated inFig. 1, which show radial-axial planes and plasma ﬂowsalong with the axial direction. In Fig. 1(a), ﬁelds onlyby permanent magnets (pm) are shown. As shown in theﬁgure, there exist two cusp ﬁelds near both ends of themagnets. When ﬁelds by coil currents are superposed,those cusp ﬁelds are cancelled. In a small current con-dition of Fig. 1(b), the cusp ﬁeld in the upstream iscancelled and that in the downstream still exists. Elec-trons ﬂowing into this ﬁeld are deﬂected along the ﬁeldlines and arrive at the downstream edge of the magnet,where an electron collector (P1) exists. On one hand, ina large current condition of Fig. 1(c), the cusp ﬁeld inthe downstream is also cancelled. In this ﬁeld, electronsalong the ﬁeld lines are not deﬂected and go straight tothe point cusp with ions, where an ion collector (P2)exists. In order to separate electrons and ions, an appro-priate guiding ﬁeld as Fig. 1(b) is required.In a plasma source device, a prototype PM-CuspDEC was composed and tested. By using a sim-ilar structure to that in Fig. 1, motion of electrons wasexamined. Figure 2 shows samples of voltage-currentcharacteristic of the electron collector (P1). Current ofcoil B (IB) was varied for 0A, 10A, and 40A while cur-rent of coil A (IA) was kept constant to be 40A. Ac-cording to Fig. 2, electron current once increases as IBincreases from 0A to 10A. However, it decreases as IBincreases from 10A to 40A. This is consistent with thevariation of ﬁeld line structure. In the variation of IBfrom 0A to 10A, the null point in the upstream is movedradially while that in the downstream remains on theaxis, thus ﬁeld lines arriving at P1 increase and electroncurrent of P1 increases. In the variation from 10A to40A, the null point in the downstream is moved, thusﬁeld lines arriving at P1 decrease and electron currentof P1 decreases. The fundamental characteristic of PM-CuspDEC was conﬁrmed experimentally.0-20 20 40 60z [cm]02010r [cm] coil Acoil Bpmcusp(a)0-20 20 40 60z [cm]02010r [cm] coil Acoil Bpmcusp(b)P1P2plasma0-20 20 40 60z [cm]02010r [cm] coil Acoil Bpm(c)P1P2plasmaFig. 1. Variation of ﬁeld line structure by external mag-netic ﬁeld.−80 −40 0 40 80−120−80−400V1 [V]I 1 [µA]IB=0AIB=10AIB=40AIA=40AFig. 2. Voltage-current characteristics of P1.1) H. Momota, et al., Proc. 7th ICENES (1994) 16.2) H. Takeno, et al., Trans. Fusion Sci. Tech. 63(1T)(2013) 131-134.514§15. New Structure of Cusp-type Direct Energy  Converter Using Permanent MagnetsTakeno, H., Yasaka, Y. (Kobe Univ.),  Miyazawa, J.,  Nakashima, Y., Katanuma, I. (Univ. Tsukuba)

New Structure of Cusp-type Direct Energy Converter Using Permanent Magnets

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