N. Sato

R. Hatakeyama

S. Ishiguro

W. Oohara

National Institute for Fusion Science (NIFS-Repository)

§21. Formation of Negative Potential Structure Accompanied by Localized Negative lon Generation Ishiguro, S., Oohara, W., Hatakeyama, R., Sato, N. (Dept. Electronic Eng., Tohoku Univ.) When neutral particles which have large affin-ity for electrons are in a plasma, electrons attach to them, and, as a result, negative ions are gen-erated. We have been investigating characteris-tics of negative ion plasmas which are produced by introducing SF 6 or C60 molecules into a Q-machine potassium plasma at Tohoku Univer-sity. C60 and SF 6 have large collision parameter for attachment of 0.2 e V electrons which are in Q machine, so negative ion plasma which con-sists of positive ions, electrons and negative ions are produced. Recent experiments in which C60 molecules are introduced in a Q machine have shown that a localized structure accompanied by a wide band low frequency oscillation has been created. In order to investigate plasma characteris-tics in this situation, we have developed one-dimensional particle simulation code in which attachment of electrons to C60 molecules is intro-duced. We assume C60 ions are produced with constant rate in time in a specified region. Since the mass of C60 is much larger than that of the electrons, C60 stay there for a long time. This gives rise to an accumulation of negative charge in this region. Figure 1 shows the typical po-tential profile which is averaged over the time much longer than the ion plasma period. Here, electrons are replaced by C60 ions in the region 30 < z /Ave < 50. We can see a negative poten-tial dip in the negative ion production region. Its depth is about e¢/Te "' 0.5 where e and Te are the electronic charge and electron tern pera-ture, respectively. This potential profile is sim-ilar to that obtained in Q-machine experiment, as shown in Fig. 2. Time evolution of potential structure in the simulation have shown that a negative potential structure which is created in the negative ion production region moves to the downstream region (positive z direction) over and over again. The propagation of the neg-ative potential structure exhibits a wide band low frequency spectrum. There is a possibility that this corresponds to the wide band low fre-quency spectrum observed in the Q-machine ex-periment. Fig. 1. Potential profile of particle simulation. 100 110 120 z (em) Fig. 2. Potential profile of Q-machine experiment. References 130 1) Sato,N. et al.: Phys. Plasmas 1 (1994) 3480. 267 

Formation of Negative Potential Structure Accompanied by Localized Negative Ion Generation

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