In this work, a self-consistent model has been used to estimate the effective secondary electron emission coefficient (γE) of the cathode in typical abnormal dc glow discharge conditions. Using this model, the value of γE has been obtained for tungsten (W), copper (Cu), and stainless steel (SS304) cathode samples for argon (Ar) and nitrogen (N2) discharges. The γE for W is lower than the Cu cathode under identical operating conditions. The results show possible dependence of γE on the Fermi energy of the cathode material since it influences the probability of electron to be emitted by the incident ion. In addition to this, we found, significant contribution of cathode directed species other than ion to γE. Further, the effect of pressure on γE for the N2 discharge has been investigated in the pressure range of 0.5 mbar to 2.0 mbar and its value increases from 0.38 to 0.47 with pressure for the SS304 cathode. The knowledge of γE successfully explains the governing processes in abnormal glow discharge plasma that cannot be explained by the value of the ion induced secondary electron emission coefficient γi. The measurement of the γE value of the cathode material in typical abnormal glow discharge plasma conditions presents possibilities of exciting advancement in various applications by accurate estimation of discharge characteristics including flux of species, fraction of power carried by ions and electrons, plasma density, discharge current density, etc.
We present direct evidence for the existence of self-organized critical behavior in cold plasma. A multiple anodic double layer structure generated in a double discharge plasma setup shows critical behavior for the anode bias above a threshold value. Analysis of the floating potential fluctuations reveals the existence of long-range time correlations and power law behavior in the tail of the probability distribution function of the fluctuations. The measured Hurst exponent and the power law tail in the rank function are strong indication of the self-organized critical behavior of the system and hence provide a condition under which complexities arise in cold plasma.
The role of self-organized criticality (SOC) in the transformation of multiple anodic double layers (MADLs) from the stable to turbulent regime has been investigated experimentally as the system approaches towards critical behavior. The experiment was performed in a modified glow discharge plasma setup, and the initial stable state of MADL comprising three concentric perceptible layers was produced when the drift velocity of electrons towards the anode exceeds the electron thermal velocity (νd ≥ 1.3νte). The macroscopic arrangement of both positive and negative charges in opposite layers of MADL is attributed to the self-organization scenario. Beyond νd ≥ 3νte, MADL begins to collapse and approaches critical and supercritical states through layer reduction which continue till the last remaining layer of the double layer is transformed into a highly unstable radiant anode glow. The avalanche resulting from the collapse of MADL leads to the rise of turbulence in the system. Long-range correlations, a key signature of SOC, have been explored in the turbulent floating potential fluctuations using the rescaled-range analysis technique. The result shows that the existence of the self-similarity regime with self-similarity parameter H varies between 0.55 and 0.91 for time lags longer than the decorrelation time. The power law tail in the rank function, slowly decaying tail of the autocorrelation function, and 1/f behavior of the power spectra of the fluctuations are consistent with the fact that SOC plays a conclusive role in the transformation of MADL from the stable to turbulent regime. Since the existence of SOC gives a measure of complexity in the system, the result provides the condition under which complexity arises in cold plasma.
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