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° of phase shift in order to launch a peak parallel refractive index (N_|| 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°±10° and 1/3±1/15, respectively. The N _|| 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 ~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 η_CD of 3.4×10¹⁹ M^-2A/W. Experimental results verify the adequacy of the launcher´s method of fabrication and its satisfaction of requirements