Plane-parallel Gap Research Articles

Early stages of channel formation in a short-gap breakdown

A description and analysis of the solution of a two-dimensional model for a HV breakdown of a short gap is presented. The model consists of the electron, ion, and excited-atom conservation and Poisson equations and is applied to a plane-parallel gap with an electrode separation of 0.48 mm in helium gas at atmospheric pressure and a temperature of 293 K subjected to an electrical field of 10 kV cm−1. Two-dimensional plots of the charged and excited-particle densities and electric field components are presented and discussed. It is shown that in the first, diffusion-controlled, stage density profiles are close to a Gaussian distribution with an effective radius increasing in time. The subsequent stage is controlled by the space-charge field, causing prominent constriction of the electron density channel. In consequence, a high ionization near the discharge axis results in a virtual narrowing of the ion and excited-atom profiles as well, and the forming conductive chanel exhibits a tendency towards constriction. Calculations were conducted up to a maximum time of t=1139 ns, when maximum electron, ion, and excited-atom densities reached values of 3.1×1010, 3.7×1011, and 2.5×1012 cm−3. Among the ionization processes the direct and Penning interactions are dominant, accounting at average for approximately 70% and 30% of the total at time t=1139 ns; ionization frequencies are substantially affected by space-charge field and vary considerably in time and space near the end of calculations.

Finite-size effects in plane layers of liquid 4He near the λ point

Quantitative measurements of the liquid 4He density temperature dependence are taken near the λ point in a thin plane-parallel gap of thickness d=3.3×10 −5 cm with a well controlled geometry. Evidence is obtained that there are three effects contributing to the average helium density in the gap: (i) the shift of the λ transition temperature in the gap as compared to the bulk helium, (ii) the contribution to the average density from the inhomogeneity of the equilibrium order parameter distribution across the gap below T λ , (iii) the rounding of the logarithmic anomaly of the helium thermal expansion coefficient in the gap due to changes in the spectrum of the order parameter fluctuation in the direction across the gap. A comparison is given with predictions of the phenomenological Ψ-theory of superfluidity near the λ point as well as with those of scaling theory.

Breakdown model of a short plane-parallel gap

A two-dimensional model of the high-voltage breakdown has been developed, consisting of the Poisson equation and the conservation equations for electrons, ions, and excited particles. The model is based on the assumption of a low degree of ionization, so that the transport coefficients of the gas are uniquely determined by the local electric field, i.e., the model is limited to the initial stages of the channel formation. It is applied to a short plane-parallel gap in He at atmospheric pressure. The discharge is started by releasing the cloud of electrons near the cathode, and its sequence is followed up to 1.11 μs. Cathode emission is taken to include that due to ion and metastable impact and the photoeffect. The discharge develops from the initial Townsend-type discharge governed by cathode emission and the direct and Penning volume ionization, and progresses to the space-charge dominated stage. At the moment when the calculations are terminated, the maximum densities have already attained values of 1.6×1010, 2.5×1011, and 1.8×1012 cm−3 for the electrons, ions, and excited particles, respectively. The maximum axial component of the space-charge electric field is approximately 3.3 kV cm−1.

D.c. electrical breakdown of saturated liquid helium at 0.1 MPa in the presence of thermally induced bubbles

Breakdown phenomena initiated by thermally generated bubbles in liquid helium are investigated experimentally. The process is considered to be of importance for the design of electrical insulation of superconducting devices. The results of the investigations show that the boiling processes are significantly affected by the application of an electrostatic field and that the bubble shape varies complicatedly with the magnitude of electric field, heater power for generation of bubble and the arrangement of electrode system. Thus the breakdown voltage of liquid helium in a parallel plane gap depends on those experimental conditions. It is argued that the breakdown strength of saturated liquid helium at atmospheric pressure in the presence of thermal bubbles becomes lower than that of saturated helium vapour at the same pressure.

Neutral heating in glow to spark transition in air and nitrogen

The heating of the neutral molecules in the glow to spark transition in air and nitrogen has been theoretically studied. A hydrodynamic first-order model has been used for electrons (density, momentum) and a second-order model (density, momentum, temperature) for neutral molecules. The interaction between the electron dynamics and the neutral dynamics has been compared in two kinds of discharges: a fast discharge (170 ns) in an overvolted plane parallel gap in nitrogen, and a discharge of longer duration (1.15 mu s) in a positive point-to-plane gap in air. The kinetics of the discharge, which is linked to the possible feedback of the neutral gas on the electron cloud, is mainly a function of the duration of the energy injection. In the case of the fast parallel gap discharge, the background gas plays a passive role; it is an energy absorber, so its temperature increases continuously. In the case of the point-to-plane discharge, lasting longer, the dynamics of the interaction between the electron cloud and the background gas constitutes the fundamental process of the discharge. The gas heating gives rise to a neutral depopulation in the core of the discharge. This leads to an increase of the reduced electric field E/N, thus to an increase of the electron production and thus of the current. In this kind of discharge, the heterogeneity of the neutral gas leads to the breakdown, owing to a constriction of the current core induced by the depopulation mechanisms.

Corona Pulse Charge Transfer at Elevated Frequencies

A technique is described for measuring corona pulse charge transfer per cycle as a function of frequency. It is found that, with a parallel-plane gap geometry employed, the corona charge transfer per cycle exhibits a non-linear decrease with frequency. The result implies that in accelerated life tests on insulating materials and systems, the degradation rate in the presence of corona discharges may not necessarily be a lineraly increasing function of the test frequency.

Analysis of microdischarge threshold conditions between a conducting sphere and plane

Geometrical coefficients for the determination of the potential difference across the grounded plane and an arbitrarily charged conducting sphere in the presence/absence of an electric field are analysed and calculated numerically. An estimation method of microdischarge thesshold conditions in a gap between the free sphere and the plane using the Paschen curve and the coefficients is proposed and is applied to two cases: a free conducting sphere placed in a parallel plane gap in SF 6 gas; water drops dripping from a high voltage conductor and approaching the grounded plane. It is found from the applications that the influence of gas pressure is less pronounced in the case of microdischarge threshold field strength in SF 6 as compared with the breakdown field stregth of the main gap, and the critical radius of the sphere for reducing the breakdown field strength of the main gap is nearly the same as that for the microdischarge threshold. Moreover, the experimental minimum charge on the water drop to produce microdischarge near the grounded plane agrees well with the present estimated value.

A method for prediction of gaseous discharge threshold voltage in the presence of a conducting particle

In this paper, calculations are presented of the field strength near a small conducting sphere floating in a parallel plane gap, and also the force acting on the sphere when the sphere is placed at an arbitrary position in a parallel plane gap. A method is proposed for the prediction of the gaseous discharge threshold voltage on the basis of the calculations. It was found that the breakdown mechanisms in a non-uniform field gap with a free conducting sphere can be classified into four kinds with regard to the particle movement and the occurring position of breakdown. Moreover, the breakdown voltage for each of the four mechanisms has a different pressure dependence. The predicted values were confirmed by using a concentric hemispherical gap and steel spheres, and are in fairly good agreement with experimental values.

Experiments on plane overvolted discharges in N2, and comparison with calculations

In this paper experimental and theoretical results for the development of overvolted discharges in N2 between a plane-parallel gap are described. By suitable initial conditions laterally extended (plane) discharges can be realized instead of the usually observed filamentary streamers. In this discharges which were large in diameter compared with the gap distance, a quantitative comparison of the experimental results with one dimensional calculations is possible. It is shown that the discharges can be described by the well-known build-up theory of instationary discharges but an additional far-reaching electron generating mechanism (gas ionizing radiation) must be included. The development of our discharges is compared with that of the streamer.

A method of preparing thin sheets of AgCl and AgBr by capillarity

The paper describes a method of preparing thin sheets of AgCl and AgBr making use of capillarity. The melt is “sucked up” into a narrow vertical plane-parallel gap between glass slides as a result of excess pressure. After cooling to room temperature the glass slides must be separated from the sheet of AgCl (AgBr) which has been produced. The AgCl sheet has a high-quality surface. When the temperature conditions are suitable the sheet consists of large (several cm2) grains. The dependence of the quality (of the relief) of the grain surface on the crystallographic orientation was determined.