A new LHRF multijunction launcher with oversized waveguides for JT-60U

The main objective of the LHCD (lower hybrid current drive) on JT-60U (JAERI Tokamak-60 Upgrade) is to drive plasma current and to control plasma current profile with high efficiency. A new multijunction launcher is being prepared to further improve the efficiency. The launcher features 4×4 multijunction modules, each of which is composed by 12 sub-waveguides and an oversized waveguide The RF power in the module is divided into 12 at a junction point in the toroidal direction. This design makes the launcher structure very simple. The RF properties of a prototype module are studied by using dummy loads. It is shown that the phasing and power dividing rate are well explained by the scattering matrix method when the reflection coefficient of the subwaveguides is less than ~10%. This launcher is expected to improve current drive efficiency by 20-30% compared with the previous JT-60 LHCD multijunction launcher