Experimental test of a two-port decoupler in the DIII-D fast wave current drive system

The DIII-D fast wave current drive (FWCD) system employs a pair of resonant loops of transmission line to reduce the four element phased antenna array to a two-port system. The equivalent two-port can be characterised by a 2×2 admittance matrix Y_ij, the off-diagonal elements of which are non-zero due to the reactive coupling between the elements in the antenna array. By connecting another two-port network (a “decoupler”) in parallel with the two feedlines, it is possible to reduce the off-diagonal elements of the admittance matrix describing the resulting two-port device to zero, thereby effectively producing a system with no coupling between the two feedlines. In this paper we report a comparison of the theoretically predicted and experimentally determined characteristics of a decoupler in a high power ICRF system