Xibo Zhang’s research while affiliated with Nuclear Power Institute of China and other places

The difference between transient-field distribution (TFD) and electrostatic-field distribution (EFD) is investigated by establishing models for a coaxial line with a supporting dielectric in it and a vacuum insulator with an equivalent load. By simulating the coaxial line model, it is found that the TFD is totally different from the EFD in that the maximum electric field strength (E _max ) of the TFD is almost 2 times of the E _max of the EFD. It is considered that the wave reflection leads to the increase of E _max of the TFD. By simulating the vacuum insulator model, it is found that the TFD on the insulator surface is dependent on the impedance of the load (Z _load ), whereas the EFD keeps constant as Z _load changes. Factors such as load impedance, gap between insulator and metal, pulsewidth, and pulse rise time are selected to further research the differences between the TFD and the EFD methods. It found that the load impedance and the gap between insulator and metal influence the TFD greatly, whereas the pulsewidth and pulse rise time have less influence on the TFD. The difference of TFD and EFD can be used to calculate the impedance distribution of an insulator in practice.

Vacuum surface flashover switch is a promising technique scheme of high repetitive frequency fast-closing switch applied to dielectric wall accelerator. In this paper, the switch is experimentally investigated, and a nanosecond pulse with 2.02-ns effective rise time is obtained. For a cylinder-fused quartz dielectric, the breakdown field of the switch is near 55 kV/cm, and the value for the frustoconical-shaped dielectric exceeds 120 kV/cm with a lifetime up to 69,000 discharges. The repetitive frequency ability of the switch is better than 100 Hz, and the breakdown voltage increases slightly with higher frequency.

The parameter for indicating the characteristics of the surface flashover voltage in vacuum is studied. Experiment results show that, compared with another conditioning method, the flashover voltages of ablative materials obtained by the multi-round stepped increasing voltage method (termed MRSIVM) are increased by 100-200%. This flashover voltage is defined as the intrinsic-like surface flashover voltage (termed ILSFV). The mechanism and significance of ILSFV are studied. After the testing process of MRSIVM, dielectric surface achieves a relatively stable state and the influence of random factors is reduced. The amplitude of ILSFV is higher and the scatter of that is smaller. In addition, the ILSFV stands for an attainable insulation level. So it is more appropriate to represent the surface flashover characteristic of the insulating material by ILSFV.

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.

A novel structure of transmission line transformer (TLT) with mutually coupled windings is described in this paper. All transmission lines except the first stage of the transformer are wound on a common ferrite core for the TLT with this structure. A referral method was introduced to analyze the TLT with this structure, and an analytic expression of the step response was derived. It is shown that a TLT with this structure has a significantly slower droop rate than a TLT with other winding structures and the number of ferrite cores needed is largely reduced. A four-stage TLT with this structure was developed, whose input and output impedance were 4.2 Ω and 67.7 Ω, respectively. A frequency response test of the TLT was carried out. The test results showed that pulse response time of the TLT is several nanoseconds. The TLT described in this paper has the potential to be used as a rectangle pulse transformer with very fast response time.

Low-density domains (LDDs) developed in polystyrene (PS) samples under nanosecond pulses in a quasi-uniform electric field are observed using an on-line transmission microscope. The test samples are 2 mm in thickness with a cylindrical profile, and immersed in clean transformer oil. The nanosecond pulse is trapezoidal with a pulse width of 10 ns. Images taken by the microscope show that the LDDs always appear in the vicinity of the cone electrode, expand as the pulse number increases, and gradually vanish when the output pulses are stopped, only leaving small voids and cracks in the samples. When the pulses are imposed on the sample again, the LDDs emerge and begin to expand again, and finally lead to bulk breakdown of the samples. The observation of LDDs under this condition gives a support for the pre-breakdown and breakdown model by K. C. Kao when applyied to a nanosecond time scale.

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 (EBD) and the flashover time delay (td) would be longer than the pulse width (τ), both of which make the dielectrics' cumulative breakdown occur easily; whereas under long pulses, that Ef is much lower than EBD and td 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.

Based on a nanosecond-pulse generator and dozens of polyethylene samples, the role of electrodes in dielectric breakdown under nanosecond pulses is experimentally investigated. The test factors include electrode material, electrode configuration, and pulse polarity. For the electrode material effect, metals of copper, stainless steel, aluminum, and tungsten are manufactured and investigated. The experimental results show that the larger the work function of the metal, the greater the electric breakdown strength (EBD). For the electrode configuration effect, electrodes with radius of 1 mm and 30 mm are respectively employed. By comparing the relevant experimental results, it is found that the smaller the radius of the electrode, the larger the EBD. The experimental results on pulse polarity show that there is a `weak' pulse polarity effect for the breakdown of PE, and the ratio of EBD under positive pulses to that under negative pulses is 0.8-0.9. All the experimental results reveal that the electrode plays a role of generating seed electrons/holes in dielectric breakdown in nanosecond time scale. In addition, based on the experimental results, a mechanism for solid dielectric breakdown under nanosecond pulses is also proposed in this paper.

An approach for producing a long pulse up to 100 ns is presented. The generator based on this approach consists of a Tesla transformer and a set of pulse-forming networks (PFNs). The Tesla transformer is used to charge pulse-forming lines (PFLs) and PFNs which are in parallel. When the voltage increases to a certain value, the main switch will close, and the PFLs and PFNs will discharge rapidly to the load. Therefore, a high-voltage long pulse is formed on the load. The amplitude of this pulse is dependent only on the charging voltage and the matching state between the load and the PFL (PFN). The pulsewidth is determined by the transmission time of the PFL and PFN. The rise time is determined by the working state of the main switch and the impedance of the PFL and is independent of the parameters of the PFN. The PFN is multistage and assembled in series. The single-stage PFN is formed with ceramic capacitors placed between two unclosed annular plates. The total series impedance is equal to the sum of every single-stage PFN's impedance. A nine-stage PFN is used in the generator, and the total impedance is 40 Omega. Experimental results show that a high voltage of an amplitude of 300 kV, current of 6.9 kA, and duration of 110 ns is obtained at a repetition rate of 10 Hz, with a rise time of approximately 7 ns.