Cathode Boundary Condition Research Articles

Cathodic Protection Simulation of Linear Anode Alongside Coated and Uncoated Pipe

Linear anode-pipe arrangement cathodic protection system with a complete nonlinear cathode boundary condition is analyzed by finite element method numerical simulation to extract the effective parameters for both coated and uncoated pipe. The results show that defect size ratio, linear anode-to-pipe distance, and pipe diameter are fundamental parameters to design an effective cathodic protection system able to guarantee a proper current distribution. Soil resistivity has a significant effect on the current distribution of both coated and uncoated pipe. The results show that the 100 mV potential shift criterion for a single linear anode-pipe arrangement can be ineffective and a two anode system may be required.

The influence of DC stray current on pipeline corrosion

DC stray current can cause severe corrosion on buried pipelines. In this study, firstly, we deduced the equation of DC stray current interference on pipelines. Next, the cathode boundary condition was discretized with pipe elements, and corresponding experiments were designed to validate the mathematical model. Finally, the numerical simulation program BEASY was used to study the corrosion effect of DC stray current that an auxiliary anode bed generated in an impressed current cathodic protection system. The effects of crossing angle, crossing distance, distance of the two pipelines, anode output current, depth, and soil resistivity were investigated. Our results indicate that pipeline crossing substantially affects the corrosion potential of both protected and unprotected pipelines. Pipeline crossing angles, crossing distances, and anode depths, our results suggest, have no significant influence. Decreasing anode output current or soil resistivity reduces pipeline corrosion gradually. A reduction of corrosion also occurs when the distance between two parallel pipelines increases.

Arc-Cathode Coupling in the Modeling of a Conventional DC Plasma Spray Torch

The plasma torch is the basis of the plasma spray process and understanding of the electric arc dynamics within the plasma torch is necessary for better control of torch and process instabilities. Numerical simulation is a useful tool for investigating the effect of the torch geometry and operating parameters on the electric arc characteristics provided that the model of arc dynamics is reliable and the boundary conditions of the computational domain are well founded. However, such a model should also address the intricate transient and 3D interactions between the electrically conducting fluid and electromagnetic, thermal, and acoustics phenomena. Especially, the description of the electrode regions where the electric arc connects with solid material is an important part of a realistic model of the plasma torch operation as the properties of electric arcs at atmospheric pressure depend not only on the arc plasma medium, but also on the electrodes. This paper describes the 3D and time-dependent numerical simulation of a plasma arc and is focused on the cathode boundary conditions. This model was used to investigate the differences in arc characteristics when the cathode is included into the numerical domain and coupled with the arc. The magnetic and thermal coupling between the cathode and arc made it possible to get rid of the current density boundary condition at the cathode tip that is delicate to predetermine. It also allowed a better prediction of the cathode flow jet generated by the pumping action induced by the interaction of the self-magnetic field with the electric current and so it allowed a better description of the dynamics of arc. It should be a necessary step in the development of a fully predictive model of DC plasma torch operation.

Cathode boundary conditions for fluid model discharges on the right-hand branch of the Paschen curve

It is shown that a fluid description of discharges on the right-hand branch of the Paschen curve must take into account a change in the effective secondary emission coefficient γeff of the cathode, which is related to the electron backscattering effect. Simple kinetic expressions for γeff are obtained which indicate that the effective secondary emission is determined by the initial energy of emitted electrons and by the applied field amplitude.

A numerical study of chaos in transferred electron oscillators and amplifiers with the use of an improved computer model

A model of a transferred electron device including reactive circuit elements and realistic boundary conditions is used to study chaos. We find that any simulation of chaos is suspect if the external circuit and cathode boundary conditions are ignored; including them reduces the range of parameters in which chaos occurs. © 1996 John Wiley & Sons, Inc.

Modeling and experimental study of a transferred arc stabilized with argon and flowing in a controlled-atmosphere chamber filled with argon at atmospheric pressure

For a transferred arc with a flat anode working at atmospheric pressure in an argon atmosphere, the influence of the gas injector design close to the cathode tip has been systematically studied for arc currents below 300 A, gas flowrates between 5 and 60 slm, and anode-cathode distances between 10 and 46 mm. Two types of injector configurations hare been studied: a cylindrical one with its wall parallel to the cathode axis and a conical one with the same cone angle as that of the cathode tip. The arc temperature was measured using flit, absolute intensity of ArI and ArII lines. Beside the roltagc and arc current, the losses at the cathode and at the anode were continuously recorded. An elliptic model was used to calculate the flow velocity, the temperature, and the current density close to the cathode and in the arc column. This model was either laminar or turbulent (K - ɛ), with the empirical constants being functions of the Reynolds nunther of turbulence. A cathode sheath with nonequilibrium conditions was used to obtain accurate cathode boundary conditions. Experiments and modeling hart shown the benefits of using conical injectors which constrict drasfically the plasma_ flow and enhance the gas velocity and the current density, thus increasing the heat flux to the anode. With the cylindrical injector, recirculations close to the cathode lip modify deeply its heating and reduce the plasma jet constriction: velocities and temperatures are lower when the recirculation velocity is higher. This results in lower heat fluxes to the anode compared to the conical injector.

The influence of cathode contacts on near-micron InP transferred electron devices

We have performed an experimental investigation of twenty-five 1.5 to 2.0 μm long InP TED's in tunable resonant structures at frequencies in the 75–100 GHz regime, and have simulated their behavior, and that of shorter devices, numerically. We have found that the cathode boundary condition determines the operation of near-micron InP TED's in the following ways: it controls the threshold conditions; it controls the required dc bias power; it affects the transit-time frequency of the device; it influences the electric field profile for an appreciable distance from the cathode.

Reflection amplification in thin layers of n-GaAs

Theoretical expressions are developed for the stability criterion and admittance of a thin coplanar GaAs reflection amplifier. The traveling-wave dispersion relation includes diffusion. The cathode boundary condition includes velocity modulation, injection, and distributed inhomogeneity effects. The admittance includes displacement currents running outside the semiconducting layer. The results show that the second to fifth harmonics may be more nearly unstable than the fundamental. Experiments on a coplanar epitaxial sample yield stable linear net gain at frequencies in the range 2 GHz-8 GHZ.

Noise properties of the injection-limited Gunn diode

The small-signal impedance and noise figure of a Gunn diode with a homogeneous field profile have been calculated, including diffusion. The importance of the nL product, d.c. electric field and cathode boundary condition are examined. Comparison is made with reported measurements at high nL products.

Bulk Negative Differential Conductivity in a Near Cathode Region in n-Type Germanium

Sinusoidal current oscillations up to 0.2 A (rms) are observed in n-type germanium at 77°K. The oscillations occur at a frequency near 1 GHz and for an average threshold electric field of approximately 1 kV/cm. The germanium samples were cut from single crystal n-type germanium with carrier concentration 6×1014/cm3, and were approximately 2.0×0.5×0.5 mm. Using a capacitive probe, the oscillation is found to occur in a region of bulk negative differential conductivity which causes traveling high electric field domains. The domain movement confines itself to a 0.3-mm region starting 0.1 mm beyond the cathode contact. The electric field in this region, at the threshold applied voltage, is approximately 2800 V/cm, the same as that previously measured for bulk negative differential conductivity in germanium. A discussion of the effect of a cathode boundary electric field is included. It is shown that cathode boundary conditions can lead to a stable high electric field at the cathode, which in turn gives rise to traveling high-field domains which are confined to a region near the cathode.

An observed r.f. hysteresis characteristic of particular gunn diodes

Certain Gunn diodes, when mounted in half-wave resonant cavities, exhibited marked hysteresis in r.f. power when the bias voltage was increased above threshold and then reduced. Observations and measurements are described of the dependence on circuit loading of this phenomenon, and the associated I-V curve of the diode. Explanations of the phenomenon are proposed, based on recent investigations into the effect of imperfect cathode boundary conditions on the behaviour of Gunn devices. Some possible practical applications of the hysteresis are suggested, assuming that controlled manufacture of such devices might eventually be possible.

The Gunn effect under imperfect cathode boundary conditions

The usual assumption in the theory of the Gunn effect of a well-behaved cathode is replaced by a more general set of boundary conditions adapted to the treatment of cathode contacts with some interface resistance. The treatment is in terms of a characteristic: that is, the current-field characteristic of the cathode interface itself. For contacts with an interface resistance, the control characteristic lies at least partially below the bulk current-field characteristic. This leads to the primary nucleation of depletion rather than accumulation layers. Depending on the detailed shape of the control characteristic and the location of its crossover point (if any) with the bulk characteristic, a large variety of device behavior can be explained. Examples are Gunn's preoscillation cathode fall, Kennedy's prethreshold amplification behavior, the occurrence of excessive threshold fields for the onset of oscillations, and oscillations with lower amplitude and higher frequency than for well-behaved transit time oscillations, despite good doping homogeneity.

The Effects of the Electrodes on an Electron Avalanche†

ABSTRACT The influence of the electrodes of a uniform field discharge gap on an electron avalanche ore discussed in. this paper. The presence of both electrodes reduces the number of electron/atom collisions in their vicinity thereby increasing the mean electron energy above the equilibrium valuo (∊). There is an associated decrease in the transit time and an increase in the amplification of the electrons across the gap. The spatial variation of the drift velocity, diffusion coefficient and amplification coefficient have been calculated. It has been shown that for a tube voltage greater than lOe that the electrons are no longer influenced by the electrodes when they reach the centre of the gap, and the energy and associated parameters attain equilibrium conditions. The cathode boundary condition has been analysed in detail, and has yielded an integral expression for the loss of electron current by ‘back diffusion’ which is more general than either the Thomson (1028) or Varney (1962) equation. The electron current source is shown to consist of both pole and dipole sources, which whilst escaping from the cathode surface undergoes a change in size, mean energy and energy distribution governed by the electron reflection coefficient from the cathode surface and the initial electron energy distribution.