High Voltage Vacuum Insulation Research Articles

Effect of selected parameters on the electrical strength of a high-voltage vacuum insulation system

Effect of selected parameters on the electrical strength of a high-voltage vacuum insulation system

A Multifunctional Long-Lifetime High-Voltage Vacuum Insulator for HPM Generation

A multifunctional, long-lifetime, compact, and coaxial high-voltage (HV) vacuum insulator for Tesla-type generators is proposed for high-power microwave (HPM) generation. This vacuum insulator has two features. First, it is multifunctional since it can not only support the two conductors of the transmission line (TL), supply a vacuum environment for load, but also can eliminate a prepulse from the main pulse, which is realized via a grounded inductor. Second, it has a long lifetime. This feature is realized by optimizing the grounded inductor on the switch side and by grooving on the vacuum side. A 50-cm 700-kV compact coaxial HV vacuum insulator of such kind was designed, which was applied to a 1-MV Tesla-type generator to drive the relativistic backward-wave oscillator (RBWO) for HPM generation. Significantly, it has operated for a lifetime longer than 3 00 000 pulses at a repetition rate of 50 Hz without failure, which verifies the design.

Formation of asperities on electrode surfaces of high-voltage vacuum insulation systems by striking microparticles

The authors analyzed the ability to produce asperities on the surface of discharge electrodes vacuum insulation systems by hitting the electrode surface small clumps of material − microparticles. Insulation systems with electrodes made of copper, aluminum and titanium were analyzed. The dependence on the distance between the electrodes and the minimum voltage value at the terminals of the insulation system was determined, under the influence of which accelerated spherical microparticles with different radii cause deformations of the electrode surface.

A multi-functional high voltage experiment apparatus for vacuum surface flashover switch research.

A multifunctional high voltage apparatus for experimental researches on surface flashover switch and high voltage insulation in vacuum has been developed. The apparatus is composed of five parts: pulse generating unit, axial field unit, radial field unit, and two switch units. Microsecond damped ringing pulse with peak-to-peak voltage 800 kV or unipolar pulse with maximum voltage 830 kV is generated, forming transient axial or radial electrical field. Different pulse waveforms and field distributions make up six experimental configurations in all. Based on this apparatus, preliminary experiments on vacuum surface flashover switch with different flashover dielectric materials have been conducted in the axial field unit, and nanosecond pulse is generated in the radial field unit which makes a pulse transmission line in the experiment. Basic work parameters of this kind of switch such as lifetime, breakdown voltage are obtained.

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 Method to design composite insulation structures based on reliability for pulsed power systems

AbstractA method to design the composite insulation structures in pulsed power systems is proposed in this paper. The theoretical bases for this method include the Weibull statistical distribution and the empirical insulation formula. A uniform formula to describe the reliability (R) for different insulation media such as solid, liquid, gas, vacuum, and vacuum surface is derived. The dependence curves of the normalized applied field onRare also obtained. These curves show that the normalized applied field decreases rapidly asRincreases but the declining rates corresponding to different insulation media are different. In addition, ifRis required to be higher than a given level, the normalized applied field should be smaller than a certain value. In practical design, the common range of the applied fields for different insulation media should be chosen to meet a global reliability requirement. In the end, the proposed method is demonstrated with a specific coaxial high-voltage vacuum insulator.

Conceptual design of a high-voltage compact bushing for application to future N-NBI systems of fusion reactors

The energy of future neutral beam injector (NBI) heating systems of fusion power plants ranges from 1 to 2MeV. They are based on powerful (several tens of MW) hydrogen negative ion electrostatic accelerators where electrodes are polarized by DC high-voltage. The beam line under vacuum is supplied by HV power supplies via a transmission line pressured under SF6 and a high voltage feedthrough called bushing. The paper presents results obtained over experimental campaigns dedicated to high voltage vacuum insulation for future NBI systems (ITER). It addresses the problematic of the electron field emission and the high voltage breakdown limit under vacuum between large electrode surfaces. The paper highlights the dependence of the electron emission (dark current) with the voltage and the background tank pressure: at low pressure (∼1E−3Pa in hydrogen), an important dark current of I∼100mA has been measured at 500kV, while at higher pressure (∼0.3Pa in helium), the dark current has been nearly suppressed (less than 3mA of dark current at 970kV). The paper shows that a field induced gas adsorption process could occur on the emitting surfaces (cathode), and this process tends to lower the electron field emission current by increasing the work function of the electrode surface. The Fowler–Nordheim law applied to the measured dark current indicates about 70% of work function increase at 0.3Pa in helium. Finally, a new high-voltage bushing concept relevant to the future NBI systems is presented; it is based on these experimental findings in high voltage vacuum insulation; the main feature of the new bushing concept is to take benefit of the field induced adsorption effect, i.e., the suppression of the dark current with helium gas, in the inner part of the bushing where the electric field intensity is highest.

Characteristics of long gap surface flashover channel in vacuum under nanosecond quasi-square pulses

High-voltage vacuum insulator failure is generally due to surface flashover rather than insulator bulk breakdown. To study the characteristic of surface flashover channel, a diagnosis system to obtain the images of flashover channel is fabricated using a high speed frame camera, and results are presented by an investigation into the flashover of poly-methyl methacrylate insulator with gap spacing of 170 mm in vacuum under 180 ns quasi-square pulses. It is obtained that after formation of the flashover channel, the impedance of the flashover channel is inductive and the waveform of the measured voltage is determined by the inductance of the flashover channel directly.

1-MV Vacuum Insulation for the ITER Neutral Beam Injectors

The ITER is an international project which aims to develop an experimental reactor as a step to realize fusion energy. To inject 33 MW of 1 MeV D0 neutral beams for heating and current drive in ITER plasmas, D- ions are accelerated to 1 MeV. The electrostatic accelerator consists of five acceleration stages, and each acceleration gap has to withstand 200 kV. Gaps between the accelerator and the vacuum vessel, which is at ground potential, must sustain voltages up to 1 MV, and high-voltage vacuum insulation is a key issue. A minimum gap length of >; 900 mm was selected for the l-MV insulation distance. Development of a high-voltage bushing (HVB) which is a bulkhead and a feedthrough between the gas insulated HV transmission line and the beam source in vacuum is ongoing. The HVB consists of a stack of five large bore ceramic rings (1.56 m diameter), each 0.29 m in height. The five-stage insulation concept was applied to both the accelerator and the HVB for better voltage holding with multiple short gaps compared to fewer longer gaps. R&D on a single-stage HVB ceramic ring with the associated electrostatic screens was carried out, and voltage holding of -203 kV dc for 5 h was confirmed.

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.

Displacement current and surface flashover

High-voltage vacuum insulator failure is generally due to surface flashover rather than insulator bulk breakdown. Vacuum surface flashover is widely believed to be initiated by a secondary electron emission avalanche along the vacuum-insulator interface. This process requires a physical mechanism to cause secondary electrons emitted from the insulator surface to return to that surface. Here, it is shown that when an insulator is subjected to a fast high-voltage pulse, the magnetic field due to displacement current through the insulator can provide this mechanism. This indicates the importance of the voltage pulse shape, especially the rise time, in the flashover initiation process.

Different approach to pulsed high-voltage vacuum-insulation design

Different approach to pulsed high-voltage vacuum-insulation design

Improved design of a high-voltage vacuum-insulator interface

We have conducted a series of experiments designed to measure the flashover strength of various azimuthally symmetric 45° vacuum-insulator configurations. The principal objective of the experiments was to identify a configuration with a flashover strength greater than that of the standard design, which consists of a 45° polymethyl-methacrylate (PMMA) insulator between flat electrodes. The thickness d and circumference C of the insulators tested were held constant at 4.318 and 95.74 cm, respectively. The peak voltage applied to the insulators ranged from 0.8 to 2.2 MV. The rise time of the voltage pulse was 40–60 ns; the effective pulse width [as defined in Phys. Rev. ST Accel. Beams 7, 070401 (2004)] was on the order of 10 ns. Experiments conducted with flat aluminum electrodes demonstrate that the flashover strength of a crosslinked polystyrene (Rexolite) insulator is (18±7)% higher than that of PMMA. Experiments conducted with a Rexolite insulator and an anode plug, i.e., an extension of the anode into the insulator, demonstrate that a plug can increase the flashover strength by an additional (44±11)%. The results are consistent with the Anderson model of anode-initiated flashover, and confirm previous measurements. It appears that a Rexolite insulator with an anode plug can, in principle, increase the peak electromagnetic power that can be transmitted across a vacuum interface by a factor of [(1.18)(1.44)]2=2.9 over that which can be achieved with the standard design.7 MoreReceived 26 November 2004DOI:https://doi.org/10.1103/PhysRevSTAB.8.050401This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Design of a cylindrical high-voltage high-temperature vacuum insulator

A design for a high temperature, HV insulator is presented. This design exploits the temperature dependent thermal and electrical properties of alumina and a unique structural insulator design to increase the high voltage hold-off characteristics of the insulator, reduces thermal flux through the insulator, and alleviate stresses caused by mismatch between insulator and electrode thermal expansion coefficients. The designed insulators insulate graphite electrodes at temperatures approaching 1000/spl deg/C and potentials up to 50 kV. In this paper, the general methodology of designing these insulators is explored, and some limited performance data is presented.

Effects of geometry and polarity on the prebreakdown and flashover behavior of conical high-voltage insulators in vacuum

Concurrent optical and electrical measurements have been used to characterize the physical phenomena associated with the prebreakdown and flashover behavior of high-voltage alumina ceramic insulator samples having both conventional and unconventional conical geometries. The critical parameters are the threshold voltage for initiating flashover events, the stable interelectrode current-voltage relationship, the luminescence pattern, and the breakdown rate as a function of time. These measurements have revealed that a well-conditioned sample of unconventional conical geometry can have very low, i.e., approaching zero, breakdown rate. Physical explanations of the findings are based on the current understanding of flashover mechanisms and the prediction from simulations using a field plotting technique.

Optical studies of surface flash-over

An experimental study has been carried out into the optical activities associated with surface flash-over on high-voltage insulators in vacuum, aimed at determining the physical origin of the flash-over process. This involves using a 'transparent anode' to form, with combinations of cathode-insulator regimes, a number of electrode configurations, in order to observe the field electron emission and the optical events occurring during surface flash-over. In particular, the flash-over characteristics of two types of insulator shapes, cylindrical insulators and truncated cone insulators, were investigated. The latter allow spatially resolved imaging of the optical activities at two triple junctions and the surface of the insulators. Using this imaging technique in conjunction with the real-time recording function of a video camera system, four types of optical images have been identified. In addition, the influences of temperature and helium conditioning upon the characteristics of a flash-over were studied. Finally, a new model has been proposed to account for these experimental findings, which assumes accumulation of charges at two interface regions at two triple junctions, and that subsequent initiation of flash-over results from the breakdown of interface layers.

Vacuum breakdown

A short review is given of physical processes which limit high voltage insulation in vacuum. Prebreakdown field emission, ignition events, current flow during full breakdown and conditioning procedures are described and discussed. It is argued that cathodic phenomena are dominating for well conditioned electrodes.

Electric Surface Strengt of High-Voltage Insulators in Vacuum

Investigations were made of various factors which affect the surface flashover strength across polymethylmethacrylate, tetraflouroethylene and polyethylene surfaces in vacuum. The factors studied included specimen conditioning under DC and AC voltage, specimen's length, and the effect of surge voltage wave-shape.

The Effect of the Number of Spacer Insulators on the Breakdown Voltage of Vacuum Insulation

It was shown that when extended high voltage vacuum insulation is being considered, the effect of the number of spacer insulators on the breakdown voltage of the system has to be taken into account. Two cylindrical electrode systems with a variable number of spacers were investigated. A very distinct decrease of the breakdown voltage with the increase in the number of insulators was noted. This effect was much greater than could be predicted when the normal distribution of the flashover voltages of a single insulator was assumed. A much better approximation was obtained when the Weibull distribution was used.

Methods of measurement of electron emission currents in high-voltage vacuum insulation devices: H Moscicka-Grzesiak and J Skibinski, Elektrotech Biul Inform, 27 (4), 1973, 13–20 (in Polish)

Methods of measurement of electron emission currents in high-voltage vacuum insulation devices: H Moscicka-Grzesiak and J Skibinski, Elektrotech Biul Inform, 27 (4), 1973, 13–20 (in Polish)