Theoretical And Experimental Study Of W-band Third Harmonic Gyrotrons

Gyrotrons can deliver powerful coherent radiation at millimeter and even up to fre-quencies of1THz. Therefore, gyrotrons are important for numerous applications. How-ever, very strong magnetic fields are required for gyrotrons operating at the fundamentalcyclotron resonance. So, there are many eforts to develop harmonic gyrotrons in orderto reduce the required operating magnetic fields. It is well known that the necessary mag-netic field can be reduced by a factor of s when a gyrotron operating at the sth cyclotronharmonic. However, the undesired excitation of lower harmonic modes presents a ma-jor problem for stable operation of high harmonic gyrotrons and can preclude one fromreaching the most efcient operating regime. To suppress mode competition, mode selec-tive interaction structures (iris cavity and PBG cavity) are investigated in the dissertation.Based on the theoretical analysis and particle-in-cell (PIC) simulations, both cavities aredesigned and conducted. Using these two cavities, two W band third harmonic gyrotronsare designed, built and tested.In the harmonic gyrotron with an iris cavity, the low order whispering gallery modeTE61is chosen as the operating mode. Linear theory results show that selective increasein the difractive Q factor of harmonic modes can be expected. Therefore, an iris cavitycan trap the third harmonic mode more efectively and lowering its start current. Self-consistent theoretical analysis demonstrates that the iris structure can extend the rangeof magnetic field required for the operating third harmonic mode. And the interactionefciency can be increased significantly. PIC simulation demonstrates that a single modethird harmonic gyrotron radiation can be expected in an iris cavity. The operating fre-quency of TE61is94.8GHz. A power of8.6kW can be obtained, corresponding to anefciency of6%. In the experiment, a single mode third harmonic gyrotron radiation wasobserved with the frequency of94.87GHz. The maximum output power is5.5kW, cor-responding to an efciency of4%. The experimental results are in good agreement withthe theoretical predictions.In order to obtain higher mode selectivity, PBG structure is investigated. The disper-sion characteristics of TE modes in2D triangular lattice PBG structure are investigatedwith finite diference method. There are several global band gaps for TE modes in the P- BG structure. For TE modes, there is no cutof frequency in the PBG structure. Therefore,we can use PBG structure to design a cavity which can only confine the third harmon-ic modes in a gyrotron. Based on simulation results, TE04-like mode is chosen as theoperating mode since it is well azimuthally symmetric and similar to TE04in a cylindri-cal waveguide. A third harmonic gyrotron oscillator with a PBG cavity was conductedand tested. In the experiment, the third harmonic oscillation with step tunable frequencyfrom91.75GHz～96.62GHz was observed when the magnetic field varied from1.16T to1.22T. PBG cavities show great promise for applications in tunable harmonic millimeterwave gyrotrons.