Investigation On Related Theory And Key Problems Of Overmoded O-Type Cerenkov High Power Microwave Generators

Due to the utilization of slow wave structures（SWSs） with large diameter, overmoded O-type Cerenkov high-power microwave（HPM） generators have shown significant advantage in high output power and long pulse operations. However, further development of this kind of devices is facing difficulty in realizing mode control and high power capacity simultaneously. This dissertation investigates the problem of mode control and power capacity of overmoded O-type Cerenkov HPM generators through theorectical analysis, particle-in-cell（PIC） simulation and experimental tests. The main content and results of this dissertation are as follows.First, the characteristics of surface waves and frequency band overlapping in overmoded SWSs are investigated. The analysis reveals the following disciplines. The phenomenon that the fundamental spatial harmonic is slow wave is necessary, not sufficient condition under which the field distribution of one mode becomes surface waves. For the frequently used rectangular periodic SWSs, we add an empirical sufficient condition under which the p mode of TM₀₁ wave is surface wave only if the fundamental spatial harmonic is slow wave. When the diaper of SWSs increases, or the periodic length and corrugation depth of SWSs decreases, the problem of frequency band overlapping between higher order TM_0n modes and TM₀₁ modes deteriorates, yet the higher order TM_0n modes are always volume waves. TM₀₁, EH₁₁ … and EH_n1 mode become surface waves in sequence as the diameter of overmoded SWSs increases. The lower order surface wave, i.e., EH₁₁ mode is quite close to TM₀₁ mode in frequency band.Second, the growth rates of synchronous interactions between space charge waves and TM_0n modes are calculated. The calculation indicates that the growth rate of TM₀₂ wave is about one tenth of that of TM₀₁ mode, so the TM₀₂ wave can hardly be the dominant mode to interact with electron beam synchronously.Third, a method of purifying the output mixed modes of overmoded Cerenkov devices is presented. This method is based on the adjustment of the phases and power coupling between TM_0n modes. With this method, the output TM₀₁ and TM₀₂ mixed modes can be converted to pure TM₀₁ mode without varying the total output power. The output TM₀₁ and TM₀₂ mixed modes of a typical overmoded HPM generator are purified in PIC simulation. The simulation results suggest that the purification effect is not sensitive to diode voltage, so this purification method is valuable in practice.Fourth, a numerical method of determining the modes composition of HPM generators through the PIC simulation results is presented. With this method, the output modes composition of all kinds of HPM generators can be calculated in both two dimensional and three dimensional PIC simulations.Fifth, the mode competition aroused by asymmetric surface waves in O-type Cerenkov devices with relatively large value of D/λ（here, D is the average diameter of SWSs, and λ is the generated wavelength） is analyzed. The analysis indicates that the mode competition between EH₁₁ and TM₀₁ modes interrupts the single frequency and single mode operation of the overmoded devices seriously. Three dimensional PIC simulations are essential for validating the effect of mode competition suppression. Adjusting the feedback at two ends of the beam-wave interaction area could suppress EH₁₁ mode competition.Sixth, the power capacity characteristics of single and mixed TM_0n modes in SWSs are numerically calculated. The calculation suggests that the power capacity of TM₀₁ mode is apparently lower than that of higher order TM_0n modes. The power capacity of SWSs grows as the diameter increases, the corrugation depth decreases, or the frequency drops, no matter the field distribution is surface wave or volume wave. Under the condition that the diameter of SWSs and beam voltage are fixed, the power capacity of TM₀₁ wave is still dropping as the periodic length shortens and the generated frequency rises, although the value of D/λ is increasing significantly. As for mixed TM_0n modes with arbitrary phases, higher modes purity means higher power capacity. Power capacity of SWSs with smaller value of D/λ and higher mode purity could be higher than that of the SWSs with larger value of D/λ and lower mode purity.Seventh, the radio frequency（RF） breakdown characteristics of cold structure SWSs of travelling wave type are explored in experiments. The experimental results are as follows. The output pulse width is less than 100 ns unless the maximum surface electric field is lower than 1 MV/cm. The main effect of RF breakdown on microwave energy in travelling wave structures is absorption, not reflection. Increase of the area covered by peak electric field leads to apparent decrease of output pulse width. Both the power capacities and the output pulse widths of brass and 304 stainless steel structures are close to each other. The microwave energy absorption and reflection in RF breakdown process do not varies apparently with the variance of external magnetic field.The intensive and detailed study mentioned above lays foundation for the precise investigation and further increase of output level of overmoded Cerenkov devices.