Session QI3: Invited: Low Temperature and Basic Plasma Physics

Kinetic Modeling of Non-Equilibrium Plasmas for Modern Applications

We have studied several non-equilibrium plasma devices where kinetic effects determine plasma self-organization: neutralization of ion beams and electron cloud effects in accelerators, negative hydrogen Ion Sources, ExB discharges (plasma switch and Penning discharge), thermoelectric converters. Neutralization of positive ion beam space-charge by electrons is important for many accelerator applications, i.e., heavy ion inertial fusion, and ion beam-based surface engineering. Past experimental studies showed poorer ion beam neutralization by electron emitting filaments, compared with neutralization by plasmas. Now researchers have found that reduced neutralization may be related to the generation of electrostatic solitary waves (ESWs) during the neutralization process, as the ion beam passes through the electron-emitting filaments. [1]. We have also developed a Global Model Code for Negative Hydrogen Ion Sources, GMNIS [2]. The codes ultimate goal is to aid developing optimized negative ion beams for ITER. The code solves volume-averaged equations: continuity for plasma species and electron energy equation for the electron temperature, and include more than 1000 volumetric and surface reactions for interactions of electrons, ground-state atomic and molecular hydrogen, molecular ions and atomic ions, negative ions, 14 vibrationally-excited states of molecular hydrogen, and excited atoms. Results of the code are benchmarked against another code [2]. Convenient analytical solution for vibrational spectrum of H2 was also derived. We performed particle-in-cell simulations and developed analytical model that can explain experimentally observed Pashen curve [3]. We have also preformed studies of rotating spoke in a Penning discharge and proposed analytical scaling law for its frequency [4]. Efficient thermal electric converter is proposed in Ref.[5]. [1] C. Lan and I. D. Kaganovich, Phys. Plasmas 26, 050704 (2019). [2] W. Yang, et al.", Phys. Plasmas 25, 113509 (2018). [3] Liang Xu, et al, Plasma Sources Sci. Technol. 27, 104004 (2018). [4] Andrew T. Powis, et al., Phys. Plasmas 25, 072110 (2018). [5] A. S. Mustafaev, et al, Journal of Applied Physics 124, 123304 (2018).   

Microparticle Dynamics in the Presence of Externally Imposed, Ordered Structures in a Magnetized Low-Temperature Plasma

A complex, or dusty, plasma is a four-component plasma comprised of electrons, ions, neutral gas atoms, and microparticles of several tens of nanometers to tens of micrometers in diameter. In laboratory settings these microparticles can collect hundreds to thousands of elementary charges on their surface. However, because the mass of the microparticles is much larger than the mass of the electrons and ions, the charge-to-mass ratio of the microparticles is very small. Because of this small charge-to-mass ratio, large magnetic fields (B $\ge $ 1 T) are required in order to observe the direct effect of the magnetic field on the microparticles. The Magnetized Dusty Plasma Experiment (MDPX) at Auburn University is a 4 Tesla class superconducting magnet system that is used to study dusty plasmas in these extreme magnetic field environments. One of the early discoveries on MDPX was the observation of imposed, ordered structures within the dust cloud. This is because the microparticles followed a patterned structure of a conducting wire mesh embedded in one of the bounding electrodes, a behavior which was strikingly different from the typical self-organization of a dusty plasma crystal. This presentation will summarize recent studies of this ordering phenomenon that use a two-dimensional parameter space to describe the particle organization and confinement of the imposed, ordered structures and to identify the experimental conditions at which these structures are observed~[1]. New results of dust behavior in experiments which use a large grid-like electrode will also be discussed. This new electrode allows for various spatially resolved probe diagnostics to measure properties such as electric fields, temperatures, and densities beneath the electrode which give a more detailed understanding of the imposed, ordered structure phenomena. Finally, preliminary PIC and fluid simulations of these plasma conditions will be shown. This work is a collaboration of the author with Edward Thomas, Jr. (Auburn), Lenaic Cou\"{e}del (Univ. of Saskatchewan), Khare Avinash (Univ. of Delhi), Robert Merlino (Univ. of Iowa), Marlene Rosenberg (UCSD), and members of the Magnetized Plasma Research Laboratory at Auburn University. [1] T. Hall, E. Thomas, K. Avinash, R. Merlino, and M. Rosenberg, Phys. Plasmas 25, 103702 (2018).   

Three-dimensional measurements of fundamental plasma parameters in pulsed ICP operation

Radio frequency inductively coupled plasma sources are widely used in low temperature industrial processing. Having reached a limit in process improvement available with steady state plasmas, the semiconductor industry is turning to temporal modulation of the rf generators in these sources. We performed 3D measurements of fundamental plasma parameters in a pulsed Argon plasma in a modified industrial etch tool. The ICP and bias RF generators are independently pulsed at arbitrary repetition rates and duty cycles. This work reports spatially and temporally resolved probe measurements of fundamental plasma parameters. RF antenna current is switched on in less than 50 micro-seconds. Initially the peak of the electron temperature appears under the antenna, then moves towards the center of the machine. The induced plasma current is primarily concentrated directly under the antenna. At a 1 kHz rep rate, the plasma does not have time to reach a steady state. Nevertheless, the density is always peaked at the center. We also report profile dynamics of pulsed operation at different repetition rates and duty cycle. Results will be presented with and without interleaved sequenced bias voltage.   

Reduction of the Plasma Electron Temperature to the Emitted Electron Temperature Near Thermionic Surfaces with Inverse Sheaths

Understanding the effects of thermionic surfaces on plasmas is important in many research areas. Examples include tokamak divertors, emissive probes, Hall thrusters, hypersonic vehicles, the Large Plasma Device, and arcs. The conventional view from Langmuir's era through modern times was that emitting sheaths are classical or space-charge limited (SCL). Classical and SCL sheath models [1,2] assume that plasma electrons near the surface are confined and can have any temperature, independent of the thermionic temperature. However in recent works we showed that under strong enough emission, conventional theory breaks down and the sheath inverts. The inverse regime is unique because no plasma electrons are confined. Also, because thermoelectrons are not accelerated by an inverse sheath, they flood the quasineutral region with electrons at the thermionic temperature Temit. This forces Te near the surface to equal Temit (generally below 0.3eV) regardless of how hot the upstream plasma is [3]. We confirm the extreme cooling effect of inverse sheaths in continuum kinetic simulations by comparing classical, SCL and inverse sheath regimes in plasmas with equivalent hot upstream temperatures. One potential application is that thermionic divertor plates with inverse sheaths could constrain the target plasma to a sub-eV temperature sufficient for detachment and thereby mitigate the plasma-wall interaction [3]. Inverse sheath detachment may have advantages over conventional detachment scenarios that rely on injecting neutrals, which are liable to compromise the core plasma.~Other recent advances, experiments, and open questions regarding inverse sheath effects in plasma physics will be reviewed with applications to hot cathode devices, emissive probe measurements, and negative ion sources. [1] S. Takamura et al., Contrib. Plasma Phys. 44, 126 (2004). [2] J.P. Sheehan et al., PRL 111, 075002 (2013). [3] M.D. Campanell and G.R. Johnson, PRL 122, 015003 (2019).