S. R. Gocić

Memory effects in the afterglow: open questions on long-lived species and the role of surface processes

The memory effect, the phenomenon that some active species survive very long afterglow periods and affect subsequent breakdown, was observed more than 40 years ago. The effects have been observed even over periods of several hours. Attempts to explain the memory effect in nitrogen were mostly based on hypothetical metastables and on the A3Σ state. However, such explanations had to neglect some quenching processes which are known to be very effective under the conditions of the experiments. The explanation based on atoms remaining from the previous discharge and recombining on the cathode to produce initial electrons was shown to be fully consistent with all the experimental data for nitrogen including a wide range of pressures and the addition of oxygen impurities. The memory effect was also shown to be sensitive to the work function of the cathode material. Thus, an attempt was made to use the memory effect as a diagnostic tool to establish the data on the dominant loss of nitrogen atoms from the discharge which is recombination on the walls of the tube. However, a possible role of higher vibrational levels has not been fully addressed, mainly due to the shortage of data. On the other hand, the memory effect which was observed for rare gases cannot be explained on the basis of the standard data unless the presence of molecular impurities is invoked. Another open issue would be the role of charges accumulated on the glass surfaces and whether those may be released to the gas phase. The aim of this paper is to summarize the achievements of the model based on atom recombination and to point out how the breakdown model associated with the memory effect may be completed and how it may be applied in practical discharges.

Study of relaxation kinetics in argon afterglow by the breakdown time delay measurements

In this paper the afterglow kinetics in argon is studied by the breakdown time delay measurements as a function of relaxation time t¯d(τ) (“memory curve”). Measurements were carried out at the pressure of 1.33mbar in a gas tube with gold-plated copper cathode and approximate and exact numerical models are developed to follow metastable and charged particle decay. It was found that the early afterglow kinetics is governed by the charged particle decay up to hundreds of milliseconds, extending from ambipolar to the free diffusion limit. Quenching processes reduce the effective lifetime of metastable states several orders of magnitude below that relevant for the time scale of the observations if realistic abundances and processes are included in the model. Nitrogen atoms originating from impurities and recombining on the cathode surface can determine the breakdown time delay down to that defined by the level of cosmic rays and natural radioactivity.

Metastable and charged particle decay in neon afterglow studied by the breakdown time delay measurements

Memory effect—the long time variation of the electrical breakdown time delay on the relaxation time td¯(τ) in neon—was explained by the Ne(P23) (1s5) metastable state remaining from the preceding glow [Dj. A. Bosan, M. K. Radovic, and Dj. M. Krmpotic, J. Phys. D 19, 2343 (1986)]. However, the authors neglected the quenching processes that reduce the effective lifetime of metastable states several orders of magnitude below that of the memory effect observations. In this paper the time delay measurements were carried out in neon at the pressure of 6.6mbar in a gas tube with gold-plated copper cathode, and the approximate and exact numerical models are developed in order to study the metastable and charged particle decay in afterglow. It was found that the metastable hypothesis completely failed to explain the afterglow kinetics, which is governed by the decay of molecular neon ions and molecular nitrogen ions produced in Ne2+ collisions with nitrogen impurities; i.e., Ne2++N2→N2++2Ne. Charged particle decay...

FLUCTUATIONS AND CORRELATIONS OF THE FORMATIVE AND STATISTICAL TIME DELAY IN NEON

The fluctuations and correlations of the formative tf and statistical time delay ts in neon are studied by electrical breakdown time delay measurements. The measurements were carried out at different preionization levels (afterglow periods) and the Gaussian distributions for the formative time were obtained to about 20?ms in afterglow. The formative time increases linearly with the afterglow period consistently with an exponential decay. After that, double-Gaussian (bi-normal) distributions for the formative time were obtained corresponding to and decay. For the statistical time delay Gaussian, Gauss-exponential and exponential distributions were obtained. It was found that the formative and statistical time delay are dependent variables for ts < tf and their correlations are estimated. The linear correlation coefficient is ? ? 1 at high electron yields (rates of electron production) and ? ? 0 at low electron yields.

New distributions of the statistical time delay of electrical breakdown in nitrogen

Two new distributions of the statistical time delay of electrical breakdown in nitrogen are reported in this paper. The Gaussian and Gauss-exponential distributions of statistical time delay have been obtained on the basis of thousands of time delay measurements on a gas tube with a plane-parallel electrode system. Distributions of the statistical time delay are theoretically founded on binomial distribution for the occurrence of initiating electrons and described by using simple analytical and numerical models. The shapes of distributions depend on the electron yields in the interelectrode space originating from residual states. It is shown that a distribution of the statistical time delay changes from exponential and Gauss-exponential to Gaussian distribution due to the influence of residual ionization.

Determination of correlation coefficient of the statistical and formative time delay in nitrogen

The determination of the correlation coefficient between the statistical and formative time delay of dc electrical breakdown in nitrogen is presented in this paper. Starting from a bivariate normal (Gaussian) distribution of two random variables, the analytical distribution of the electrical breakdown time delay is theoretically founded on correlation of the dependent statistical and formative time delay, contrary to convolution of independent variables. Within the limits, a Gaussian density distribution of the electrical breakdown time delay goes to a Gaussian of the formative time or to a Gaussian of the statistical time delay depending on electron yields (preionization level) in the interelectrode space, while the correlation coefficient is determined in the transition region from tf to ts dominated statistics.

Experiment for measurements of the gas breakdown statistics by ramp voltage pulses

In the first part of this article the electronic automatic system for the measurements of dynamic breakdown voltages Ub with linearly rising (ramp) pulses is presented. It generates the sequence of ramp pulses with subvoltage level Usub≈0 during the relaxation time τ of the tube, and the ramp pulses start from the static breakdown voltage Us, thus enabling the correct study of electrical breakdowns and relaxation in gases. In the second part the measurements in argon with and without a voltage during the off period of the pulse are analyzed. The influence of the subvoltage on the mean value of the breakdown voltage Ub¯ as a function of the rise rate k, on the statistical Ub distributions and on the afterglow kinetics is also discussed.

Influence of the cathode surface conditions on V?A characteristics in low-pressure nitrogen discharge

In this paper we demonstrate and analyse the influence of cathode surface inhomogeneities on the breakdown, volt?ampere (V?A) characteristics and the spatial structure of the low-pressure non-equilibrium discharges. The idea for this work came from the need to explain the non-typical V?A characteristics (positive slope of the characteristics in low-current regime of the discharge) that we observed in a parallel-plate, dc discharge in nitrogen. It was found that the cathode was locally conditioned by the discharge that operated in the constricted glow regime. Spatial inhomogeneity of the cathode surface strongly affected the subsequent operation in the breakdown-Townsend regime where discharge is supposed to be uniform.

New phenomenology of gas breakdown in DC and RF fields

This paper follows a review lecture on the new developments in the field of gas breakdown and low current discharges, usually covered by a form of Townsends theory and phenomenology. It gives an overview of a new approach to identifying which feedback agents provide breakdown, how to model gas discharge conditions and reconcile the results with binary experiments and how to employ that knowledge in modelling gas discharges. The next step is an illustration on how to record volt-ampere characteristics and use them on one hand to obtain the breakdown voltage and, on the other, to identify the regime of operation and model the secondary electron yields. The second aspect of this section concerns understanding the different regimes, their anatomy, how those are generated and how free running oscillations occur. While temporal development is the most useful and interesting part of the new developments, the difficulty of presenting the data in a written form precludes an easy publication and discussion. Thus, we shall only mention some of the results that stem from these measurements. Most micro discharges operate in DC albeit with complex geometries. Thus, parallel plate micro discharge measurements were needed to establish that Townsends theory, with all its recent extensions, is still valid until some very small gaps. We have shown, for example, how a long-path breakdown puts in jeopardy many experimental observations and why a flat left-hand side of the Paschen curve often does not represent good physics. We will also summarize a kinetic representation of the RF breakdown revealing a somewhat more complex picture than the standard model. Finally, we will address briefly the breakdown in radially inhomogeneous conditions and how that affects the measured properties of the discharge. This review has the goal of summarizing (rather than developing details of) the current status of the low-current DC discharges formation and operation as a discipline which, in spite of its very long history, is developing rapidly.

Multi-component non-stationary exponential distributions of the breakdown voltages and time delays in neon ramp breakdown experiments

The concept of physically based distributions used in studies concerning gas electrical breakdowns is introduced in this paper. The non-stationary exponential distribution of the breakdown voltages and time delays with time dependent distribution parameter is theoretically derived based on physical grounds starting from a binomial distribution for electron occurrence in the interelectrode gap. The experimental distributions of breakdown voltages Ub and time delays td are obtained by applying linearly rising (ramp) voltage pulses to the discharge tube with a hard galvanic layer of gold on the cathode and modeled by multi-component non-stationary exponential distribution, as well as by a Weibull distribution for the sake of comparison. In order to fit the experimental data, the multi-component voltage/time dependent distribution parameter YP is introduced, where Y is electron yield (number of generated electrons in the interelectrode gap per second), and P is breakdown probability (the probability of one el...