Fig. 2-13 Negative ion neutral particle injector installed in the JT-60 hall The power supply is the 500 kV, 64 A with the fast acting circuit break. The 10 MW power (the efficiency of 50 %) is injected to the plasma of JT-60, using two large ion sources (the outer diameter of 2 m).

Fig. 2-14 Progress of the performance of the negative ion neutral particle injection facilities

Neutral particle injection is one of the methods to inject energy to the high temperature plasma confined by a magnetic field. The high speed neutral particles arrive at the core plasma without interruption by the magnetic field where they give up their energy to the plasma. The ion accelerated by the electric field is neutralized by passing through the same gas (in a neutralizer cell), becoming a high speed neutral particle. In the case of positive ions the efficiency of neutralization deteriorates if the electric field is higher than 500 kV. This means loss of total efficiency. In the case of negative ions the neutralization efficiency does not decrease at this energy. However, negative ions are technically difficult to produce, because of these instability. We succeeded to overcome this defficulty by proposing ideas for the structure and gas composition of the ion source. We made the neutral particle injection facilities for the JT-60 (Fig. 2-13). On the way to commission the facilities, the 400 kV, 13.5 A operation, etc. were achieved as shown in Fig. 2-14. These data are all world records. The duration time will be increased with aging operations of the ion source. New technology was developed, that is the high frequency transformer in the power supply system which has the low voltage circuit breaker in the primary circuit to realize the fast acting break of the high voltage secondary circuit. It is expected that this heating facility will contribute to the high performance operation of the JT-60 because of its remarkable characteristics. Furthermore, the neutral beam injection technology has a wide range of applications such as the formation of the homogeneous surface composition of large structures.

Reference M. Kuriyama et al., High Energy Negative-ion Based Neutral Beam Injection System for JT-60U, Fusion Eng. Des., 26, 445 (1995).

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