Jiancang Su

To put forward the application of an overmoded Cherenkov oscillator in Ku-band, it is significant to purify the mixed output modes into the single TM₀₁ output mode. A dual-cavity mode convertor (DMC) is introduced to convert the generated mixed modes into the pure TM₀₁ output mode. By considering the factors of mode ratio and phase difference between the mixed TM₀₁ and TM₀₂ modes, the structural parameters of a DMC are analytically optimized. Through the method of particle-in-cell (PIC) simulation, the performance of the DMC integrated into the oscillator is studied. The influence of the waveguide length on the mode conversion efficiency is focused, which shows a periodic effect on the conversion efficiency. Finally, the experiment on the structure of “Oscillator + DMC” is conducted with a 0.5-T permanent magnet. Under the conditions of 570 kV for the diode voltage and the repetitive pulse manner of 20 Hz, the device is operating stably, and its output power is about 740 MW. The radiation pattern is diagnosed as TM₀₁ mode, and the conversion efficiency is about 97.1%.

The inter-turn breakdown phenomenon of the secondary winding in Tesla transformers is studied in order to fabricate a 2.7-MV oil-immersed Tesla-transformer secondary winding. Microscopic photos show that the gradual inter-turn discharges are one of the main reasons causing the secondary winding of the Tesla transformer to be open-circuit. General improvements are implemented during the fabrication: 1) the ratio of turn diameter to the inter-turn distance is set about 0.4; 2) a new type of wire, three-layer-insulation wire, is used to replace the original enamel wire; 3) the insulation liquid where the wire is immersed is fresh and periodically purified; 4) the glue is slightly heated to preclude the voids. With these measures, a 3500-turn secondary winding for a pulsed power generator, TPG2000, is designed, fabricated and tested, in order to boost a primary voltage of 900 V to a secondary voltage of 2.7 MV. Application of this secondary winding in TPG2000 shows that the 2.7-MV secondary voltage can be realized and this winding can work reliably for an output power of 40 GW and a pulse duration of 60 ns.

To develop a compact and lightweight high-power microwave system operating at Ku-band, an overmoded Cherenkov oscillator is studied theoretically and experimentally. In the moderately overmoded structure, the issues of mode competition and electric field strength around the slow wave structure (SWS) surface are specially focused to improve the device’s stability. Both 2-D and 3-D simulation results show that the optimized structure operates stably with the output power of 1 GW under the conditions of 545 kV and 7.1 kA for the beam. With a 0.5 T permanent magnet (PM), the experimental research is conducted. Guided by the PM, the electron beam is effectively transported to the collector, and the device is operating stably. The output power reaches 750 MW under the conditions of 580 kV and 8.3 kA. The pulse duration is 21.8 ns and the frequency is 15.07 GHz. By tuning the specified structural parameter such as “anode–cathode distance” in experiment, the output power increases over 800 MW. At last, the appearance of the high-order asymmetric mode in experiment is analyzed as well.

Rogowski coil is an effective tool, which is capable of testing kA-class nanosecond-scale pulsed current. The basic principle is the Faraday Law of electromagnetic induction. In practical application, it is found that the test current waveform of a Tesla-type generator via the Rogowski coil is strongly distorted. It is conjectured that the width of coupling slot in the generator contributes to the distortion. A circuit model including the slot of the Rogowski coil is proposed. Via this model, the conjecture about the slot is verified. In addition, it is found that the oscillation superimposed on the measured waveform becomes large as the slot becomes narrow. To further study the effect of the slot width on the measurement waveform, the practical 3-D model of the Rogowski coil assembled on the generator is also established and the oscillation dependent on the capacitance of the slot is obtained. In the end, an empirical formula to design the width of the coupling slot of the Rogowski coil is presented. With this formula, the new width of coupling slot is calculated out and the ideal measurement waveform of the Rogowski coil in the Tesla-type generators is obtained.

This article presents the research on repetitive-frequency V/N-type gas switch triggered by high-voltage photoconductive semiconductor switch (PCSS). The working states of V/N-type gas switch in three cases are analyzed theoretically. At underdamping case, the V/N-type gas switch has a shorter trigger delay and a large overvoltage, with a larger triggering current. While at critical-damping case and overdamping case, it has a longer trigger delay and a smaller overvoltage, with a smaller triggering current. Simulations of the working states of the V/N-type switches in three cases were carried out, and the simulation results were consistent with the theoretical results. A V/N-type gas switch triggered by PCSS at the underdamping case was designed. The experimental result of the switch showed that the switch working at the mode of 50 Hz/70 kV/1000 pulses had a trigger delay jitter of 988.7 ps, corresponding operating coefficient of 0.7. After 60000 pulses, there was a continuous breakdown chain, which connected the electrodes of the PCSS. The damage leading to the breakdown occurred after the close stage of the PCSS, including the damage caused by large conduction current and the damage caused by the energy releasing from the reversal inductor during the insulation recovery of PCSS.

Multi-stage gas switches (MSGSs) are widely used in megavolt pulsed power systems due to their advantages of high withstand voltage, fast establishing time and relatively high operating frequency. Multiple operation modes, which are induced by the different breakdown sequences at each stage, are found to affect the breakdown characteristics of the gas switch. Based on the theoretical analysis of the working principle, the classification of the operation mode and the factors determining the operation mode of the MSGS are proposed. Methods for investigating the output waveform for MSGS are established by simulating lumped parameters and studying the breakdown time sequence of individual stages. In comparison with the intended mode of operation, the disadvantages of the unintended modes of operation for MSGS, including error-breakdown and non-triggering mode, are revealed. For unintended modes, the distortion of the output waveform, which determines the efficiency of energy transmission through the switch and the increased delay time jitter, which determines the synchronization of the gas switch, are further investigated. The possibility of non-triggering and its mechanism on surface flashovers induced by drifting electrons are also analyzed. Based on this study, design principles and methods are proposed to ensure the intended operation of the MSGS.

Owing to their wide applications in the field of pulsed power technology, further improvement of quasi-square pulses with fast risetime has attracted increasing attention. In the present work, an optimal double-matched Marx generator (DMG), which combines both impedance matching and time matching, is proposed to generate fast-rising quasi-square pulses. First, a theoretical analysis of the pulse-forming characteristics of the DMG consisting of the pulse-forming line (PFL) module is performed, which demonstrates that fast-rising square-wave pulses are obtainable because the energy stored in all the PFL-modules of the DMG is superimposed and output to the load most effectively. However, when the energy stored in the connection lines (CLs) is output after the main pulse, a trailing edge inevitably appears. In addition, circuit simulations consisting of totally matched and typical mismatch cases are performed to verify optimal dual matching. Finally, a coaxial DMG adopting the annular PFL module has been developed, that successfully generates fast-rising quasi-square pulses with power, voltage amplitude, risetime, and pulse width of 5.5 GW, 525 kV, 4 ns, and 45 ns.

The miniaturization, lightweight, and solidification of pulse forming lines (PFLs) are of prime significance during the evolution of pulsed power technology. In this paper, an all-solid-state annular pulse forming line (APFL) based on film-insulated coaxial transmission lines is developed to generate fast-rise time quasi-square pulses. First, a coiled coaxial transmission line (CCTL) comprised of multilayer polypropylene films with outstanding insulating properties is constructed. It can withstand direct current voltages up to 200 kV, with a cross section diameter of 7.4 mm. In addition, in order to turn the pulse transmission direction from circumferential to axial, a compact insulated terminal with a 90° bend structure is designed for CCTL. Although single terminal inductance can slow down the rising edge of the output pulse, their parallel connection in an APFL can weaken such an effect. The APFL, with a characteristic impedance of 2.95 Ω and a transmission time of 13 ns, is composed of three CCTLs with six terminals, which can run over 100 thousand times under the pulse voltage of 75 kV. Finally, 15 series APFL modules are employed to assemble a multi-stage PFL for the Tesla-type pulse generator. When charged to a voltage of 1 MV, the mixed PFL consisting of a coaxial line and the multi-stage PFL outputs quasi-square pulses with a voltage amplitude, rise time, and width of 510 kV, 4 ns, and 41.5 ns, respectively, and the fluctuation of the flat top is about 6%.

A MV-class SF ₆ -insulated supporting board (ISB) for Tesla-type generators is proposed, which has advantages of compact structure and light weight, but its lifetime is short, only 30,000 pulses. Therefore, various techniques are employed to prolong the lifetime, which include grooving on the board surface, elimination of triple junction (TJ) regions, high-temperature drying, and control of water content in the surrounding SF ₆ environment. Each method is respectively researched. Small-scale experimental results demonstrate that grooving can increase the flashover voltage ( U_f ) by 6.3-13.8% and drying at 120°C can increase U_f by 4.3%. Simulation results reveal that eliminating the TJ regions of the assembled ISBs can reduce the maximum applied field ( E_op ) in this region by 50%. Literature review suggests that controlling the water content in the neighboring SF ₆ gas under 500 ppm (part per million) can increase U_f by 2.5%. All the four techniques are applied to a pair of ISBs in a Tesla-type generator, effectively extending the lifetime from 30,000 to million class with an enhancement factor greater than 30.

By describing the gas breakdown phenomenon from the view of probability density function (pdf) estimation, a numerical calculation model is proposed to apply for the design of a 1 $+$ n-type multistage gas spark switch (GSS). The physical principle, numerical calculation model, design procedures, and experimental test for GSS are hence described in considerable detail in the text. Various key operating parameters for GSS, that is, nontrigger rate $\xi $ , error breakdown rate $\chi $ , mean breakdown voltage, and breakdown voltage jitter $\sigma $ , are deduced from the calculation model. Optimization of operation parameters helps to design reliable compact GSS for pulsed power systems. The validity of the calculation model is verified and its suitability to a more general switch design is discussed.