The influence of frequency of AC component on space charge behaviours in polyethylene under combined AC and DC electric fields
Combined ac and dc voltages may occur in high voltage power transmission system, which bring challenges to the dielectric materials. Generally speaking, space charge is well recognised under high voltage dc conditions but has received little attention under high voltage ac conditions. Under ac electric fields, the formation space charge in the material depends strongly on the frequency. The knowledge of space charge behaviour in dielectric materials under combined voltages is desirable in terms of both experimental and simulation. In this paper, a numerical model based on the bipolar charge injection/transport model is used to analyse characteristics of space charge in polyethylene under combined electric fields at room temperature. The DC component ratio and the overall root mean square (RMS) are kept the same, while the frequency of the AC component is altered from 0.05 Hz to 50 Hz. It has been found that the amount of the accumulated charges reduces with increasing frequency due to the more frequent polarity reversal. Moreover, the time to reach the steady states of charge amount increases with frequency as well.
A new numerical model applied to bipolar charge transport, trapping and recombination under low and high dc voltages
In this paper, we present a new numerical formulation that we have applied to the model for bipolar charge transport, trapping and recombination in polymeric insulators, such as low density polyethylene. The numerical model is based on the high precision Runge–Kutta method for determining mobile and trapped charge local density within the sample, during the application of a dc voltage. In addition, we have applied the finite element method considered as the most general one, which can be applied to any sample shape and boundary conditions. It is applied to resolve Poisson's equation, thus providing the potential and its gradient within the sample and at the dielectric–electrode interfaces. In a first step, the new formulation is validated for the charge dynamic under low dc applied voltage. In a second step, model outputs are analyzed under high dc applied voltage. The appearance of charge packets is revealed for the first time in modelling works although they have long been reported in experimental studies on polyethylene materials. The conditions for the appearance of such packets are discussed.
Long Ceramic Beam Tubes for Accelerator Magnets
The ceramic beam tubes for the fast and abort kickers and the bucker and pinger magnets used at Fermilab must meet a number of exacting requirements. The tubes must be long and sufficiently straight so as not to limit magnet design. They must have wall strength capable of withstanding the atmospheric and mechanical forces encountered during construction and operation. Vacuum tight ceramic to stainless steel transitions must be of reasonable cost; they must also withstand high temperature processing and remain vacuum tight to the 10/sup -9/ Torr range. To remove the possibility of static charge buildup each tube must have a surface coating of indium oxide applied to the inner wall. This thin coating is difficult to achieve and requires the most careful attention to detail. The methods used at Fermilab to achieve these goals are presented in this paper.