Liang Zhao

A ceramic coaxial vacuum insulator has been designed and fabricated to deliver 600-kV nanosecond pulses for high-power microwave (HPM) generation. Compared to previously reported ceramic vacuum insulators in the literature, this design incorporates two distinctive features: a 45° conical angle, a grooved surface, and high reliability. The 45° conical angle is used to maximize the vacuum surface flashover threshold. The grooved surface, featuring rectangular grooves with a width of 4 mm, depth of 5 mm, and pitch of 8 mm, is engineered to further enhance the insulation performance. Theoretical calculations estimate the overall reliability of the insulator structure to exceed 90%. Preliminary experimental tests confirm that the ceramic coaxial vacuum insulator operates reliably, with no observable signs of failure.

A coaxial vacuum insulator assembled on a Tesla-type generator developed mechanical cracks when subjected to gigawatt-level electrical pulses. The insulator was conical in shape, made of nylon, and designed to transmit 1.1-MV, 45-ns rectangular pulses. Investigation revealed that the failure was not solely caused by high mechanical stress, but also by intense electric fields and material impurities. To address these issues, several improvements were implemented, including increasing the purity of the nylon material and reducing both the mechanical stress and the electric field. Through a redesigned geometry, the electric field at the oil interface was reduced by 24%, while the mechanical stress at the vacuum interface decreased by 52% compared to the original design. Using a multiphysics field-based lifetime evaluation model, the operational lifetime of the improved insulator was estimated. The results show that the lifetime increased from $4.67\times 10^{\mathbf {4}}$ pulses to $4.74\times 10^{\mathbf {5}}$ pulses—a more than tenfold improvement. Preliminary experimental results confirm the effectiveness of the design enhancements.

Surface flashover damage across stage insulators persists as a primary bottleneck for improving insulation strength in compact Tesla-type pulse generators, even when conventional rectangular surface grooving is applied under repetitive microsecond pulses in SF6. To enhance the insulation performance of stage insulators in a multi-stage gas switch (MGS) operating at 1 MV (130 kV per stage), this study systematically investigates the influence of the surface grooving method and conducts the verification of the optimized design. Cylindrical, rectangle-grooved, and wave-grooved insulator samples were compared experimentally under simulated MGS conditions to evaluate the effectiveness of wave grooves in increasing flashover voltage (Uf). Electric field analysis and flashover path observations show that wave grooves suppress streamer development and improve surface insulation strength. Electric field distributions along the surface and within the gas–solid interaction region indicate that the wave-grooved structure promotes surface charge drift and exhibits a stronger normal electric field, which favors deviation of the flashover path from the insulator surface. Consequently, streamer propagation shifts from a surface-dominated to a gas-dominated mode. Observations confirm that wave grooves extend the effective creepage distance and increase the probability of a “gas” path—a dual modulation mechanism that explains their superior improvement in surface flashover characteristics. Based on these findings, the stage insulator in the MGS was redesigned with wave grooves, raising the Uf improvement from 0.89% to 7.70%, extending the operational lifetime by an order of magnitude, and mitigating abrupt insulation failures in SF6.

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.

Presents corrections to the paper, (“Investigation Into Increase Process of High-Power Microwave With S Curve”).

The $S$ curve, with an analytical expression of $y(t) = Y$ /[ $1+ A$ exp(−kt)], has a slow-fast-saturation growth tendency, where $Y,A$ , and $k$ are constants. The formation mechanism of the rise time and the fall time of nanosecond rectangular pulses is investigated in perspective of the $S$ curve. It is found thattherise time of a nanosecond pulse is not cliffy but takes on the $S$ -curve feature, which is due to the electron avalanche in the switch-on process. In addition, the fall time of a nanosecond pulse is also not cliffy too but appears like an inverted $S$ -curve, which is due to the pulse reflection and the closure of the switch. A theoretical formula for the nanosecond rectangular pulses is presented and the output pulses of a Tesla-type generator and an LTD-type generator are reconstructed by referring to the theoretical formula. In the end, the formation mechanism of nanosecond rectangular pulses is discussed from the viewpoint of the self-feedback process.

An improved formula between the breakdown voltage ( $U_{\textit {b}}$ ) and the initial electrons ( $N_{0}$ ) in gas discharges is presented based on the Meek–Raether criterion. This formula shows that $U_{\textit {b}}$ is not only determined by the product of the electrode separation and the gas pressure (pd) but is also influenced by $N_{0}$ . The variation in $N_{0}$ results in an extension of $U_{\textit {b}}$ for a fixed pd. $U_{\textit {b}}$ extends toward smaller values as $N_{0}$ increases and toward to larger values as $N_{0}$ decreases. Two sets of experiments are designed to support the formula. The first is to condition the gas-insulated electrodes, in which $U_{\textit {b}}$ increases as the conditioning cycles increase. The second is to stabilize the gas-gap switch, in which $U_{\textit {b}}$ decreases when the electrode materials change to those can easily emit electrons. As a conclusion, the formula presents a quantitative connection between $U_{\textit {b}}$ and $N_{0}$ in the gas breakdown phenomenon.