Fig. 2-8 The relation between the negative ion current density and the working gas pressure of an arc discharge in the ion source A value of 20 mA/cm2 is achieved at less than 0.3 Pa.

Fig. 2-9 The 3 mA, 80 kV negative ion beam emerging from the single extractor hole The diameter of the extractor hole is 11.3 mm and the beam diameter after 2 m of flight is 8 mm. This means that the beam divergency angle is 1.5 milli-radian.

Neutral particle beams are useful because they can penetrate and heat a plasma without being bent by the magnetic field. Such a beam, of more than 1 MeV, can be obtained from a negative ion beam with a conversion efficiency of more than 50%. For ITER, the beam requirements are that the working gas pressure should be as low as 0.3 Pa, to reduce negative ion losses in the beam line, and the negative ion current density should be as high as 20 mA/cm2so that a high power can be injected in a restricted window area. We have developed an ion source (a KAMABOKO-type ion source) that satisfies these requirements, using surface ionization of cesium to enhance the negative ion production efficiency, and using an optimized vessel shape and optimized magnetic field lines. The resulting negative ion current and beam density are shown in Fig. 2-8, as a function of the working gas pressure. The negative ion beam also has a small divergence, so neutral beams produced from negative ions will have wide applicability to long distance parallel beam transport, which is necessary in large facilities (Fig. 2-9). The small divergence is mainly due to small thermal motion of the beam particles, and a stable acceleration mechanism at the extractor electrode.

Reference Y. Okumura et al., High Power Negative Ion Source for Fusion at Japan Atomic Energy Research Institute, Rev. Sci. Instrum., 67(3), 1092 (1996).

Select a topic in left column