Three-dimensional electromagnetic characteristic of overmoded coupling pattern for the cut-off extended interaction field in THz sheet beam resonant system

The concept of overmoded coupling pattern for matching with the cut-off resonant condition in a THz sheet beam resonant system is proposed to build up super double-overmoded extended interaction field for high power THz radiation. A physical design procedure is proposed to support the matched overmoded coupling pattern. The three-dimensional electromagnetic characteristic of the super overmoded field is studied to demonstrate the effectiveness of the procedure for clarifying the relationship between the overmoded coupling pattern and cut-off resonant condition. This relationship is analyzed by focusing on (1) the distribution characteristic of the overmoded coupling pattern associated with the specific boundaries inside the structure, (2) the equivalent method for describing the interaction impedance in sheet beam system, (3) magnitude of the internal field strength, and (4) variation of the ohmic loss. The simulation results show the typical cut-off resonant condition can be established and maintained with same distribution pattern and same frequency, as in the case of the systems with the proposed overmoded coupling pattern and without it. Multiple sections of coupling patterns, which are distributed arbitrarily in coupling cavities, also have no effect on the cut-off condition only if they are based on the design procedure. The important fact is that the introduction of the proposed overmoded coupling pattern has the capability of reducing the ohmic loss and mitigating the strong field density inside the systems. This is significant for developing high power THz radiation. (paper)