Theoretical Basis and Experimental Validation of the BIRD Model Spada, Emanuele; De Lorenzi, Antonio; Pilan, Nicola ...
2018 28th International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV),
2018-Sept., Letnik:
1
Conference Proceeding
At high de voltage (>100 kV) and with long gap (>10 −3 m) (p<10 −2 Pa), the current flowing in vacuum between metallic electrodes follows the Fowler-Nordheim (FN) emission, with superimposed random ...bursts during the conditioning processes, eventually ending in a breakdown. Despite the effort made to get a solid understanding of these phenomena, satisfactory explanation hasn't been so far attained. A novel approach is here presented, based upon the hypothesis that the electrode surface is not an ideal metal but instead it is covered by a dielectric compound layer (oxides mainly). According to the model the process leading to the current burst is associated to electron depletion of the cathode layer, due to FN-like emission. Under certain conditions, the electric field inside the layer exceeds its dielectric strength, producing a micro-breakdown and a burst of current. The evaluation of the FN-like current has been set and solved imposing quantum indetermination relations to a solution based on classical theory. The burst can therefore end up into a full breakdown, provided the voltage is high enough to start avalanche process at the anode.
This overview summarizes the R&D activities aimed to secure for the ITER Neutral Beam Injectors a stable voltage holding, one of the most challenging issue for the NBI systems operation. The huge ...size of the accelerator and the highest voltage (1 MV) ever used in such systems make prominent some aspects that are, in other application, almost not influent or well under control, like the Area Effect, Total Voltage Effect, the highly complex electrostatic configuration and, last but not least, the cleaning degree of the electrodes surfaces. A clear explanation of the mechanism leading to the voltage breakdown in these conditions is far from being well established. But in the last years a great effort have been deployed by the NBI community to reliably achieve 1 MV voltage holding that is required for the NB injectors for ITER. Theoretical models and new tools have been derived to explain the mechanism of voltage conditioning and for the design of the complex electrodes geometries, that supersede the simple electric field optimization. The 1 MV across a single gap has not yet been achieved but, on the basis of the results so far obtained, it is now considered to be achievable.
In this paper, in order to investigate the lifetime of oil-paper insulation, specimens were artificially aged with thermal and electrical multiple stresses. Accelerated ageing factors and equivalent ...operating years for each aging temperatures were derived from results of tensile strengths for the aged paper specimens. Also, the evaluation for the multi-stress aged specimens were carried out through the measurement of impulse breakdown voltage at high temperature of 85℃. The lifetimes of the oil-paper insulations were calculated with the value of 66.7 for 1.0 mm thickness specimens and 69.7 for 1.25 mm thickness specimens throughout the analysis of impulse BD voltages using equivalent operating years, which means that dielectric strengths would not be severely decreased until the mechanical lifetime limit. Therefore, for the lifetime evaluation of the oil-paper insulation, thermal aging would be considered as a dominant factor whereas electrical degradation would be less effective. KCI Citation Count: 3
This paper describes a long-term operation of the 2.45-GHz microwave proton source at Peking University. The DC proton beam of 50-55 mA with energy of 35 keV has been run for 306 hours continuously. ...Total beam availability, defined as 35-keV beam-on time divided by elapsed time, is higher than 99%. Water cooling machine failures cause all the downtime, and no plasma generator failure or high voltage breakdown is observed. The longest uninterrupted run time is 122 hours.
The article covers the research of the electric strength and fracture of rocks that are undergone multiple breakdowns generated by electric discharge rock fracture and drilling technologies. The ...research outcomes allow proposing an experimental technology able to determine operating high voltage pulse values in the electric discharge drilling process where one should take into account such factors as the rock re-crushing and the drill bit shift cyclicity.
This paper describes a long-term operation of the 2.45-GHz microwave proton source at Peking University. The DC proton beam of 50-55 mA with energy of 35 keV has been run for 306 hours continuously. ...Total beam availability, defined as 35-keV beam-on time divided by elapsed time, is higher than 99%. Water cooling machine failures cause all the downtime, and no plasma generator failure or high voltage breakdown is observed. The longest uninterrupted run time is 122 hours.
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•Colloidal silica Layer-by-Layer assemblies have been deposited on polycarbonate.•These assemblies influenced the nature of the trapped space charges significantly.•Silica ...nanoparticles turned out to affect the resistance of polycarbonate to laser ablation.
This study focuses on the dielectrical characterization of polycarbonate films coated with silica nanoparticle bilayers assembled through the Layer-by-Layer (LbL) technique. This is the first attempt of dielectric characterization performed on LbL-treated plastic substrates. To this aim, LbL coatings consisting of oppositely charged colloidal silica have been built on a polymeric substrate (polycarbonate). Then, dielectric features such as space charge accumulation, electrical voltage breakdown, and resistance to corona discharge (through laser ablation) have demonstrated that the colloidal silica nanoparticle assemblies can influence the nature of the trapped space charges and affect the resistance of polycarbonate to corona discharge, changing the distribution of the laser energy on impact.
An arc fault is the discharge of electricity through the air between two conductors creating large quantities of heat and light. It is widely accepted that there are many variables which affect the ...behavior of an arc in real switchboards, and thus, understanding the phenomena is difficult. This paper is based upon the results of several years of arc testing with circuits from 155 Vdc to three-phase 450 Vac and at power levels from tens of kilowatts to several megawatts. The goals of the testing were to understand the general behavior of arcing faults in low-voltage switchboards-well enough to predict the motion of arcs-and to design electronic monitoring systems that are capable of protecting switchboards against arcs. This paper will focus on the arc behavior, whereas our other papers will discuss its implications upon forensic investigations and upon the arc-fault protective systems.
The results of theoretical modeling of the process of formation of a nanosecond discharge in a coaxial discharge gap filled with a high-pressure gas are presented. Two cardinally different evolution ...scenarios of the nanosecond discharge are addressed: A) in a uniformly volume pre-ionized gas medium and B) in a strongly spatially-nonuniform initially-ionized region near the cathode with a small curvature radius. Relying on the minimal mathematical model of a high-voltage discharge and the description of the physical kinetics of runaway electrons, it is shown using the Boltzmann kinetic equation that the amplitude and duration of a current pulse of runaway electrons and their energy spectrum strongly depend on the mode of gas preionization in the gap. In particular, the other conditions being equal, near-cathode initiation gives rise to the generation of a large group of low-energy runaway electrons within the late current-switching stage. The volume-homogeneous gas preionization can reduce the number of fast electrons by nearly two orders of magnitude compared to the regime without preionization.
Several types of C-band broadband multibeam klystrons (MBKs) are under development at the Institute of Electronics, Chinese Academy of Sciences. These MBKs operate at various C-band frequencies and ...have peak powers of 30-200 kW, average powers of 2-10 kW, and bandwidths of 4%-7%. The design considerations for obtaining high power and wide instantaneous bandwidths and test results for these MBKs are presented in this paper. The main technical problems, including a power sag in the band, high voltage breakdown, and non-operating mode oscillation, are also discussed. Further research work for improving the performance of these MBKs is described
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