Improvement for lower hybrid coupler with step transformers and power dividers

PPPL and MIT are preparing an experiment of current profile control using lower hybrid waves in order to produce and sustain advanced tokamak regimes in steady state conditions in Alcator C-Mod. This new C-Mod coupler will have similar characteristics as the multijunction coupler used in other LH systems in order to obtain the full control of the phasing. This is achieved by using 3-dB vertical power splitters and a stack of laminated plates into which the waveguides are milled. Since the 3-dB vertical power splitter is in the form of the reduced height waveguide, the matching between standard WR187 waveguides and reduced height waveguides is very important in order to guarantee the power splitter performance and reduce the matching loss. 3-section quarter-wave transformers are designed for this purpose to replace the tapers in the system. Furthermore, the 3-dB vertical power splitter is redesigned with a post in the center of the coupling region, so that the splitter performance is greatly improved at the desired operating frequency.