A long-pulse generator TPG700L based on a Tesla transformer and a series pulse forming network (PFN) is constructed to generate intense electron beams for the purpose of high power microwave (HPM) generation. The TPG700L mainly consists of a 12-stage PFN, a built-in Tesla transformer in a pulse forming line, a three-electrode gas switch, a transmission line with a trigger, and a load. The Tesla transformer and the compact PFN are the key technologies for the development of the TPG700L. This generator can output electrical pulses with a width as long as 200 ns at a level of 8 GW and a repetition rate of 50 Hz. When used to drive a relative backward wave oscillator for HPM generation, the electrical pulse width is about 100 ns on a voltage level of 520 kV. Factors affecting the pulse waveform of the TPG700L are also discussed. At present, the TPG700L performs well for long-pulse HPM generation in our laboratory.

The breakdown of polymer insulators at a nanosecond scale is a serious problem for high-voltage devices and pulsed power systems. This paper reviews recent developments in research on the mechanisms and characteristics of the breakdown of polymers at a nanosecond scale at the Northwest Institute of Nuclear Technology. An improved avalanche breakdown model is proposed, and a formula for the strength of the electric breakdown (E_BD) is formulated based on it. The formative time lag of the avalanche breakdown model is then developed to verify it. Two research groups were formed to study the characteristics of the breakdown. One focused on the characteristics of single-pulse breakdown, such as the effects of size, polymer type, electrode, pulse width, and pulse polarity on E_BD, with the aim of deriving expressions for E_BD. The other research group examined cumulative (multi-pulse) characteristics of the breakdown with the aim of deriving a formula for the lifetime of the polymer. A method to design solid insulation structures is finally proposed.

To shorten the length of the pulse-forming line (PFL) and generate pulses with good flat-top quality, a 5-GW Tesla-type pulse generator based on a mixed PFL is developed in this paper to produce intense electron beams and generate high-power microwaves (HPMs). The mixed PFL is composed of a coaxial PFL and a multistage series annular PFL, which, in turn, consists of 18 coaxial-output capacitor-loaded annular PFL modules in series. The generator can produce quasi-square electrical pulses with a width of 43 ns and a peak power of 5 GW on a matched load. In experiments where it is used to drive a relative backward-wave oscillator to generate HPMs, the results show that the HPM frequency is 16.15 GHz and the power is 1.06 GW with an efficiency of 25% when the voltage of the diode is 620 kV and the beam current is 6.9 kA.

On a nanosecond time scale, solid insulators abnormally fail in bulk rather than on surface, which is termed as the “worm-hole” effect. By using a generator with adjustable output pulse width and dozens of organic glass (PMMA) and polystyrene (PS) samples, experiments to verify this effect are conducted. The results show that under short pulses of 10 ns, all the samples fail due to bulk breakdown, whereas when the pulse width is tuned to a long pulse of 7 μs, the samples fail as a result of surface flashover. The experimental results are interpreted by analyzing the conditions for the bulk breakdown and the surface flashover. It is found that under short pulses, the flashover threshold would be as high as the bulk breakdown strength (E_BD) and the flashover time delay (t_d) would be longer than the pulse width (τ), both of which make the dielectrics' cumulative breakdown occur easily; whereas under long pulses, that E_f is much lower than E_BD and t_d is smaller than τ is advantageous to the occurrence of the surface flashover. In addition, a general principle on solid insulation design under short pulse condition is proposed based on the experimental results and the theoretical analysis.

Theoretical analysis for the volume (V) effect on electric breakdown strength (E_BD) and the operating field (E_op) effect on lifetime (NL) of solid dielectrics on a nanosecond time scale is presented in the perspective of the Weibull distribution. It is derived that the general formula for the volume effect is E_BD=kV^-1/m and the formula for the lifetime effect is N_L=(E_BD/E_op)^m, where m is a parameter which is determined by the dielectric quality and influenced by the dielectric types. Besides that, it is found that the volume effect and the lifetime effect correlate with each other via m. By summarizing the experimental results, it is concluded that m is averaged to be 8 for polymers with a normal quality under short pulses and that m will be larger (or smaller) than 8 when polymers are with better (or poor) quality. It is suggested that m can be defined as a parameter to describe the dielectric's quality and m of 8 can be regarded as a criterion to choose polymers as insulation materials.

This study proposes a unified formula for five basic forms of discharge—gas, liquid, solid, and vacuum breakdown, and vacuum surface flashover—under short pulses in an electric field. This formula considers the effects of the number of dimensions and pulsewidth on the electric field. It is verified by using the results of experiments reported at the Aldermaston Weapon Research Establishment (AWRE) and the Northwest Institute of Nuclear Technology. The ranges of application of this formula to different discharge forms are also summarized. The proposed formula can be used to transform experimental data at a small scale under a known pulsewidth into those at a large scale under the application pulsewidth and is thus important for designing insulation.