A. Watson

Pulsed Flashover in Vacuum

Experiments on the vacuum flashover of dielectric frusta subjected to 35 and 75 nsec pulses show the strong influence of cone angle on insulation strength. A theory is presented to explain the formative process leading to flashover for one polarity. It is proposed that positive surface charge grows by emission of hot electrons during the application of the electric field. When the surface field intensity is sufficient to draw other electrons into it, they will multiply by secondary emission until the critical breakdown condition is achieved. Flashover is thus initiated by secondary emission, but the formative time lag is governed by thermionic emission of electrons. The experimental data are analyzed and interpreted in the light of this theory.

Emission Processes Accompanying Megavolt Electron Irradiation of Dielectrics

Experiments are described in which thick dielectric samples were irradiated with electrons of about 1 MeV energy. The temporal growth of the external electric field from the trapped charge was punctuated by either total or partial collapses associated with electron emission and followed by pauses before the process repeated itself. Secondary electrons are thought to be heated by the growing internal space charge field until thermionic emission induces a regenerative growth of positive surface charge sufficient to oppose it. Electron emission then ceases until recombination can restore the outside surface of the dielectric to a field‐free condition from which it grows again. Analysis of this model shows that it appropriately describes the observed phenomena.

Electrical current conduction and electrohydrodynamically induced fluid flow in an AW type EHD pump

Abstract An experimental investigation was conducted to study current conduction phenomena and electrohydrodynamic (EHD) induced flow for the air wave (AW) type EHD pump. An AW type EHD pump consists of a corona wire in gas phase and a grounded plate electrode inclined to gas–liquid interface, where mineral oil was used as the working fluid. Time-averaged discharge currents and corona discharge current waveforms in corona wire and plate electrodes under negative polarity were measured at the gas phase corona wire and at the liquid phase grounded plate to study current conduction phenomena. EHD induced liquid flow rate and liquid film thickness were measured as well as gas–liquid interfacial geometry. The results show that no significant electrical charge was carried by an EHD flow of oil beyond the pump sections and only slight current waveform difference was observed between corona discharge current and conduction current through the gas, gas–liquid interface and liquid phases. The effect of applied voltage is discussed in terms of dimensionless parameters in detail.

The influence of crossed electric and magnetic fields upon the breakdown voltage and prebreakdown current in vacuum

An experimental study is presented for the dependence of the breakdown voltage and the prebreakdown current on the crossed electrical and magnetic fields in vacuum. The magnetic field is varied in the range 0–250 G and the gap separation 0.5–3 cm. Bruce profiled copper electrodes which have been prebaked either in vacuum or hydrogen were employed. The breakdown voltage and the prebreakdown current are presented as functions of the magnetic flux density at fixed gap separation as well as at fixed flux density as functions of the gap separation. Comparison is made between these functions and curves calculated from the theory of the magnetic perturbation of the field emission from protrusions covered with semiconducting oxide layers and good agreement is obtained.

A Fundamental Characteristic and Image Analysis of Liquid Flow in an AW Type EHD Pump

Non-intrusive two-phase fluid pumping based on an electrohydrodynamically (EHD) induced flow phenomenon with free liquid surface exposed to gas-phase corona discharges is experimentally investigated. Dielectric liquid flow generated near a corona discharge electrode progresses toward an inclined plate electrode, and then climbs up the surface against the gravitational force for an air-wave (AW) type EHD pump. The AW type EHD pump is operated on ionic wind field along the inclined plate electrode. The pumping performance of time-averaged liquid flow rate and the liquid-phase flow motion are characterized. The liquid flow characteristics related to a dimensionless parameter of corona discharge fields are presented.

Prebreakdown current just prior to breakdown in a vacuum gap as a function of gap separation: A factorially designed experiment

Abstract In a three-factor, two-level, full-factorial experiment the variation of the ultimate prebreakdown current I B has been studied when changes were made in the preparatory treatment of each of two copper electrodes by firing in hydrogen or vacuum. An increase in the electrode area was found to reduce I B for the complete range of gap spacing. Log I B was greatly reduced when the gap was progressively extended from 0.5–1.5 cm, but the reverse was true for the gap range 2.5–3.0 cm. The roles played by pretreatment of each electrode were revealed and the influence of cathode pretreatment alone was found to be consistent with the theory of field emission from oxide coated surfaces. A successful interpretation of the area effect and of the manner in which it is influenced by electrode pretreatment has been achieved by consideration of the discharge of the electrode capacitance during the voltage collapse.

The displacement current from ramped voltage application to dielectric liquids

During ramped voltage application to dielectric liquids the displacement current has been measured and displays two kinds of anomalous behavior. First of all it may be positive or negative according to the type of fluid and it reverses sign at high field strength in transformer oil. Secondly, the displacement current is proportional to a simple power of the applied voltage. This property is shown to be related to the parameters governing the propagation of a Sine-Gordon soliton launched into the liquid by electron injection at the cathode. The first anomaly moreover is believed to be due to a displacement current component due to the annihilation of homocharge or heterocharge polar layers by electron injection at the cathode.

Ramped voltage testing of transformer oil

It has been shown that, compared with dimethyl siloxane, transformer oil has similar reproducible current characteristics as a function of ramped voltage applied up to a high prebreakdown electric stress, where it obeys the equation I varies as V/sup s/. Moreover, at a very high electric stress, the displacement current component apparently reverses, which was not previously observed in silicone oil. This abnormal phenomenon is very dependent upon the conditions. From an earlier study in silicone oil, the average current response to the rising and falling voltage appears on a log-log scale with two linear portions which are sharply divided. Transformer oil has similar characteristics under similar testing conditions, but with increasing voltage amplitude of the applied ramp the slopes of the linear portions are changed. These two lines may become one if the maximum voltage and ramp rate are very high. This is true for copper and aluminum electrodes at differing gap separations.<<ETX>>

The influence of electrode material upon prebreakdown current in viscous dielectric oil

Silicone fluids are now being used as an insulator. Watson,2 Hakim,3 and Hosticka4 have carried out some investigations on this liquid. There is still, however, a great deal that is unclear about the breakdown mechanism under high electric field. It is of importance to know the behavior of the current stress characteristic in the region below breakdown, since the breakdown itself in the liquid dielectric is the ultimate point of the conduction stage.1 It is, therefore, worthwhile characterizing more fully their dielectric performance in this region of electric field. Watson2 has studied the influence of electrode separation upon current conduction in silicone oil and has shown that a linear relationship between the slope of the log voltage vs. log current [d(ℓnV)/d(ℓnI)] ani gap separation exists. He has also shown this phenomenon to be consistent with a model involving Fowler-Nordheim emission which initiates a convective charge transport mechanism with the fluid flowing in a vortex cell enclosing a steady irrotational funnel flow through its core at the cathode. In his interpretation, negative charge will be trapped in the moving core of liquid dielectric, and the space charge is contained laterally by the tendency of the vortex ring to resist stretching by the outward component of the electrostatic Maxwell stress. He reports that the Maxwell stress from this space charge will increase and enlarge the vortex by raising the applied voltage. This suggests that the phenomenon may be influenced by the viscosity of the liquid. The effect of electrode materials was also considered from the viewpoint of their capability to inject charge under stress.

Image analysis of internal liquid flow in an AW type EHD pump

In this work, an experimental investigation of internal liquid flow in a non-intrusive gas-liquid two-phase electrohydrodynamical pump was conducted. The electrode system consists of a corona wire electrode in gas-phase and an inclined plate electrode in a gas-liquid interface. An operation of the DC corona discharge electrohydrodynamically induces gas-liquid two-phase stratified flow, and pumps the liquid out of its tank along the plate electrode surface against the gravitational force. An analysis of the internal liquid flow in the tank is carried out by an image processing technique from the visualized liquid flow patterns. The velocity distribution of the liquid flow inside the tank and along the gas-liquid interface is quantitatively shown.