0883-6272/86 $3.00 + .00 Copyright ® 1986 SUN SAT Energy Council A RIDGED WAVEGUIDE SLOTTED COUPLER FOR LARGE TOKAMAKS HIROYUKI ARAI and NAOHISA GOTO Department of Physical Electronics Faculty of Engineering Tokyo Institute of Technology Ohokayama, Megura-Ku, Tokyo 152, Japan INTRODUCTION A ridged waveguide was proposed to transfer the waves into the plasma through the small cross section for the ICRF heating experiments (1,2). It can be fabricated compactly with all-metal, which is free from the damage of insulators in the coupler. In order to apply the ridged waveguide to the tokamak, this paper newly proposes the slotted coupler which can reduce the VSWR in the ridged waveguide. THE RIDGED WAVEGUIDE SLOTTED COUPLER The maximum transmission power Plm is the power in the case of no reflection from the load. However, the load is not matched to the ridged waveguide, the maximum launching power to the load Pm is given as follows (1). where T, p are the reflection coefficient from the load and the VSWR in the waveguide, respectively. Equation (1) shows that the maximum launching power is inversely proportional to the VSWR in the waveguide. The following experiments are devoted to reduce the VSWR in the ridged waveguide. Simulated Coupler Geometry A ridged waveguide slotted coupler has a short circuit in the mouth and a series of slots like a Faraday shield. Figure 1 shows the overall configuration. The frequency of ICRF heating in JT-60 is planned to be 110 MHz-130 MHz. The simulated coupler was about one-tenth of actual size, and the VSWR in the waveguide was measured around 1 GHz. Design parameters of the coupler are shown in Fig. 2. A polyethylene water tank (260 mm x 260 mm x 300 mm) filled with 20 liter water is used as dielectric instead of the fusion plasma, where the thickness of the polyethylene is 3 mm. In our experiments, there was not a large flange around the mouth for the simplicity of set up. Additionally, the distance to the water tank d could be varied in order to simulate the change in distance to the plasma.
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