Electromagnetic Analysis of a Disk-Loaded Coaxial Waveguiding Structure for MILO

An all-metal disk-loaded coaxial waveguiding structure, for its application in a magnetically insulated line oscillator (MILO), as a slow-wave interaction structure, excited in the TM mode, has been field analyzed. In the analytical model, modal matching technique has been used considering all the space harmonics generated due to the structure axial periodicity in the free-space region inside the structure and stationary modal harmonics caused by the reflections from the disk walls in the disk-occupied region of the structure. The dispersion relation of the structure is obtained considering the continuity of the fields at the interface between these two regions. A system of homogeneous equations is formed in the Fourier components of the field constants. The condition for the nontrivial solution of the equations gives the dispersion relation of the structure in the form of a determinantal equation. The analysis is further extended for the interaction impedance, which has direct relevance with the temporal RF signal growth in the device. The derived dispersion characteristic has been validated as special cases for the known results and has been also validated with those results published in the literature. Furthermore, the structure has been simulated using commercial code “CST Microwave Studio” for the dispersion as well as the interaction impedance characteristics. The simulated values have been found to be in agreement with the theoretically derived results. The dispersion characteristics and the axial interaction impedance of the structure have been plotted for a wide range of structure parameters. Furthermore, the desired operation of the MILO device, the method of structure parameter selection, and the effect of parameter variation have been also discussed. This study would help the design engineers in understanding the physics and in selecting the coaxial disk-loaded structure parameters for their successful use in MILO devices having reasonable RF growth rate in synchronism with the electron beam.