| Due to the potential use of the Ku-band high power microwave(HPM) in communication, radar, remote sensing and other applications, the investigation of the Ku-band HPM generater technology has been one of the hottest topics in the HPM domain. However, for the traditional single-moded generators, their dimensions and the generated wavelength are of the same order. This means Ku-band generators are of the smaller size and support less power capacity. Hence, it is difficult to improve the output power of the traditional HPM device at Ku-band. In order to increase the power capacity, the overmoded Cerenkov generator has been put forward. Nevertheless, its large input voltage, high guiding magnetic field and complicated mode control limit its practicability and application prospects. Thus, investigations on the compact Ku-band HPM device with high power capacity, moderate input voltage, low guiding magnetic field and single mode operation become more and more attractive. Based on the above discussion, a novle Ku-band coaxial transit-time oscillator is proposed and designed in this dissertation. With the low guiding magnetic field and moderate input voltage, this device can output GW-level HPM. Besides, it is also suitable for repetitive and long-pulsed operation. The main content and innovative work are listed as follows.Firstly, the coaxial transition structures are investigated systematically. Compared with the non-coaxial structure, the coaxial structure has lower electric potential energy and higher space-charge limiting current, and thus, the deivice with coaxial structure is in favor of lowering its impedance, guiding magnetic field and improving its conversion efficiency. By the theoretical analysis of the coaxial cavity, we get the methods of improving the power capacity, making the working mode have the trait of quais-body wave and approximately calculating the resonant frequency of the multi-cavity structure. In addition, the buncher and extractor are investigated theoretically with the small signal theory and equivelant circuit theory to get the design consideration of the coaxial transit-time oscillator.Secondly,based on the transition mechanism and design requirements of the high frequency band device with low magnetic field, a novel Ku-band coaxial transit-time oscillator is proposed. The design idea of its each part is described, and the novel buncher, coaxial reflector and new electron collector are analyzed emphatically. To make the Ku-band device more compact, the ways for further lowering guiding magnetic field are investigated, and by these ways, the proposed device’s feasibility of low magnetic field operation is demonstrated.Thirdly, the mechanism of Ku-band HPM generation from the device is investigated thoroughly by the particle-in-cell(PIC) code and related physical images are presented. Main structure parameters of the device are optimized by PIC simulations. The typical simulation result is that, with a 410 k V, 8 k A beam guided by a magnetic field of 0.7 T, 1 GW HPM pulse at 14.2 GHz is generated, yielding a conversion efficiency of about 30%. In order to forecast the experimental results, according to the experiment conditions, the simulation of the proposed device is conducted. Moreover, to make the device work with lower guiding magnetic field, after a cathode with improved structure employed in the device, the generator can output 920 MW microwave pulse at 14.2 GHz under 0.3 T magnetic field, which demonstrates the device’s feasibility of lower magnetic field(~0.3 T) operation.Fourthly, after the engineering design and fabrication of the device, the experiments are carried out on the Torch01 accelerator. With the diode voltage of 430 k V, the current 8.3 k A and the guiding magnetic field of 0.7 T, the radiated microwave with frequency of 14.3 GHz, power of 790 MW, pulse duration of 26 ns and conversion efficiency of 22% is generated. Meanwhile, the device obtains the maximum microwave power 1 GW, when the diode voltage and current are increased to 500 k V and 10 k A, and this experimental result demonstrates the abilities of the Ku-band device to generate GW-level Ku-band HPM with low guiding magnetic field. In addition, the stimulation and suppression of the asymmetric competition mode in the device are analyzed emphatically, and the function of the novel electron collector for suppressing the mode competition is investigated by experiments. And the experimental results testify that this collector can effectively suppress the asymmetric mode competition.At last, studies of the Ku-band coaxial device are further widened. By employing a coaxial TM02 mode dual-resonant reflector and a resonant cavity located before the extractor in the proposed device, the conversion efficency is further improved and the power saturation time is lowered effectively. To illustrate the abilities of this improved device, a simulation study is conducted. The typical simulation result is that, with a 380 k V, 8.2 k A beam guided by a magnetic field of about 0.7 T, 1.15 GW microwave pulse at 14.25 GHz is generated, yielding a conversion efficiency of about 37%, and compared with the device without the resonant cavity located before the extractor, this improved device lower the saturation time obviously. Moreover, a novel coaxial transit-time oscillator without external guiding magnetic field is designed to generate high power microwave at Ku-band. Compared with other traditional HPM generators without guiding magnetic field, this novel device has higher power capacity, space-charge limiting current and conversion efficiency. Although the foils are employed in the device to guide the beam, if the pulse duration is short and the repetition rate is low, this device is still attractive in certain applications because of its simplicity, ligh weight and low energy consumption.
|
PDF Full Text Request