For Ku-band Gyrotron Traveling Wave Tube Coupled Output Transition Section Of The Study

The high power and broad bandwidth capability of Gyrotron traveling wave tube (Gyro-TWT) make it attractive coherent radiation source in the millimeter wave range. Gyro-TWT has received a great deal of attention for its bright future on the fields of high-power radar,communication,precision guidance,material processing,and particle accelerators and so on.There is a disparity between our country and the others in the development of Ku band long distance radar,for the lack of appropriate high power microwave resources.It is a feasible and potential way to choose Gyrotron traveling wave tube to be the power resource of the transmitter of Ku band super-distance radar.Gyrotron is a fast wave device,which has a quite high power capacity at a high operation frequency.The design of the high frequency structure of a Gyrotron traveling wave tube is different from the other high frequency transition systems because of the specialty of its size and frequency.In this paper,we made theoretical analysis and synthesis of the output waveguide taper of a Ku band Gyrotron traveling wave tube according to the demand and actuality of Gyrotron traveling wave amplifier.The output waveguide tapers are usually used as the transitions of the interaction structure and the output waveguide which has a bigger radius.These tapers are required to have good performance on transition and the suppressant of spurious modes,and at the same time,a relatively short length.With regards of these factors,the output taper of a Ku band Gyrotron traveling wave tube is a radius gradual changing waveguide transition,which suppresses the spurious modes and introduces no discontinuity of the impedance.Comparing with highly oversized waveguide tapers,there are special difficulties in designing gyrotron tapers:1) The main mode is close to cut-off at the input of the taper so that the input reflection has to be considered;2) The radio a₂/a₁ is so large that there is strong excitation of many unwanted modes;3) As well as coupling to the TE_mp modes,TE_mp modes excite TM_mp modes.In this paper,firstly,we analyzed the coupling of the transmit modes in overmoded waveguide tapers,and got the three-mode-coupling wave equations as well as the coupled wave equations of non-uniform waveguides,and we also gave a brief introduction of taper contour analysis and synthesis.Secondly,we studied the coupled wave equations of the waveguide tapers which near cutoff in terms of the waveguide taper of Ku band gyro-TWT,derived the coupling coefficient,and analyzed the composition design the horn transitions.We tried to bring the design of horn transition to the waveguide tapers near cutoff,made our design according to VSWR and mode conversion separately.We designed a three-section parabolic taper and a two-section modified Dolph-Chebyshev taper,discussed the characteristic of both tapers.We made theoretical evaluation of both tapers with Matlab,decided the contour and length.The simulations of RF code HFSS shows,composing the complete taper with several sections provides a good way to ensure the transmit performance of the waveguide taper. Without regard to the influence of electron beam,and the modified Dolph-Chebyshev taper surely has superiority in shorter length.