A. Komori

N. Ohyabu

T. A Watanabe

National Institute for Fusion Science (NIFS-Repository)

§48. Local Island Divertor for LHD Komori, A., Ohyabu, N., Watanabe, T. A local island divertor (LID) has been proposed for the edge plasma control of the Large Helical Device (LHD). However, the LID functions are not obvious, so that they have been studied in detail [ 1]. To estimate the number of particles that strike upon the back side of the divertor head, we calculate the percentage P of the field lines that strike upon the target plate, located in the back side of the divertor head, among a total of 240 field lines that circulates around the torus 15 times. The field lines start from the poloidally equally spaced positions at distances of 4, 8 and 12 mm from the last closed flux surface and toroidally at the same angle as the center of the plate. This distance between the plate and starting points is the longest along the field lines. The perpendicular spreading of the starting points takes into account the perpendicular diffusion of the particle flux to some extent. Figure 1 shows the magnetic surfaces formed by these field lines, before they strike upon the plate, on the plane, whose normal direction is parallel to the direction of the helical coils at the outside (large major radius side) of the equatorial plane. The plate with the width of D is on this plane in our calculation. Figure 1 (a) shows the magnetic surfaces obtained without the plate, while Fig. 1(b) and l(c) represent the magnetic surfaces with the plates of D = 30 and 60 em, respectively. It is clearly shown that the field lines pass through the plate and tend to circulate around the entire island flux surface when D = 30 em, but almost all n1agnetic field lines strike upon the plate during the first island circulation when D > 60 em. Thus, Pis almost proportional to D, and begins to saturate at D------- 50 em. It reaches 90o/o at D------ 60 em. The relation between P and N was also obtained with D = 40 em, as shown in Fig. 2. This figure indicates that N must be larger than I 0 for realizing the large P in this case. Then, the width of the gap between the pumping duct and divertor head, b. , necessary for the high efficiency pumping, is obtained to be > ------- 5 em using the equation b. > ------- / 2 rr RND 1 /v//i when the major radius R = 3. 9 m, N = 10, D1 =1 m2 /sec and Ti = 100 eV, since the particle fux diffuses radially during the toroidal circulation. Optimization of the divertor head, and design works of the pumping and cooling systems are under way. (b) -... ·. Fig. 1. Magnetic surfaces with D's of 0, 30, and 60 em, respectively. 1 00 .---....----.----.----y------r-----, ~ 50 Cl-c 10 20 30 N Fig. 2. Relation between P and N. References [l]Komori, A., et al., in Plasma Physics and Controlled Nuclear Fusion Research 1994 (Proc. 15th Int. Conf. Seville, 1994), IAEA-CN-60/F-P-4. 53 

Local Island Divertor for LHD

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