Session JO07: Low Temperature Plasmas: Sources and modeling

Live

In this work, we compare the gas-phase chemistry produced in the ablation plume of solid boron and boron-nitride targets. The targets are ablated by a nanosecond pulsed laser at sub-atmospheric pressures of nitrogen and helium gases as well at pressure 100 mTorr. Optical emission spectroscopy is used to identify the excited species in the ablation plume. Evaluation of chemical composition in the plasma plume revealed that for both boron-rich targets, emission from BN molecules is always observed in nitrogen-rich environments. Presence of BN molecules also detected when ablating a boron nitride target in helium gas and at pressure of 100 mTorr. Furthermore, the ablation of BN target features emission of B$_{\mathrm{2}}$N molecules, regardless of the pressure and surrounding gas. These results suggest that the ablation of the BN target is more favorable for the generation of complex molecules containing boron and nitrogen species and possibly hint that BN is also more favorable feedstock for high-yield BN nanomaterial synthesis. Plasma parameters such as electron temperature (peak value of 1.3 eV) and density (peak value of 2x10$^{\mathrm{18}}$ cm$^{\mathrm{-3}})$ were also investigated in this work in order to discuss the chemical dynamics in the plume.   

Live

Recently acquired high-resolution ICCD images of ns laser ablation plumes suggest a strong correlation between the formation of internal plume structures and the type of material being ablated. This observation brings up the possibility of inferring the composition of an ablation plasma based on the observed plume dynamics. However, the details of this relation are currently not well understood. In this work, we attempt to explore this correlation using a 2D reactive compressible fluid model to study the dependence of internal plume structure formation on the ablation material. Spatio-temporal emission maps and plume expansion velocities from experimental measurements are compared with the model predictions, and the impact of plasma and gas phase material parameters on the plume dynamics is explored. This effort constitutes a continued development towards a predictive model of ablation plume dynamics and chemistry for various materials in extreme environments.   

Live

Micro- and nanoscale electronic device design is often driven by space charge, both positively and negatively. A higher space-charge limit allows more current to flow through a vacuum diode, while lower limits may be sought to avoid significant current flow. While space-charge limited emission (SCLE) is well understood for a one-dimensional planar diode, a general solution independent of geometry has only recently been derived using variational calculus [1]. One may also vary SCLE by introducing an external magnetic field perpendicular to the electric field. The emitted electrons travel in curved paths, spending longer in the gap, contributing to increased space-charge; for magnetic fields above the Hull cutoff, electrons no longer reach the anode, but travel in cycloidal orbits back to the cathode [2]. This paper extends our previous work with SCLE for general geometry to the crossed-field case, looking specifically at planar and cylindrical geometries. [1] A. M. Darr and A. L. Garner, Appl. Phys. Lett. 115, 054101 (2019). [2] P. J. Christenson and Y. Y. Lau, Phys. Plasmas 1, 3725-3727 (1994). This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-19-1-010.   

Live

In order to understand nanomaterial synthesis in carbon arc, we performed atomistic simulations, thermodynamic and fluid dynamics (CFD) modeling of complex processes occurring in the arc. We performed validated arc modeling to predict how the arc can provide feedstock for nanomaterial synthesis. A complicated setup was implemented into ANSYS and included many complex effects: radiation, sheath boundary conditions near emitting electrodes, ablation/deposition of carbon on electrodes, and coupling of the thermal transport through electrodes. In addition, we developed several analytic models for key phenomena: 1) nonlinear dependence of the ablation rate as a function of arc current and interelectrode gap, 2) anode spot formation, in which the arc channel is constricted near anode, 3) radial narrow arc jet emanated from the arc.   

Live

Multiple applications, including combustion, flow control, and medicine, have leveraged nanosecond pulsed plasma (NPP) discharges to generate plasma generated reactive species (PGRS). While a one moment fluid model has been developed to examine NPP discharges, a detailed assessment of the effect of electric pulse (EP) parameters, such as electric field intensity and pulse shape, on PGRS formation remains incompletely characterized. Here, we assess the influence of EP conditions on the electric potential in the gap and PGRS by coupling a quasi-one-dimensional model for a parallel plate geometry with a Boltzmann solver (BOLSIG$+)$ to improve plasma species calculations [1]. We consider a single discharge for a low-pressure gas (3 Torr) using a five-species model for argon [2]. We examine variations in PGRS for changes and the voltage and pulse width of the applied EP. By fixing the energy delivered by the EP, we further examine the implications of pulse width on species generation and sheath formation. Implications of pulsed power parameters will be discussed. 1. G. J. M. Hagelaar and L. C. Pitchford, Plasma Sources Sci. Technol$.$ 14, 722--733 (2005). 2. T. Piskin, V. Podolsky, S. Macheret, and J. Poggie, J. Phys. D. Appl. Phys. 52, 304002 (2019).   

Live

In atmospheric pressure plasma jets (APPJs) ignited in helium, electrons can collide with ambient helium atoms to form metastable states with an energy well above the ground state. These helium metastables can be of considerable significance, especially in discharges operating in the absence of a molecular precursor [1]. To examine the production and importance of helium metastables in a micro-scale APPJ, a COST Reference Jet [2] was investigated using absolutely calibrated optical emission spectroscopy (OES). The emission of two rotational bands of nitrogen -- N$_{\mathrm{2}}$(C--B) and N$_{\mathrm{2}}^{\mathrm{+}}$(B--X) -- was used to determine the relative contribution of helium metastables for a variety of discharge parameters. Additionally, small amounts of a molecular precursor were added to the feed gas to identify a transition point where the role of helium metastables could be considered negligible. [1] K Niemi et al 2011 Plasma Sources Sci. Technol. 20 055005 [2] J Golda et al 2016 J. Phys. D: Appl. Phys. 49 084003   

On Demand

A particular challenge for low temperature plasma (LTP) research is the diversity of parameter space and conditions. For plasma systems where the densities are not low, the kinetic theory is time-consuming and becomes unrealistic. In this regime, the particle-in-Cell (PIC) method is appropriate where the evolution of a particle system at every time step consists of an Eulerian stage and a Lagrangian stage. The PIC method can deal with complex geometries and large distortions in the field. The PIC solver, Starfish, is a two-dimensional plasma and gas simulation code operating on structured 2D/axisymmetric Cartesian or body fitted stretched meshes. The purpose of this study is to use the Starfish Plasma Simulation Code for modeling a large area (30 cm x 30 cm) microwave plasma chemical vapor deposition system. With the exact geometries and experimental results being provided, numerical simulations of the applications are ongoing. The study is beneficial to many current hypersonic challenges due to the limitations of materials degradation under extreme conditions (i.e. thermal, electrical, magnetic, acoustic, shear, or pressure fields in hypersonic conditions).   

Live

An atmospheric pressure arc discharge is the simplest method for the industrial-scale production of carbon nanoparticles. In this work, we developed a self-consistent model to study the properties of short carbon arcs. The model accounts for the transport of heat and current in both plasma and electrodes as well as multiple surface processes including sheath, carbon ablation and deposition, thermionic emission, and radiation. These processes are similar to those in tokamak diverter, thus ablating arc can be used as a benchmark for diverter codes. Results show that the arc is constricted at the electrodes, leading to the spot formation. We conclude that the anode spot formation is not caused by a plasma instability, as commonly believed in case of other constricted discharges, but occurs due to the highly nonlinear nature of heat balance in the anode. We prove this point with an analytical model and by showing that changing the anode heat conduction affects spot size. We also show that the spot size increases with the arc current. This behavior was confirmed in our experiments. Due to the anode spot formation, a large gradient of carbon gas density occurs near the anode, driving a portion of the ablated carbon back to the anode. This consequently reduces the total ablation rate.   

Live

Radiation enhancement phenomenon of a thin inhomogeneous plasma layer covered metal antenna can be observed by optimizing the plasma density and electron temperature for giving antenna--plasma layer configuration. Based on the collisional plasma assumption, theoretical results obtained by solving the eigenmode dispersion relation of symmetric waves propagating along the antenna--plasma layer interface indicate that, when the wave frequency is 1 GHz, the radius of the metal antenna is 3 times of the plasma skin depth, and the thickness of the sheath is about one tenth of the thickness of the main plasma layer, there is a ring-like radiation enhancement phenomenon where the plasma frequency is about 1.4 times of the wave frequency. This radiation enhancement phenomenon may induced by the impedance resonance of the antenna-plasma layer configuration, and which may have great potential in radio frequency antenna communication.   

Quantum chemistry modeling of B$_{\mathrm{n}}$N$_{\mathrm{2}}$ clusters formation in reaction between small B$_{\mathrm{n}}$ clusters (n$=$2-4) and N$_{\mathrm{2}}$ molecule for boron nitride nanotubes synthesis.

We study precursors for the boron nitride nanotubes (BNNTs) formation that can effectively convert molecular nitrogen into boron nitride. The data have been obtained by using a DFT (density function theory) method with unrestricted WB97X-D functional with 6-311$+$G(2dp) basis set. Using quantum chemistry methods, we discovered that formation of linear BNBN, cyclo-BNBNB and cyclo-BNBNB$_{\mathrm{2}}$ species from B$_{\mathrm{n}}$ (n$=$2-4) and N$_{\mathrm{2}}$ proceeds through sequential steps, and activation barrier of the rate-limited step is near 1.1 eV for all considered B$_{\mathrm{n}}$ clusters. On the other hand, the highest barrier towards dissociation of BNBN, cyclo-BNBNB and cyclo-BNBNB$_{\mathrm{2}}$ species on B$_{\mathrm{n}}$ and N$_{\mathrm{2}}$ increases and equals 2.5, 4.8 and 5.6 eV respectively. Thus, based on our calculations we can conclude that N$_{\mathrm{2}}$ is able to react with small B$_{\mathrm{n}}$ clusters producing new B$_{\mathrm{n}}$N$_{\mathrm{2}}$ clusters with BN bonds, and these B$_{\mathrm{n}}$N$_{\mathrm{2}}$ clusters can be accumulated in the gas phase even at high temperature providing contribution in the BNNTs growth.