The creation of a spatially extended stable DC complex plasma crystal is a big experimental challenge and a topical area of research in the field of dusty plasmas. In this paper we describe a newly built and commissioned dusty plasma experimental (DPEx-II) device at the Institute for Plasma Research. The device can support the formation of large sized Coulomb crystals in a DC glow discharge plasma. The plasma in this L-shaped table-top glass chamber is produced between a circular anode and a long tray shaped cathode. It is characterized with the help of various electrostatic probes over a range of discharge conditions. The dust particles are introduced by a dust dispenser to form a strongly coupled Coulomb crystal in the cathode sheath region. The unique asymmetric electrode configuration minimizes the heating of dust particles and facilitates the formation of crystalline structures with a maximum achievable dimension of 40 cm × 15 cm using this device. A larger crystal has numerous advantages over smaller ones, such as higher structural homogeneity, fewer defects, lower statistical errors due to finite size effects etc. A host of diagnostics tools are provided to characterize the Coulomb crystal. Results of a few initial experiments aimed at demonstrating the technical capabilities of the device and its potential for future dusty plasma research, are reported.

In this work, a self-consistent model has been used to estimate the effective secondary electron emission coefficient (γ_E) of the cathode in typical abnormal dc glow discharge conditions. Using this model, the value of γ_E has been obtained for tungsten (W), copper (Cu), and stainless steel (SS304) cathode samples for argon (Ar) and nitrogen (N₂) discharges. The γ_E for W is lower than the Cu cathode under identical operating conditions. The results show possible dependence of γ_E 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 γ_E. Further, the effect of pressure on γ_E 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 γ_E 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 γ_i. The measurement of the γ_E 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.

In this work, the concept of plasma–metal junction (pm-junction) is presented, in addition to well established physics of pn-junction, metal–semiconductor junction, semiconductor–insulator junction, etc. We demonstrate experimentally the dependence of I-V characteristics on work function of the metal for a pm-junction. The experiment is done using tungsten (W) and stainless steel (SS) having different work functions (${\Phi }_m$ ) of 4.55 eV and 4.30 eV, respectively. The W and SS planar metal probes of identical geometry were exposed to plasma simultaneously, making two distinct pm-junctions. The plasma exposed is identical for both the metals, and therefore, the difference observed in the I-V characteristics of these two pm-junctions is attributed only to the difference in an energy-band diagram of these junctions, owing to respective work function. The proposed concept of the pm-junction may simplify the complex plasma–material interactions in all laboratory plasmas, including cold glow discharges to hot fusion plasmas.

In this paper, we present experimental observation of the transition of a 2D dust crystal to a non-equilibrium solid–liquid phase coexistence state. The experiments have been carried out in an L-shaped dusty plasma experimental device in a DC glow discharge argon plasma environment. Initially, a monolayer crystalline structure is formed, which is later transformed to a two-phase coexistence state using the background neutral pressure as a control parameter. Self-excited horizontal oscillations are found in the center of the monolayer prior to the appearance of the coexistence state. It is observed that a molten center coexists with a solid periphery. Various structural, thermodynamic, and dynamical quantities are used to characterize the phase state. The surface tension at the solid–liquid circular interface is also determined. A detailed parametric study is made to delineate the existence region of such a state. It is found that melting caused at the core is due to the onset of a localized Schweigert instability in the presence of a few stray particles beneath the top layer in that region.

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 (ν_d ≥ 1.3ν_te). The macroscopic arrangement of both positive and negative charges in opposite layers of MADL is attributed to the self-organization scenario. Beyond ν_d ≥ 3ν_te, 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.