N. Sato

R. Hatakeyama

S. Ishiguro

W. Oohara

National Institute for Fusion Science (NIFS-Repository)

§21. Evolution of Plasma Structures Accom-panied by Local Production of Negative Ions Ishiguro, S., Oohara, W., Hatakeyama, R., Sato, N. (Dept. Electronic Eng., Tohoku Univ.) We have investigated plasma dynamics associated with local production of negative ions by a laboratory experiment and a numerical simulation. Our prelimi-nary measurements1) are performed in our Q-machine plasma (density np ~ 5 x 109 em - 3 , electron tem-perature Te ~ 0.2 eV), where fullerene C60 (cage-like molecule composed of 60 carbon atoms, mass number ~ 720) particles of temperature Tc60 ~ 0.058 e V are introduced.2) In the experiment, negative ions C60 are locally produced in the K+(mass number ~ 39, tem-perature T+ ~ Te)-electron plasma column as a result of sublimation characteristic and large electron affinity of the C60 particles. A computer simulation has been performed by means of a one-dimensional electrostatic particle code based on a Q-machine configuration. As electrons and pos-itive ions are emitted from the plasma emitter placed at one end of the system, they fill the system, provid-ing a quasistationary state. After that, negative ions start to be produced in a localized region of the plasma in such a way that some of electrons attached to C60 particles are replaced by negative ions. While the plasma is stable for a small rate of negative ion production, a large fluctuation is found to appear in some physical quantities for a large rate of negative ion production. Figure 1 shows the potential profiles (a) and the phase space distributions of negative ions (b) at Wpst = 127.6 (top), 127.8 (second), 128.0 (third), and 128.4 (bottom) x 103 , which demonstrate the gen-eration process of a large-amplitude solitary wave with negative potential around the center of the negative ion production region (z/ ADs rv 130). A double layer is observed to be intermittently formed there. Nega-tive ions are strongly accelerated by the double layer and a large-amplitude pulse of negative ion density is created by their bunching. The potential profile in the negative ion-rich region created in such a way be-comes depressed or concave (d2 ¢/dz2 > 0). This po-tential pulse propagates as a large-amplitude solitary wave with negative potential. Detailed observations have shown that the double layer formation is triggered by a small negative poten-tial pulse which is generated in the upstream region and propagates to the negative ion production region. This pulse is created by a positive-negative ion two-stream instability. Figure 2 shows the velocity space-time distributions of positive ions f+(v)(top), negative ions J_(v)(middle), and electrons fe(v)(bottom) dur-244 ing Wpst = 114 rv 154 X 103 in the region of 10 < z /ADs < 80. When the negative potential pulse is gen-erated, the drift velocities of positive and negative ions approach each other. The created double layer acceler-ates negative ions and a large amplitude solitary wave with negative potential is thereby generated. (a) oops t = 127.6 x l03 -4~~--~~--~~ to~27·8 "t -2 -4 ;_:~ -4~ t 0 E::J2s.4 "t-2 -4 0 200 400 z/'Aos (b) 0.06 Jl 0.03 > -; 0.00 -0.03 0.06 } 0.03 -; 0.00 -0.03 ~::::::=~~~~==~ 400 z/'Aos Fig. 1. Potential profiles (a) and phase space distribu-tions of negative ions (b) at Wpst = 127.6 (top), 127.8 (second), 128.0 (third), and 128.4 (bottom) x 103 . &;"" 150 0 ~ 140 ... , 130 8c. 120 &;"" 150 0 ~ 140 ... 130 ! 120 &;"" 150 0 ~ 140 ... ;g, 130 8 120 -0.2 -0.02 -3 0.0 0.2 0.00 0.02 0 3 VI Ythe i:oo ...: 0 i:;oo i:o ~ 0 Fig. 2. Velocity space-time distributions of positive ions (top), negative ions (middle), and electrons (bot-tom) in the upstream region. References 1) Oohara, W. et al.: in Double Layers- Potential For-mation and Related Nonlinear Phenomena in Plasmas edited by Sendai "Plasma Forum" (World Scientfic, Singapore, 1997) p. 149. 2) Sato, N. et al.: Phys. Plasmas l (1994) 3480. 

Evolution of Plasma Structures Accompanied by Local Production of Negative Ions

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