Design and fabrication of multijunction launcher on the JT-60

A multijunction-type launcher was installed on the JAERI (Japan Atomic Energy Research Institute) Tokamak-60 (JT-60). It is composed of 32 modules consisting of 96 secondary waveguides, in which E-plane junctions are used to divide each main waveguide into three secondary waveguides at its grill mouth. The waveguides are equipped with geometrical phase shifters yielding 70 degrees of phase shift in order to launch a peak parallel refractive index (N/sub /// peak) of 1-2.5 at 2 GHz. In the course of fabricating the multijunction launcher, the main technological problem was to fabricate multijunction modules with good RF properties (phase angle, power dividing ratio, etc.). A TIG (tungsten inert gas) welding method was adopted after a study of applicable fabrication methods. The phase shift and the power dividing ratio were verified to be 70 degrees +or-10 degrees and 1/3+or-1/15, respectively. The N/sub /// spectrum of the multijunction launcher calculated from measured values of phase shift was not so different from the design value. Using a test module, the authors also performed a high-power transmission test up to a power level of approximately 700 kW without breakdown. After installation of the multijunction launcher in the JT-60 tokamak, it was quickly conditioned up to a power level of 2.75 MW and maximum current drive efficiency eta /sub CD/ of 3.4*10/sup 19/ M/sup -2/A/W. Experimental results verify the adequacy of the launcher's method of fabrication and its satisfaction of requirements.<>