Session EM1: US DOE-Funded Collaborative Low Temperature Plasma Research Facilities

Workshop Welcome Remarks: DOE/FES Program Activities in Basic and Low-Temperature Plasma

An update will be provided for the General Plasma Science (GPS) program of the DOE Office of Science Fusion Energy Sciences (FES). Specifically, the talk will cover an overview of the program and fiscal year activities in low temperature and basic plasma science.   

Overview of Princeton Collaborative Research Facility (PCRF)

The Princeton Collaborative Low Temperature Plasma Research Facility (PCRF) provides the scientific communities and industry access to state-of-the-art research capabilities, including advanced diagnostics, plasma sources and computational codes, theory support, and expertise for comprehensive characterization of low temperature plasmas (LTPs) with focuses on 1) plasma-liquid and plasma-solid interactions, 2) collective phenomena in LTP, and 3) use of LTP in modern applications (e.g. nanomaterials synthesis and processing, microelectronics and quantum systems, energy, sustainability, aerospace, bio/med/agro). Since the beginning of the facility operation in 2019, 86 collaborative users from the plasma and other scientific communities, including from universities, national labs, and industry, were awarded with runtime at the PCRF. In this presentation, we will overview PCRF research, capabilities and opportunities for collaboration.   

Sandia National Laboratories Plasma Research Facility (PRF)

This presentation will provide an update on the activities and structure of Sandia’s Low-Temperature Plasma Research Facility (PRF), funded by the DOE Office of Science, Office of Fusion Energy Sciences, General Plasma Science. The PRF is a resource available to anyone in the international Low-Temperature Plasma (LTP) community to access the advanced resources available at Sandia (not restricted to US collaborators). Capabilities are accessed through an annual proposal process which opens immediately following the GEC conference. Available resources include both experimental and modeling capabilities that represent many person-years and millions of dollars of development through DOE and other investments (some for decades). Examples of real-time diagnostics include Laser-Induced Fluorescence (LIF), Laser-Collision-Induced Fluorescence (LCIF), Photofragmentation LIF (PF-LIF), and Molecular Beam Mass Spectrometry (MBMS). Advanced modeling capabilities are also available, including state-of-the-art PIC-DSMC modeling tools along with access to Sandia’s high-performance computing (HPC) capabilities (many 10K’s of cores).

 

The presentation will include an overview of expertise and capabilities available, an explanation of the proposal process, and a summary of select recent projects.

 

 

This work used the capabilities of the SNL Plasma Research Facility, supported by DOE SC FES.  SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.  SAND2023-04477A   

Atomic oxygen densities in the gas and liquid phase: utilizing ps- and fs-TALIF to interrogate the transport of O from the gas into the liquid phase

Recently, there has been an emerging realization of the importance of atomic oxygen in plasma-induced chemistry, particularly in the aqueous phase. Unlike many reactive species generated by plasmas, atomic oxygen is not found in biological systems in nature, so its effects remain largely unknown. However, several studies have alluded to its potential, documenting atomic oxygen’s central role in deactivating multiple cancer cell lines, cleaving DNA, and degrading a variety of organics. Quantifying solvated O atoms, critical for isolating its effects, has proven exceedingly difficult as chemical probes used for its detection suffer from a number of shortcomings. Here we show that aqueous O can be measured directly by employing two-photon absorption laser induced fluorescence (TALIF) with a femtosecond laser [1]. We demonstrate that given a sufficiently fast laser pulse, solvated O atoms can be excited without appreciable heating of the liquid and at a requisite efficiency for detection of a fluorescence signal despite the highly collisional aqueous environment. These measurements establish the proof of concept for an experimental technique to directly quantify atomic oxygen in liquid without the need for inherently problematic chemical probes. The measurements were conducted at the PCRF and combined with gas phase measurements conducted with a picosecond laser at PRF [2,3]. Gas phase measurements were performed in open atmosphere and with and without a liquid interface; to account for quenching in this heterogeneous and collisional environment, the effective lifetimes of excited atomic oxygen atoms were measured directly in situ.

[1] B. Myers, A. Dogariu, B. Beeler, K. Stapelmann, "Imaging solvated oxygen atoms with a femtosecond laser", Nature Comm. in revision

[2] B. Myers, E. Barnat, K. Stapelmann, J. Phys. D: Appl. Phys. 54 (2021) 455202

[3] K. Stapelmann, B. Myers, M. H. Quesada, E. Lenker, P. Ranieri, J. Phys. D: Appl. Phys. 54 (2021) 434003   

From plasma self-organization to in situ utilization of extraterrestrialmolecules for propulsion: Enabling plasma science with advanced diagnostics

Low temperature plasma science discovery is enabled by advanced diagnostics and high-performance computing.  The Princeton PCRF provides such capabilities for single investigators enabling broader insight and understanding in a wide array of plasma science problems. Three science projects from UM’s Plasma Science and Technology Laboratory have been hosted by the PCRF. These varied topics include: 1) plasma-based depolymerization of plastics, featuring advanced FTIR and OES analysis, 2) modeling the plasma extraction sheath of a plasma cathode operating on molecular gases using EDIPIC, and 3) the study of the role of negative ions in self organizing plasmas. These studies provide insight into long standing science problems and inform engineering solutions to environmental challenges here on Earth and resource utilization in space.  Here we report on results from these studies featuring PCRF diagnostic and modeling tools and put into context the invaluable insight that they have revealed.   

Collaborative Research on Particle-in-Cell Modeling of Materials-Processing Plasmas

Kinetic effects are important in many low-pressure plasma processing applications. Although fluid or hybrid models are computationally efficient, they are often not able to adequately describe electron transport and non-local phenomena. Full kinetic treatment of these plasmas is therefore necessary. We discuss results from our collaborative research on particle-in-cell (PIC) modeling of materials processing plasmas. These projects have been used to understand the physics of several plasma sources, facilitated the development of faster kinetic modeling methodologies, and provided innovative solutions to processing problems. This presentation describes the outcome of some of this research. Plasmas generated by energetic electron beams are known for their low electron temperature. PIC modeling in conjunction with analytical models was used to understand electron transport in magnetized electron beam plasmas. Magnetized capacitively coupled plasmas (CCP) can exhibit instabilities producing non-uniform and rotating structures. PIC simulations of low-pressure magnetized CCPs were used to delineate the limits of stable plasma operation and devise methodologies for extending the stable regime. Pulsing is widely used to control plasma chemistry and ion energy in processing plasmas. Synchronous pulsing in low-pressure dual-frequency CCP was examined to develop strategies to further regulate the plasma during the on and off states. We also discuss methods to accelerate kinetic simulations including semi-implicit PIC, the use of graphical processing units, and hybrid PIC–fluid models.   

Pulsed laser induced photoemission and its effects in plasma discharge

Laser-induced photoemission of electrons offers opportunities to trigger and control plasmas and discharges. However, the underlying mechanisms and their effects on the resultant plasmas and discharges are not well characterized. Photoemission itself is a highly nonlinear process through mechanisms including multiphoton absorption, above threshold ionization, field enhanced hot electron emission, and optical field emission or tunneling, etc. The dominant process depends on the work function of the material, photon energy and associated optical fields, surface heating, background fields, etc. To characterize these effects, breakdown experiments are performed and interpreted using a 0D discharge model and a quantum model of photoemission. In the low-current regime considered, it is found that laser-induced photoemission is sufficiently de-coupled from space charge effects to be observable. The breakdown voltage is found to be lower with photoemission than without. When the applied voltage is insufficient for ion-induced secondary electron emission to sustain the plasma, laser driven photoemission can still create a breakdown where a sheath is formed. This photoemission induced plasma persists and decays on a much longer time scale (~10 μs) than the laser pulse length (30 ps). The effects of different applied voltages and laser intensities on the breakdown voltage and current waveforms are investigated.   

The characterization of Ar metastables in electron beam generated plasmas

Electron beam generated plasmas are attractive for low-damage surface processing applications (deposition and etching) because of their ability to produce plasmas with large densities (>10¹⁰ cm^-3) at low electron temperatures (< 1 eV) and thus deliver a large fluence of very low energy (< 5 eV) ions to adjacent surfaces. These inherent features originate from the use of high energy electrons beams, rather than the heating of the plasma electrons via the use of electric fields, to drive plasma generation. The production and the diffusion of ions from the beam volume to adjacent substrates has been extensively studied, while neutrals, particularly metastables have received less attention. The interest in metastable species is facilitated by the fact that they have long lifetimes and large internal energy and can therefore distribute energy and drive reactions far from their point of origin. .  Previous modeling work has indicated the production and density of Ar metastables in electron beam generated plasmas produced in mixtures of argon and nitrogen is sensitive to the relative concentration of nitrogen. In this work, we have investigated the spatially-resolved density of metastable Ar species in electron beam generated plasmas produced in pure and diluted Ar using laser-induced fluorescence (LIF). Specifically, the density of the 1s₅ excited, metastable level was measured radially with respect to the beam axis at pressures of about 25 and 60 mTorr in Ar/N₂ gas backgrounds, where the relative N₂ concentration was varied up to approximately 20%. The measurements are compared to a zero-dimensional model of the system in an effort to understand the kinetics.   

Temperature and methyl radical measurements in NRP glow discharges for combustion enhancement

While nonequilibrium plasma discharges have showed promising abilities to enhance combustion, the mechanisms of their effect on the reactive front remain under investigation. These effects can be classified into three main categories, thermal, chemical, and transport, that are usually coupled. In this study we investigate in a canonical configuration the thermal effect as well as the production of methyl radical (CH₃) by nanosecond repetitively pulsed (NRP) glow discharges, applied across a laminar methane-air flame at atmospheric pressure. A wall-stabilized configuration with the discharges generated on the symmetric axis of the burner is chosen to allow phase-locked averaging as well as comparisons with two-dimensional axis symmetric simulations. Hybrid fs/ps rotational Coherent Anti-Stokes Raman Scattering (CARS) measurements are used to determine the temperature of ground state nitrogen and the oxygen-to-nitrogen concentration ratio in the discharge region. Photofragmentation Laser Induced Fluorescence (LIF) is utilized to image the spatial distribution of methyl in the discharge region. Results show that in this configuration, the thermal effect is minimal while the production of methyl radical upstream of the flame front can reach 1200 ppm, i.e., twice the concentration that is predicted in the flame without plasma. These results are put into perspective to previous measurements performed on this system, more specifically on the spatial distribution of atomic oxygen (O), atomic hydrogen (H), hydroxyl radical (OH), and methylidyne radical (CH). A discussion on the chemical pathway of plasma-assisted oxidation of methane is then proposed.   

Advancing Sheath Physics Using Particle-In-Cell Simulations

This talk will summarize recent advances in sheath physics that were enabled by particle-in-cell simulations conducted in collaboration with the Plasma Research Facility at Sandia National Laboratory. Three results are emphasized: (1) Ions in low pressure discharges can be heated substantially by electron-ion energy relaxation when it is enhanced by ion-acoustic instabilities excited near sheaths. Observations from PIC simulations indicate that when the electron-to-ion temperature ratio exceeds approximately 28, the electron-ion energy equilibration rate dramatically increases to such an extent that the temperature ratio at the sheath edge cannot substantially exceed this threshold. Because ion-acoustic waves are reflected from the sheath, ion heating extends into the bulk plasma as well. (2) The second result concerns how sheath and presheath properties change as neutral gas pressure is increased. Many fluid-based models have been proposed to describe this. Tests using particle-in-cell simulations show general agreement with the trends of the models, but also show inconsistencies in how they treat different properties, such as the collisional Bohm criterion, edge-to-center density ratio, sheath width, and sheath potential drop. A new fluid model is proposed to consistently include each of these properties and is tested with the simulations. (3) The third result shows that a new type of electron plasma wave instability can be excited by an ambipolar electric field, as in the presheath near a sheath. This instability is fundamentally different than commonly understood electrostatic instabilities in that it does not satisfy the Penrose criterion. This is possible because of the steady background electric field. The instability wavelength is tens of Debye lengths with a growth rate that is proportional to the electric field strength. This result many have broad implications for electron scattering in the presence of ambipolar electric fields.