Session SR53: Glow Discharge and Dusty/negative-ion Plasma

Unexpectedly high electron temperatures in argon:silane plasmas measured by double probe

Nonthermal plasmas are attractive sources for nanoparticle synthesis. A double probe is utilized to measure the plasma properties of an argon:silane plasma and exhibits good reproducibility despite the chemically reactive environment. However, unexpectedly large electron temperatures, as high as 10 eV, are measured with increasing the silane mole fraction. Here, we discuss that these high electron temperatures can be explained based on the non-Maxwellian electron energy probability function (EEPF) and the fact that a double probe indicates an “apparent” electron temperature, which corresponds to the slope of the EEPF around the probe’s floating potential. We utilize a zero-dimensional global model, in which nanoparticles are modeled as quasi-negative ions with fractional negative average particle charges, to determine the reduced electric field in the plasma. First, using the plasma electronegativity as an independent parameter, the corresponding average particle charges and particle densities are found. Positive ion losses to the particles and reactor walls determine the reduced electric field, which is used to derive the EEPF for different plasma electronegativities. The apparent electron temperatures computed from this model are in good agreement with experimental results.   

Not Participating

A modeling approach to investigate the charge distribution of particles exiting flow-through non-thermal plasmas and the afterglow region is presented. Understanding the effect of plasma parameters, diffusivity of charged species, and reaction rate constants in the resulting particle charge distributions is critical to material synthesis and relevant applications. In this work, ion-flux coefficient models developed using Langevin Dynamics based simulations are incorporated into species transport equations for ions, electrons, and charged particles in the afterglow. Charge predictions from the developed models are compared against measured values in stationary, non-thermal plasmas from past PK-4 campaigns. Experiments of Sharma et al. to probe particle charge distributions are modeled using particle-ion collision rate constant models and the calculated charge fractions are compared with measurements. The comparisons reveal that the plasma concentration and gas temperature in the afterglow region critically influence the particle charge and the predictions are generally in qualitative agreement with the measurements.   

Nitrogen Vibrational Population Measurement in Non-Self-Sustaining DC Discharge Plasma Source

A Non-Self-Sustaining DC (NSS DC) discharge plasma source aiming for efficient nitrogen vibrational excitation by apparent reduced electric field (E/N) control has been developed. This plasma source utilizes a nanosecond pulse plasma generator and a DC power supply. The superimposed DC voltage controls the apparent E/N suitable for efficient nitrogen vibrational excitation. The temporal evolution of nitrogen vibrational distribution during/after the discharge burst is estimated by Laser Raman Spectroscopy. The vibrational excitation is enhanced with the increase of the apparent E/N, and vibrational level more than v=8 is observed. The rotational temperature is estimated with an emission from the nitrogen second positive system. Further details such as the influence of apparent E/N to generation and loss mechanism of vibrationally excited nitrogen will be discussed at the meeting.   

Spectroscopic results on low-current micro-arcs between cadmium-tungsten contacts for intrinsic safety in explosion protection

There exists a spark test apparatus to support the certification of electrical devices to fulfil requirements on explosion protection. This spark test apparatus generates low-current micro-arcs with a length of 150 µm on a time scale of 1000 µs at around 60 mA constant current. The standardised contact material combination consists of a bulk cadmium cathode and a tungsten wire with 100-200 µm diameter as anode. The physics of the contact discharge is not well understood. Radiation is dominated by metallic cadmium. Most reasonable explanation is an a-spot contact bridge explosion that initiates the discharges and that provides initial material with low ionization potential. Results from optical emission spectroscopy are presented to provide deeper insights into the mechanism of this micro-arc by spatial profiles of excitation temperatures.   

Not Participating

Dust grains in a complex plasma, typically of nanometer to several microns in size, collect charge primarily through collisions with ions and electrons. While the grain charge usually fluctuates around an equilibrium value, for particles of size<100nm, the extent of these fluctuations can be relatively significant. This work focuses on modeling the charge fluctuations considering the effect of ion concentration, gas pressure, particle size, and electron temperature. Langevin Dynamics is used to simulate the motion of multiple ions around a non-emitting spherical particle in a periodic domain. The particle, initially neutral, is assumed to lose charge through continuous electron current following the traditional Orbital Motion Limited theory and to gain charge through discrete ion collisions. Ion-ion and particle-ion interactions are assumed to be simple Coulombic. The velocity distribution of the ions is observed to be Maxwellian within our parametric space of interest. The magnitude of the fluctuations relative to the steady state charge will be presented as function of the aforementioned parameters.   

Interaction of nonlinear low frequency structures in a polarized dusty plasma

The polarization force in dusty plasma arises when the dust Debye sphere is polarized due to  interaction with the background plasma particles.The density inhomogeneity is the main cause of the polarization of the Debye sphere. Polarization force modifies the dynamics of dust, which in turns modifies characteristics of nonlinear structures. The interaction of dust-acoustic (DA) shock waves in a magnetized dusty plasma under the influence of nonextensively modified polarization force is investigated. The plasma model consists of negatively charged dust, Maxwellian electrons, nonextensive ions and polarization force. In this investigation, we have derived the expression of polarization force in the presence of nonextensive ions and illustrated the head-on collision between two DA shock waves. The extended Poincar\'{e}-Lighthill-Kuo method is employed to obtain the two sided Korteweg-de Vries-Burgers (KdVB) equations and phase shifts of two shock waves. The trajectories and phase shifts of negative potential dust-acoustic shock waves after collision are examined. The combined effects of various physical parameters such as polarization force, nonextensivity of ions, viscosity of dust, and magnetic field strength on the phase shifts of DA shock waves have been studied.   

Plasma Interactions with Complex Surfaces

Complex material surfaces (i.e., material surfaces with feature sizes from the micron to mm scale) interact with plasmas in unique ways that can lead to significantly improved plasma conditions and material lifetime. Since these complex material surfaces offer a broad design space across material composition and surface geometry, there are many opportunities to optimize surfaces for plasma applications from advanced space propulsion to fusion energy. Efficiently navigating this multi-variable design space requires understanding the mutual plasma-surface interactions (PMI) for a range of material and surface designs. This research has required a combination of experiments, computational models, and theory to accurately describe the coupled behavior of the material, surface and feature erosion, ion-induced and secondary electron yield, sputtering yield, geometric trapping, and plasma response. Our experiments and models have shown that properly designed complex surfaces can significantly lower both sputtering and electron yield, leading to improved plasma performance via reductions to both plasma contamination and plasma cooling. However, since these surface features erode away with time, persistent yield reduction for complex surfaces offers a significant challenge. In response to this challenge, we recently reported in Physical Review Letters the first-ever demonstration of persistent sputtering yield reduction of up to 80% throughout tens of hours of plasma exposure by using “volumetrically complex” metallic foams. An exciting result of this research effort was the discovery of a new “plasma‑infused” regime, which causes significant changes to the plasma-material interactions because the plasma infuses into the surface and transitions the plasma material interactions from superficial to volumetric phenomena. The applicability of this research in aerospace, clean energy, medicine, and related opportunities will be discussed.   

The formation of super rogue waves in space dusty plasmas

The plasma physicists have rejuvenated the research in the dusty plasma after the confirmation of the presence of dust grains in Saturnian rings by Cassini and Voyager space missions. The deformation of the Debye sphere around the dust in nonuniform plasma is known as polarization force. They found that the difference in positive ion density on either side of negative dust leads to the occurrence of the polarization force.   In this theoretical investigation, we have examined the combined effects of nonthermally revamped polarization force on modulational instability MI of dust acoustic waves DAWs and evolution of different kinds of dust acoustic (DA) breathers in a dusty plasma consisting of negatively charged dust as fluid, Maxwellian electrons, and ions obeying Cairns' nonthermal distribution. The nonthermality of ions has considerably altered the strength of polarization force. By employing the multiple-scale perturbation technique, the nonlinear Schrödinger equation NLSE is derived to study modulational MI instability of dust acoustic waves DAWs.  It is remarked that the outcome of the present theoretical investigation may provide physical insight into understanding the role of nonlinear phenomena for the generation of various types of DA breathers in experiments and different regions of space (e.g., the planetary spoke and cometary tails).