In-situ characterization of spurious modes in HE11 transmission lines with a 5-port coupler

Real-time in-situ measurement of spurious modes in HE11 trans- mission lines is becoming an important topic for the design of next-generation ECRH installations (e.g. ITER), because the acceptable tolerances for the align- ment of the waveguides and coupling optics are small for oversized waveguides. Also, the effects of spurious modes (ohmic heating, wrong beam parameters at the launcher) become increasingly critical. We present a method for in-situ char- acterization of 4 dominant spurious modes by using a 5-port coupler, which is integrated into a miter bend. The coupler signals can be directly transformed into the mode spectrum by a matrix multiplication. A general formalism for obtaining the coefficients of the transformation matrix is presented along with a method for optimizing the coupler positions in order to obtain the maximum dynamic range for the diagnostics.