Junna Li

Process of Surface Flashover in Vacuum Under Nanosecond Pulse

In a wide variety of high-power pulsed devices, vacuum surface flashover plays a key role in the system performance. High-speed electrical measurements with temporal resolution in the nanosecond range were employed to measure the flashover characteristics, and high-speed three-frame photography was used to image the flashover channels at different phases along the insulator surface. From these test results, the flashover under nanosecond pulse would experience a process with three typical phases: initial phase, developing phase, and extinguishing phase. Spectra luminosity of the flashover channel is also discussed for better understanding the mechanism of the flashover process.

A 3-MV Low-Jitter UV-Illumination Switch

In a wide variety of high-power pulsed devices, the megavolt switch plays a key role in the system performance. A capacitance-resistance coupling structure was designed to produce UV light which triggered the switch to decrease the breakdown jitter. High-speed electrical measurements with a circuit integrating probe in the nanosecond range were employed to measure the voltage before the switch. From the experimental results, the breakdown characteristic of a 3-MV UV-illumination switch was obtained in the range of 1.6-2.5 MV: The jitter of the breakdown voltage is lower than 25 kV, and the jitter of the breakdown time is shorter than 5 ns.

Influences of electric field on the jitter of ultraviolet-illuminated switch under pulsed voltages

In a wide variety of high-power pulsed facilities, megavolt switches play a key role in determining the system performance. In particular, ultraviolet (UV)-illuminated switches have been widely used for their high-voltage, low-jitter characteristics. In this paper, two factors that may affect the breakdown jitter of a 3 MV UV-illuminated switch under pulsed voltages were investigated, namely, the electric field enhancement factor of a UV-illuminating gap (UV gap) and the number of UV gaps. The experimental results show that the electric field enhancement factor is important for adjusting the average E/p of the main switch gap when the UV gaps breaks down under certain N2 pressures. When the electric field enhancement factor of the UV gaps is approximately that of the main switch gap, the breakdown jitter percentage goes down to less than 1.9%. Comparatively, the number of UV gaps affects breakdown voltage and jitter in a limited-way. When only one UV gap is functioning and the electric field enhancement f...

Characteristics of nanosecond pulse dielectric surface flashover in high pressure SF6

Surface electrical insulation performance is an important issue in pulse power devices. In order to investigate the surface flashover characteristics in an environment of high pressure SF6, a flashover experiment platform was established with nanosecond pulse voltage supply. The flashover characteristics of polymethylmethacrylate (PMMA) specimens with circular truncated cone shapes were subsequently determined under incremental SF6 pressures from 0.2 to 1.0 MPa. As the angle of cone increased, the flashover voltage (FV) rose under low pressure condition, but fell at first and then rose at high pressure. The surface length and the field nonuniform factor were the main influences of FV with angle. In addition, the FV rose linearly at first and then reached its maximum when gas pressure increased, and it was easier to reach the maximum when the field nonuniform factor was higher. A high speed camera was also applied to get the photos at different period of flashover, and the luminous processes of flashover along different specimens were discussed in particle perspective.

Analysis of ohmic losses in Tesla transformers utilizing the open magnetic-core

For typical Tesla transformers with open magnetic-core, a novel coaxial design of the primary side using a mechanically controlled spark gap is introduced to obtain a more compact configuration. The primary winding has several alternative ports to vary the current distribution by changing the position of the capacitor component, and the entire primary circuit is built inside the outer cylinder. For this new primary configuration, we estimate the series parameters and ohmic losses: Polypropylene film capacitors and combined metallized film capacitors are used in the verification tests, and the former capacitor exhibits a lower parasitic resistance below 10 mΩ. The ohmic losses caused by the windings and spark gap are also considered. The results of the experiments show that the voltage gain has an 8.2% drop when the parasitic resistance of C1 increased from 10 mΩ to 24 mΩ, and we deduce that the contact resistance of this primary structure can be limited to below 10 mΩ according to the circuit analysis res...

An optical measuring method to obtain the turn-on time of a MV output switch

The output switch is a key component which connects the pulse power generator and the antenna in an intense electromagnetic pulse simulator and its turning on and off can reflect the working condition of the simulator. In order to measure the turn-on time of the output switch, the formative time of the switch is analyzed based on avalanche and streamer theory, and the optical technique which provides a method via optical fiber transmission for penetrating into high voltage cavities with the advantages of insulation and electromagnetic interference is adopted. A measuring system based on optical fiber and silicon PIN detector is developed, and an experimental platform for testing the synchronous characteristics is established. The experiments are carried out using a one-to-one scale output switch model. The experimental results show that the measuring system is excellent in electromagnetic resistance, the start point of the measured optical signal is clear to recognize; the average time delay between start points of the optical signal and the current signal is 20.42ns, the range and the jitter of the time delay are 0.78ns and 0.4ns; the designed optical system can be used to monitor the turn-on time of the output switch.

Effects of Optical Pulse Parameters on a Pulsed UV-Illuminated Switch and Their Adjusting Methods

Ultraviolet (UV) illumination can effectively shorten the statistical lag and jitter in gas breakdown processes. The spark gap discharge can produce abundant spectrum including UV waveband. In this paper, a simple but reliable spark gap operating with N2 at atmospheric pressure is used to test optical radiation characteristics, such as spectrum intensity, rise time, fall time, and duration of optical pulse, which are adjusted by changing the circuit or configuration parameters. Three UV-illuminated switches with different UV spark gap configurations have been tested. The experimental results approve our assumption. With a uniform electric field UV spark gap, the breakdown voltage standard deviation percentage of the switch is 0.74%.

A 800 kV compact peaking capacitor for nanosecond generator

An extremely compact high voltage peaking capacitor is developed. The capacitor has a pancake structure with a diameter of 315 mm, a thickness of 59 mm, and a mass of 6.1 kg. The novel structural design endows the capacitor with a better mechanical stability and reliability under hundreds of kilovolts pulse voltage and an inner gas pressure of more than 1.5 MPa. The theoretical value of the capacitor self-inductance is near to 17 nH. Proved by series of electrical experiments, the capacitor can endure a high-voltage pulse with a rise time of about 20 ns, a half-width duration of around 25 ns, and an amplitude of up to 800 kV in a single shot model. When the capacitor was used in an electromagnetic pulse simulator as a peaking capacitor, the rise time of the voltage pulse can be reduced from 20 ns to less than 3 ns. The practical value of the capacitors inductance deduced from the experimental date is no more than 25 nH.

Degraded Switching Characteristics of a High-Current, High-Action Reversely Switched Dynistor

High-voltage, high-current, and high-coulomb transfer closing switches are required for many high-pulsed power systems. This letter presents the degraded switching characteristics of a high-power reversely switched dynistor (RSD). The degraded switching characteristics of the RSD were tested with a working voltage of 10 kV, the repetition rate of 0.2 Hz, and a peak current of about 107 kA. The action is about 1.45 MA2·s. The transferred charge and energy per shot are about 20 C and 100 kJ, respectively. Only minimal changes in the leakage current and voltage drop were witnessed after a serial test of 50 000 shots. The degraded switching characteristics of the RSD were investigated based on two different methods with the special parameters of static volt-ampere characteristics and dynamic voltage drop on RSD, respectively. The degraded switching model of RSD under high-current conditions for long-pulse application was established using the method of numerical fitting. The results show that these two methods agree well with each other.