Xibo Zhang

Investigation of Thickness Effect on Electric Breakdown Strength of Polymers Under Nanosecond Pulses

The thickness effect on electric breakdown strength (EBD) of four kinds of polymers under nanosecond pulses is investigated. The polymers are polyethylene, PTFE, PMMA, and nylon. The test samples are 0.5-3.5 mm in thickness (d) and are immersed in transformer oil. The nanosecond pulse is based on a Tesla-type generator, TPG200, which is with values of pulsewidth of 8.5 ns and rise time of 1.5 ns. The experimental results show that EBD is 1-2 MV/cm and decreases as d increases. The dependence of EBD on d is analyzed with the Weibull statistical distribution. It is concluded that logEBD versus log d is linear. By replotting the experimental data and by comparing with Martins results, it is found that the slope for the linear dependence is about -1/8. With this conclusion, the breakdown probability is researched. It is shown that, to get a breakdown probability as low as 0.5%, the applied field should be decreased to about half of EBD.

An 8-GW long-pulse generator based on Tesla transformer and pulse forming network

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.

Insulation Analysis of a Coaxial High-Voltage Vacuum Insulator

Insulation of a coaxial high-voltage vacuum insulator used in the pulsed power generator TPG700 has been studied in this paper. When output voltage is increased from 700 to 800 kV, breakdown happens in the insulator. With transient simulation, the region of the insulator subject to the highest electric strength is concluded. According to the experimental and simulated results, the breakdown strength has been calculated. An electric-thermal model has been proposed in which factors such as local electric field (E-field) enhancement, imperfection in dielectric, and repetition working state are analyzed. A formula to calculate the effects of these factors is suggested. Moreover, the key structures on the coaxial line which influence the E-field distribution are optimized, and useful advice for designing an insulator of this kind is presented.

Research on Reliability and Lifetime of Solid Insulation Structures in Pulsed Power Systems

Based on the Weibull statistical distribution and the thickness effect on dielectric breakdown strength EBD, a formula to evaluate the reliability of solid insulation structures (SISs) in pulsed power systems is derived. By calculating this formula, it is concluded that an increase of 1.4 times in the dielectric thickness d or a 3/4 decrease in the applied voltage U will increase the reliability of SISs by “9.” Moreover, by introducing the variable of pulse number N into the Weibull statistical distribution, a formula to evaluate the lifetime of SISs is also derived. It is concluded that when the time shape parameter a is equal to 1, an SIS is in its normal lifetime stage, and the lifetime can be described by the Martins formula; when a is not equal to 1, the Martins formula should be revised. To verify the lifetime formula, experiments were designed and conducted. In the end, useful suggestions on design of SISs are summarized.

Design, Simulation, and Experiments for an Improved Coaxial High-Voltage Vacuum Insulator in TPG700 for High-Power Microwave Generation

An improved coaxial high-voltage vacuum insulator applied in a Tesla-type generator, model TPG700, has been designed and tested for high-power microwave (HPM) generation. The design improvements include: changing the connection type of the insulator to the conductors from insertion to tangential, making the insulator thickness uniform, and using Nylon as the insulation material. Transient field simulation shows that the electric field (E-field) distribution within the improved insulator is much more uniform and that the average E-field on the two insulator surfaces is decreased by approximately 30% compared with the previous insulator at a voltage of 700 kV. Key structures such as the anode and the cathode shielding rings of the insulator have been optimized to significantly reduce E-field stresses. Aging experiments and experiments for HPM generation with this insulator were conducted based on a relativistic backward-wave oscillator. The preliminary test results show that the output voltage is larger than 700 kV and the HPM power is about 1 GW. Measurements show that the insulator is well within allowable E-field stresses on both the vacuum insulator surface and the cathode shielding ring.

A large-dynamic-range current probe for microsecond pulsed vacuum breakdown research

A large-dynamic-range current probe for microsecond pulsed vacuum breakdown research is fabricated. The basic principle of this probe is to turn the current signal to a voltage one via a load of fixed impedance and to test the voltage signal via a voltage divider of adjustable voltage ratio. With a segment of 40-Ω coaxial line, which is connected to the experimental chamber, and a self-fabricated two-stage resistor-capacitor voltage divider, the current probe is realized and is applied to test the vacuum breakdown current for 3-cm parallel-plate electrodes under microsecond pulses. The results show that the current probe is capable of responding to current signals with an amplitude of 10(-2)-10(3) A and a duration of 10(-2)-10(1) μs. Based on the current probe, the characteristics of the vacuum breakdown current under 30-μs quasi-sinusoidal pulses are summarized. The potential mechanisms for each type of current are also discussed in this paper.

A novel structure of transmission line pulse transformer with mutually coupled windings.

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.

Observation of low-density domain in polystyrene under nanosecond pulses in quasi-uniform electric field

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.

Investigation Into the Operating Characteristics of Fused Quartz Vacuum Surface Flashover Switch

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 69000 discharges. The repetitive frequency ability of the switch is better than 100 Hz, and the breakdown voltage increases slightly with higher frequency.

Intrinsic-like surface flashover voltage of insulators

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.