| The conventional HPM source based on the axial transit-time effect usually has a high diode impedance, and only the radial transit-time oscillator has been studied as a main low-impedance HPM source. Moreover, most of these HPM sources have a foil structure. The high-impedance devices can't matched well with the low-impedance pulsed power sources providing high electric power. And that the foil structure is disadvantageous for long pulse and repetitive operation. Up to now, the low-impedance oscillator without foils based on transit-time effect has not been reported over the world, and there has been no experimental research in this field. In this paper, a novel L-band low-impedance transit radiation oscillator (LITRO) was designed without foils. By use of theoretical and numerical analysis, particle simulation, and related engineering design, the LITRO has been investigated systematically and a series of useful conclusions and laws are presented.Firstly, on the basis of the small signal theory, the transit-time effect of electron beam passing through the gap with random standing wave electric field is studied, and the analytic formula of electron load conductance implying the beam-wave energy interchange is derived. Under such a condition,mathematical expressions of the frequency offset caused by the electron beam and the RF field increasing rate along with the time are discussed. Meanwhile, the high frequency features of the novel L-band LITRO have been investigated numerically. The influences of a span change of the drift-tube on the eigenmode and on the corresponding frequency have been obtained. Combined the numerical analysis and the small signal theory, the interaction between the electron beam and the eigenmode in the LITRO is investigated. Eventually, the voltage work area of LITRO is gotten and the eigenfrequency change caused by electron beam is considered.Secondly, the LITRO has been simulated by a 2.5D particle-in-cell code. With the voltage 600kV, the current 36kA and the magnetic field 0.45T in the simulation, the output power is over 5GW at the main frequency 1.60GHz with 15ns duration. The beam-to-microwave power conversion efficiency is 23.1%, and the working mode of the LITRO is p-like mode. Moreover, the oscillators on S-band, C-band and X-band have also been studied in our simulation and some primary results have been obtained. The typical S-band output power is about 4GW at the main frequency 3.175GHz with the voltage 550kV, the current 27.6kA and the magnetic field 0.8T, respectively. The beam-to-microwave power conversion efficiency is 26.4%. With the same input voltage, both the output power of C-band and that of X-band are also over 2.0GW with the efficiency about 20%.Finally, a related engineering design is proposed in our paper. The magnetic system of LITRO is designed chiefly and the number of the capacitors is calculated. By employing such a solenoid, the peak current reaches 600A at the time 22.0ms. The rising temperature of the coils is about 0.261K in 40ms. The force between any two adjacent circles is less than 144N with a distance of 0.5mm. Furthermore, some struts and a mode converter are also optimized by a simulation code. In the case of two rows struts with the distance ofλ/4 and five struts per row used, the energy transmission coefficient exceeds 99% at frequency ranged from 1.5GHz to 1.68GHz. By using the mode converter from TEM to TM01, the maximum gain is in the direction of 27±off the principal axis with the given structure parameters. |
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