1. Cold Analysis of Disc-Loaded Circular Waveguides for Wideband Gyro-TWTs Vishal Kesari vishal_kesari@rediffmail.com Centre of Research in Microwave Tubes Department of Electronics Engineering Institute of Technology, Banaras Hindu University Varanasi – 221 005

2. Applications of high power devices at millimeter wave frequency range Radar (long-range and high resolution) Communication (high information density) Electronic warfare Directed energy weaponry Material processing Waste remediation Ozone generation Atmospheric purification of admixtures like freons that destroy ozone layer

3. Conventional Microwave Tubes Increase of the operating frequency of conventional microwave tubes RF power output becomes limited due to DC power dissipation RF losses Attainable electron current density Heat transfer (restricting the average power capability) Material breakdown (arcing) (restricting the peak power capability) Difficulty of fabricating tiny parts Microwave Solid State Devices limited output power limited bandwidth low efficiency thermal considerations

4. Gyro-klystron, application in a linear accelerator limited bandwidth cavity-type interaction structures Gyro-travelling-wave tube (gyro-TWT) wider bandwidth propagating waveguide interaction structure For the communication purpose there is need to broaden the bandwidth of a gyro-TWT Unconventional high power microwave tubes operable in the millimetre-wave frequency band for instance, gyro-devices

5. Controlling dispersion characteristics of the waveguide for wideband coalescence between the beam-mode and waveguide-mode dispersion characteristics Method of broadbanding a gyro-TWT i) by dielectric lining the metal wall of a circular waveguide entails the risk of dielectric charging that results into heating if the dielectric is lossy ii) by corrugation or metal disc loading a circular waveguide optimisation of the corrugation or disc parameters brings about the desired shape of the structure dispersion for wide coalescence bandwidth for wideband device performance

6. Region I: disc-free region Region II: disc-occupied region Propagating waves Standing waves Electromagnetic boundary conditions

7. Relevant field intensities For TE mode excitation where

8. Field expressions Boundary conditions Integration within limits of validity Elimination of field constants Dispersion relation Multiplication by

9. Azimuthal interaction impedance An estimate of azimuthal electric field available for the interaction of RF with gyrating electrons in a gyro-TWT P t Total power transmitted through the structure Free-space intrinsic impedance

10. Dispersion Characteristics Interaction Impedance Characteristics

12. Region I: disc-free region Region II: disc-occupied region Propagating waves Standing waves Electromagnetic boundary conditions

13. Dispersion Relation Azimuthal Interaction Impedance As special case results passes on to that of the circular waveguide loaded with infinitesimally thin discs

14. Validation of dispersion and interaction impedance characteristics of a disc-loaded circular waveguide, excited in the TE 01 mode obtained by the present analysis against HFSS Validation of dispersion characteristics of a disc-loaded circular waveguide obtained by the present analysis against those due to Amari et al.

15. Dispersion and azimuthal interaction impedance characteristics of the disc- loaded circular waveguide typically for the TE 01 mode, taking disc hole radius as the parameter Broken curves refer to circular waveguide loaded with infinitesimally thin discs

16. Dispersion and azimuthal interaction impedance characteristics of the disc- loaded circular waveguide typically for the TE 01 mode, taking disc periodicity as the parameter Broken curves refer to circular waveguide loaded with infinitesimally thin discs

17. Dispersion and azimuthal interaction impedance characteristics of the disc- loaded circular waveguide typically for the TE 01 mode, taking disc thickness as the parameter Broken curve refers to circular waveguide loaded with infinitesimally thin discs

18. Rigorous analysis of disc-loaded circular waveguide including all the harmonics and finite disc thickness was developed for its dispersion and azimuthal interaction impedance that is valid for all the azimuthally symmetric modes Effects of structure parameters, namely, disc-hole radius, periodicity and disc-thickness were studied Periodicity was identified as the most effective parameter for dispersion shaping and hence broadbanding a gyro-TWT Conclusion