V. Lj. Marković

Surface recombination of atoms in a nitrogen afterglow

The surface recombination of nitrogen atoms in afterglow is studied by the time delay method, accompanied by the macrokinetic diffusive model. The method consists of the measurement of the dependence of the mean value of the breakdown time delay on afterglow period td=f(τ) and fitting of the data by the model that was developed. Excited N2(A 3∑+u) nitrogen molecules formed in the surface‐catalyzed recombination on cathode produce secondary electrons. The electrons entering the interelectrode space determine the time delay in electrical breakdown. The time delay method is very efficient in nitrogen atom detection down to a natural radioactivity level. By fitting the calculated curve to the experimental data, we have: (1) shown that the nitrogen atom recombination on the glass container walls is second‐order in N while the recombination on the copper electrode is the first order; (2) determined the value of the surface recombination coefficient for molybdenum glass; (3) determined the combined probability o...

Modelling of charged particle decay in nitrogen afterglow

In this paper we extend the available experimental data for breakdown delay times as a function of the afterglow time to short afterglow times for a pulsed obstructed discharge in nitrogen. Measurements were performed at 6.6 mbar in research grade nitrogen by application of a high-vacuum procedure. The delay times are separated into formative and statistical time lags and their dependence on the afterglow time is discussed. A model is developed that is able to reproduce the breakdown delay time behaviour for short afterglow times which is based on ion density decay through the diffusion. A transition from the effective diffusion coefficient of to the free diffusion of ions is observed at times of the order of 20 ms. The model was applied both in a simple approximate analytical form and in a numerical form with accurate representation of the geometry of the tube. When applied in conjunction with our earlier model of the breakdown delay appropriate for longer afterglow times the theory can predict the behaviour of the breakdown delay times between 0.5 ms and several hours.

Surface recombination in breakdown time delay experiments: Effect of different cathode materials

The late afterglow in nitrogen with iron electrode is studied by the breakdown time delay method, i.e., by measuring the breakdown time delay td as a function of the afterglow time τ. It is proposed that the cause of the secondary electrons initiating the breakdown is the energy of the surface recombination of nitrogen atoms on the iron electrode. The gas-phase and macrokinetic diffusive models are used to describe the experimental breakdown time delay data. By fitting the theoretical curve to the experimental data: (1) it has been confirmed that the recombination on the molybdenum glass is of the second order and the value of the surface recombination coefficient is determined at 4 mbar; (2) it has been shown that the surface recombination on the iron electrode is of the second order, and the effective recombination coefficients are determined; (3) the analytical form of the recombination coefficient as a function of the adsorption characteristics of surfaces and the pressure of the parent gas has been derived. In addition, the orders of surface recombination on the molybdenum-, aluminum-, and gold-plated electrode were determined by the same method.

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.

Efficiency of copper and gold cathode in initiation of secondary emission in nitrogen-filled tube

Abstract The efficiency of secondary electron emission induced by ions and atoms remaining from previous discharge during the initiation of electrical breakdown in a nitrogen-filled tube with bulk copper and gold plated electrodes has been investigated. This investigation was done by measuring the electrical breakdown time delay as a function of afterglow period τ, glow current i g and electrode gap d . The results show that the gold plated cathode is more efficient in secondary electron emission caused by nitrogen atom impact on its surface than that of bulk copper. This phenomenon can be explained by different adsorption ability of nitrogen atoms on the copper and gold surface.

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.