K. "Nakamura

N. Kasuya

Y." Takeiri

National Institute for Fusion Science (NIFS-Repository)

§26. Behavior of Negative ,Ions in Inductive RF Negative Ion Sources Nakamura, K., Kasuya, N. (Chubu University), Takeiri, Y. In a negative ion source for neutral beam injection (NBI) of nuclear fusion devices, a hot cathode DC discharge has been widely used. Instead of the DC discharge, a RF inductive discharge has been recently studied because of no usage of filaments which limit the lifetime of the ion source. In the inductive discharge, the plasma is mainly produced with electromagnetic (inductive) coupling, however, there are simultaneously capacitive coupling, and negative self bias appears on the insulated discharge antenna. Therefore, in the RF negative ion source, such a capacitive coupling may affect a production of negative ions. In the previous study, a negative ion density of the inductive negative ion source has been measured based on laser-photo-detachment technique. This paper reports influences of capacitive coupling on negative ion production in the RF negative ion source ofNIFS. In a stainless steel vacuum vessel, 30 cm x 30 cm in crosssection and 20 cm in depth, a hydrogen plasma is generated by supplying 2 MHz RF IX>wers through an insulated antenna at a pressure of 8 mTorr. As shown in Fig. 1, the antenna conductor is electrically floated by two series-connected blocking capacitors C1 and C2 • Amplitude of the RF voltage distributed along the antenna conductor can be changed with the two blocking capacitors, leading to control of the capacitive coupling between the antenna and the plasma. Laser-photodetachment technique is used to detect the hydrogen negative ions. In the center of the chamber, a Langmuir probe is located, and a flash-lamp-excited pulse Y AG laser is irradiated onto the probe tip. From an increment of electron saturation current induced by the laser irradiation, density and fraction of the negative ions are obtained. Figure 2 shows power dependence of the negative ion fraction drived from the electron saturation current and its laser-induced increment (photodeteched electron current) as a parameter of the blocking capac tor C1 for C2=8000 pF. For the both cases of C1=1333 pF and 5000 pF, the negative ion fraction is not so dependent on the discharge power. However, the fraction for C1= 1333 pF is larger roughly by a factor of two than that for Cr =5000 pF. Measurements of the antenna voltage showed that the maximum value ofthedistibutedantenna voltage is,..." 1 k~lorCl=5000 pF, but reaches ,....5 kV P,P for C1= 1333 pt<. This causes enhancement of capaci ti ve coupling wi th the plasma leading to production of high energy electrons. Actually, probe measurements revealed that both the electron temperature and plasma potential for C1=1333 pF are higher than those for C1=5000 pF. Therefore, under the present condition, such high energy electrons produced in the plasma with high electron temperature will contribute to increase the negative ion fraction, probably due to vibrational excitation of hydrogen molecules which is a essential species for dissociative attachment process. 1 .6f.! Fig. 1 Experimental Apparatus 0 0.1 ~ 0.08 • 5000 pF a: 0 0 1333 pF 0 c: 0.06 -.Q 0 0 (]) 0.04 - 0 > 0 • • '+:i CO 0.02 ~ ..... -Ol • (]) 0 I n I Z 0 5 10 15 Net Discharge Power (kW) Fig. 2 Negative ion fraction as a function of net discharge power 139 

Behavior of Negative Ions in Inductive RF Negative Ion Sources

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