Session TO6: Low Temperature and Technology

Abstract Withdrawn

Nitrogen plasmas have many applications in materials processing. They can be used harden tool steel samples as well as assist in the growth of many important semiconductor nitride films. The detailed composition of nitrogen plasmas in RF ICP (Inductively Coupled Plasma) discharges is therefore a topic of great importance in applied plasma physics. Here we report on studies of a nitrogen RF ICP discharge using Optical Emission (OES) Spectroscopy combined with Langmuir probe characterization of the discharge. We study the discharge at two different locations in the plasma chamber, one closer to and one further from the inductive coil and gas injection ring. The implications of these discharge properties for nitrogen plasma material processing applications are further explored using nitrogen plasma ion implantation into a variety of metallic and semiconductor targets.   

Measurement of low temperature plasma properties using non-invasive impedance measurements

A plasma discharge can be modeled electrically as a combination of capacitors, resistors, and inductors. The plasma, much like an RLC circuit, will have resonances at particular frequencies. The location in frequency space of these resonances provides information about the plasma parameters. These resonances can be detected using impedance measurements, where the AC impedance of the plasma is measured by sweeping the frequency of an AC voltage applied to a sensor and determining the magnitude and phase of the measured current. In this work, an electrode used to sustain a glow discharge is also used as an impedance probe. The novelty of this method is that insertion of a physical probe, which can introduce perturbation and/or contamination, is not necessary. This non-invasive impedance probe method is used to measure the plasma discharge density in various regimes of plasma operation. Experimental results are compared to the basic circuit model results. The potential applications of this diagnostic method and regimes over which this measurement method is valid will be discussed.   

Influence of Neutral Pressure on Instability Enhanced Friction and Ion Velocities at the Sheath Edge of Two-Ion-Species Plasmas

The speed at which ions enter the sheath is a critical parameter to model in low temperature plasmas. For two ion species plasmas, the Instability-Enhanced Friction (IEF) theory [1] predicts the ions' sheath-edge flow speeds based upon the presence of ion-ion two stream instabilities in the presheath which cause an enhanced friction between the ions merging their velocities up-until the sheath-edge. Here we will report two contributions advancing the IEF theory. First, we have directly calculated the ion-ion friction force in the presheath due to the two stream instability from new Particle-in-Cell Monte-Carlo Collision (PIC-MCC) simulations. This result directly links the merging of the ion velocities with the enhanced wave-particle scattering due to the ion-ion two stream instability. Our second result was that the two stream instability persisted up to 10's of mTorr as we varied the neutral pressure in the simulations. Adding an ion-neutral collision operator into the IEF theory resulted in accurate predictions for the ion sheath-edge speeds over a range of neutral pressures. This result could impact plasma based manufacturing designs which can operate in the 10's of mTorr. \\ $[1]$ S. D. Baalrud, C. C. Hegna, and J. D. Callen, PRL 103, 205002 (2009)   

Gas Composition and Input Waveform Effects on Alpha-to-Gamma Transitions in CCRF Plasma

Optimizing the production of carbon nanotubes, graphene and graphitic nanopetals is of great interest to the engineering community owing to their excellent electrical, thermal and structural properties. Roll-to-roll radio frequency chemical vapor deposition (RFCVD) uses capacitively coupled radio frequency (CCRF) plasma to grow carbon nanostructures from radical precursors generated in the plasma. The transition of the plasma from the $\alpha$ mode dominated by impact ionization in the plasma bulk and displacement current in the sheaths, to the $\gamma$ mode dominated by secondary electron emission and conduction current in the sheaths, controls the heat and precursor fluxes onto the growth substrate. Thus, characterizing these transitions under various input conditions is imperative to the optimization of the deposition. In the current work, we model CCRF plasma using the Poisson equation, hydrodynamic model with drift-diffusion approximation for electrons, modified Maxwell-Stefan equations, and the heat conduction equation. The effect of input voltage, pressure, frequency and waveform on $\alpha$-to-$\gamma$ transitions are studied for argon and hydrogen discharges. The differences in properties between monatomic and diatomic gases are explored for square wave voltage input.   

Characterizing Hypervelocity Impact Plasma Through Experiments and Simulations

Hypervelocity micro particles, including meteoroids and space debris with masses \textless 1 ng, routinely impact spacecraft and create dense plasma that expands at the isothermal sound speed. This plasma, with a charge separation commensurate with different species mobilities, can produce a strong electromagnetic pulse (EMP) with a broad frequency spectrum. Subsequent plasma oscillations resulting from instabilities can also emit significant power and may be responsible for many reported satellite anomalies. We present theory and recent results from ground-based impact tests aimed at characterizing hypervelocity impact plasma. We also show results from particle-in-cell (PIC) and computational fluid dynamics (CFD) simulations that allow us to extend to regimes not currently possible with ground-based technology. We show that significant impact-produced radio frequency (RF) emissions occurred in frequencies ranging from VHF through L-band and that these emissions were highly correlated with fast (\textgreater 20 km/s) impacts that produced a fully ionized plasma.   

Latest results and developments from the Hybrid Illinois Device for Research and Applications

The Hybrid Illinois Device for Research and Applications (HIDRA) is a five-period, $l =$ 2, $m =$ 5, toroidal fusion device operated at the University of Illinois at Urbana-Champaign (UIUC). It has a major radius $R_{0\thinspace }=$ 0.72 m and minor radius $a_{\thinspace }=$ 0.19 m. Initial heating is achieved with 2.45 GHz electron cyclotron resonance heating (ECRH) at an on-axis magnetic field of $B_{0\thinspace }=$ 0.087 T which can go as high as $B_{0\thinspace }=$ 0.5 T. HIDRA will mainly be used as a classical stellarator, but can also run as a tokamak. This allows for both steady-state and transient regime operations. Experiments on HIDRA will primarily tackle the issue of plasma-material interactions (PMI) in fusion, and focus on developing innovative plasma facing component (PFC) technologies. Currently, research on flowing liquid lithium PFCs meant to be tested inside the machine in real-time operation, is being carried on. The first experiments run on HIDRA started in early 2016 in the low field region. Now, HIDRA is also capable of running in the high field zone, allowing for more interesting experiments and meaningful outcomes. Here, we present some of the initial results coming from the machine.   

Analysis of high-speed rotating flow inside gas centrifuge casing

The generalized analytical model for the radial boundary layer inside the gas centrifuge casing in which the inner cylinder is rotating at a constant angular velocity $\Omega $\textit{\textunderscore i} while the outer one is stationary, is formulated for studying the secondary gas flow field due to wall thermal forcing, inflow/outflow of light gas along the boundaries, as well as due to the combination of the above two external forcing. The analytical model includes the sixth order differential equation for the radial boundary layer at the cylindrical curved surface in terms of master potential ($\chi )$, which is derived from the equations of motion in an axisymmetric $(r - z)$ plane. The linearization approximation is used, where the equations of motion are truncated at linear order in the velocity and pressure disturbances to the base flow, which is a solid-body rotation. Additional approximations in the analytical model include constant temperature in the base state (isothermal compressible Couette flow), high aspect ratio (length is large compared to the annular gap), high Reynolds number, but there is no limitation on the Mach number. The discrete eigenvalues and eigenfunctions of the linear operators (sixth-order in the radial direction for the generalized analytical equation) are obtained. The solutions for the secondary flow is determined in terms of these eigenvalues and eigenfunctions. These solutions are compared with direct simulation Monte Carlo (DSMC) simulations and found excellent agreement (with a difference of less than 15{\%}) between the predictions of the analytical model and the DSMC simulations, provided the boundary conditions in the analytical model are accurately specified.   

A model of early formation of uranium molecular oxides in laser-ablated plasmas

An important problem within the field of nuclear forensics is fractionation: the formation of post-detonation nuclear debris whose composition does not reflect that of the source weapon. We are investigating uranium fractionation in rapidly cooling plasma using a combined experimental and modeling approach. In particular, we use laser ablation of uranium metal samples to produce a low-temperature plasma with physical conditions similar to a condensing nuclear fireball. Here we present a first plasma-chemistry model of uranium molecular species formation during the early stage of laser ablated plasma evolution in atmospheric oxygen. The system is simulated using a global kinetic model with rate coefficients calculated according to literature data and the application of reaction rate theory. The model allows for a detailed analysis of the evolution of key uranium molecular species and represents the first step in producing a uranium fireball model that is kinetically validated against spatially and temporally resolved spectroscopy measurements.   

Developing the Polynomial Expressions for Fields in the ITER Tokamak

The two most important problems to be solved in the development of working nuclear fusion power plants are: sustained partial ignition and turbulence. These two phenomena are the subject of research and investigation through the development of analytic functions and computational models. Ansatz development through Gaussian wave-function approximations, dielectric quark models, field solutions using new elliptic functions, and better descriptions of the polynomials of the superconducting current loops are the critical theoretical developments that need to be improved. Euler-Lagrange equations of motion in addition to geodesic formulations generate the particle model which should correspond to the Dirac dispersive scattering coefficient calculations and the fluid plasma model. Feynman-Hellman formalism and Heaviside step functional forms are introduced to the fusion equations to produce simple expressions for the kinetic energy and loop currents. Conclusively, a polynomial description of the current loops, the Biot-Savart field, and the Lagrangian must be uncovered before there can be an adequate computational and iterative model of the thermonuclear plasma.   

Particle-In-Cell Simulations of a Thermionic Converter

Simulations of thermionic converters are presented where cesium is used as a work function reducing agent in a nano-fabricated triode configuration. The cathode and anode are spaced on the order of 100 $\mu$m, and the grid structure has features on the micron scale near the anode. The hot side is operated near 1600 K, the cold side near 600 K, and the converter has the potential to convert heat to DC electrical current upwards of 20\% efficiency. Affordable and robust thermionic converters have the potential to displace century old mechanical engines and turbines as a primary means of electrical power generation in the near future. High efficiency converters that operate at a small scale could be used to generate power locally and alleviate the need for large scale power transmission systems. Electron and negative cesium ion back emission from the anode are considered, as well as device longevity and fabrication feasibility.   

A CASPER STEM Oriented Educational Intervention based on Microgravity using a 1.5 sec Drop Tower

The CASPER educational research group strives to contribute to the effort of increasing students' interest in and preparation for STEM careers by cultivating partnerships between educators, industry, and state educational institutions while applying the latest innovations and available tools to curriculum development. ~ To this end, CASPER has brought together a group of educational researchers and curriculum designers to produce the CASPER Microgravity Investigators educational intervention, which is coordinated to the 21$^{\mathrm{st}}$ Century Learning framework. Material for this educational intervention is based on a newly constructed 1.5 s drop tower at Baylor University and operated by the Center.~ In this presentation we present the details of the intervention as well as the physics and STEM material which are incorporated into microgravity experiments.