Dusty plasma crystals have traditionally been observed and studied in radio frequency (RF) discharge plasmas and their formation in a DC glow discharge plasma remains experimentally challenging. We report the first ever observation of a stable dusty plasma Coulomb crystal in the cathode sheath region of a DC glow discharge plasma. The observations are made in the DPEx device where crystals of mono-disperse Melamine Formaldehyde grains are produced in the background of an Argon plasma. The crystalline nature of the structure is confirmed through a host of measurements that includes the radial pair correlation function, Voronoi diagram, Delaunay Triangulation, the structural order parameter, the dust temperature and the Coulomb coupling parameter. The special features of the DPEx device that permit such a crystal formation are delineated and some principal physical features of the crystal discussed.

We report on experimental observations on the modifications in the propagation characteristics of precursor solitons due to the different shapes and sizes of the object over which the dust fluid flows. The experiments have been performed in a Π shaped Dusty Plasma Experimental (DPEx) device where dusty plasma is created in a DC glow discharge Ar plasma using kaolin particles. A floating copper wire installed radially on the cathode, acts as a charged object in the plasma environment. The flow on the dust fluid is initiated by suddenly lowering the potential of the charged object from grounded potential to close to floating potential. The size (height and width) of the potential hill is then varied by drawing current from the wire through a variable resistance. With a decrease in the height of the potential hill the amplitude, velocity and the number of excited precursor solitons are found to decrease whereas the widths of the solitons are seen to increase. It is found that below a threshold value these solitary waves are not excited and the dust fluid simply flows over the hill. To examine the effect due to the shape of the potential profiles, the wire is replaced by a triangular object. Only trailing wakes are seen to be excited when the dust fluid faces the linearly increasing slope of the potential profile whereas both solitons and wakes get excited when the object is placed with the sharp edge facing the flow. All the experimental findings qualitatively agree with numerical solutions obtained with different source terms in the forced-Korteweg de Vries (f-KdV) model equation.

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.

The formation and melting of a mono-layered charged dust particle crystal in a DC glow discharge Argon plasma is studied. The nature of the melting/formation process is established as a first order phase transition from the nature of the variations in the Coulomb coupling parameter, the dust temperature, the structural order parameter and from the existence of a hysteresis behavior. Our experimental results are distinctly different from existing theoretical predictions for 2D crystals based on the KTHNY mechanism or the Grain boundary induced melting and indicate a novel mechanism that is akin to a fluctuation induced first order phase transition that has not been observed before in complex plasmas. PACS numbers: 52.27.Lw, 52.35.Fp, 52.35.Sb Introduction.-The phase behaviour of two dimensional structures, particularly the nature of their melting transition, has long been a subject of theoretical and experimental interest and also the source of some controversy. Some commonly studied two dimensional structures are molecular monolayers formed by surfactants spread on a water layer [1], electrons on the surface of liquid Helium [2], colloidal suspensions of charged sub-micron spheres [3] and more recently single layer crystalline structures of charged micro-particles (dust) suspended in the electric sheath of a plasma [4][5][6][7]. Two dimensional structures have also been the subject of many computer simulations based on simple theoretical model systems [8][9][10][11]. The most well known theory for two dimensional melting is the one proposed by KTHNY [12] which describes the melting as a two stage transition process with an intermediate hexatic phase. The melting begins by forming dislocations and disclinations in the 2D crystalline structure and as a result the long range translational order breaks down and leads to a hexatic phase. The transitions from the solid to the hexatic and then on to the liquid phase are both continuous in nature constituting second order transitions. Other competing mechanisms of melting consist of the Grain-boundary induced (GBI) melting theory [13,14], the density wave theory [15] and the instability triggered theory [16]. The GBI melting [13,14] proceeds through long arrays of dislocations at the lattice boundaries that drive a first order phase transition from the crystalline phase directly to the liquid phase. Density wave theory [15] deals with the change of entropy and structure factor variations, but it fails to establish the order of the phase transition. Likewise simulation studies to date do not provide a definitive picture of the nature of the 2D melting phase transition and the question still remains open.The advent of dusty plasma crystals in recent times has provided a strong impetus to the experimental study of 2D melting since the process can be well diagnosed using non-perturbative techniques and a number of such studies have addressed this problem [4,6,7,[17][18][19]. In

We present a detailed experimental study of gas flow induced motion of dust particles in a DC glow discharge plasma. The characteristics of the dust dynamics are investigated as a function of the differential gas flow rate, the background neutral pressure, the dust particle size as well as the neutral species of the gas. The experiments have been carried out in the table top Dusty Plasma Experimental (DPEx) device in which a plasma is created between a disk shaped anode and a grounded cathode in a Π-shaped pyrex glass tube. The asymptotic steady state flow velocity of the injected micron sized dust particles is found to increase with an increase of neutral flow velocity and decrease with an increase in the background pressure. Furthermore, this velocity is seen to be independent of the size of the dust particles but decreases with an increase in the mass of the background gas. A simple theoretical model, based on estimates of the various forces acting on the dust particles, is used to elucidate the role of neutrals in the flow dynamics of the dust particles. Our experiments thus provide a detailed microscopic understanding of some of the past phenomenological observations of dust flows in the DPEx device and can prove useful in future experimental implementations of dust flow experiments.