Measurement Results The dependence on the distance to the water tank is presented in Fig. 3. In Fig. 3(a), the VSWR of coupler II is always less than that of coupler I, and the increase of the former is more gradual than the latter. The VSWR of slotted coupler is less than half that of coupler I except for the large distance to water tank (d>6 cm). This result indicates that the matching between the ridged waveguide and the water load can be improved by the slotted coupler. The VSWR of g=2.5 mm is a little bit smaller than g=4 mm in the range of d>3 cm, while the VSWR varies considerably by the g for G=6 mm in Fig. 4(b). The VSWR curve of coupler I with G=6 mm is smaller than that of slotted coupler with g=6.4 mm for d>l cm. However, the VSWR of coupler II is improved by the small g, for example, that of g= 1.7 mm is less than half that of coupler I for d<4 cm. The VSWR of the slotted coupler for water load with G=6 mm is also improved, provided the gap spacing g at the mouth is smaller than the G. Though the VSWR for air load is not improved by the slotted coupler, the matching section reduces it less than one-third. One typical result of the slotted coupler with matching section is contrasted with the couplers I and II. It is shown in Fig. 4. The VSWR for air load is reduced from 20-30 less than 7 at the matching frequency. In this paper, we call the frequency of the minimum VSWR the matching frequency. In addition, the effect of matching section is more obvious for water load. The matching section reduces its VSWR less than one-third of coupler II and less than one-fourth of coupler I at the matching frequency. The slotted coupler with matching section is proved to be very effective to reduce the VSWR for the air load. DISCUSSION For the estimation of the maximum power capability of the ICRF coupler, electric fields around the short circuit at the mouth should also be considered. However, those electric fields are expected to be smaller than those in the waveguide, since a node of standing wave is located at the bottom of short circuit. At this stage, the maximum launching power of the slotted coupler by Eq. (1) represents the maximum power capability of the ICRF coupler. According to this procedure, we estimate the maximum launching power of the slotted coupler for water load using the measured VSWR in the waveguide. The maximum transmission power of the ridged waveguide with G=4 mm as in Fig. 3(a), for example, is 10 MW (3). Consequently, the maximum launching power is 3.4 MW for d = 5 mm and 2 MW for d=10 mm by substituting the measured VSWR into Eq. (1). This power capability is sufficient for the ICRF coupler to large tokamaks which need a few million watts incident power. CONCLUSION The ridged waveguide slotted coupler has lower VSWR for water load than that of the simple open ended coupler. The matching section reduces its VSWR to one-third of coupler I for air load and one-fourth for water load. These results suggest that the ridged waveguide slotted coupler is very attractive for ICRF coupler. The applicability of this coupler to the plasma load together with its optimum design is left for future problem.
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