FR2.4: Controlling LTPs via advanced plasma sources and diagnostics
Challenge: To generate LTPs and measure their properties across a broad range of operating conditions from low pressure to atmospheric pressure and from unmagnetized to strongly magnetized.
Cross-cutting activities that Integrate Transformational Technology (TT) with Foundational Research (FR1, FR2) The development of new capabilities for controlling the generation of plasmas with independent control over the plasma parameters (e.g., density, electron temperature, potential, particle charge) and advanced techniques for diagnosing the plasma is required to create new technological tools that impact all TT areas. These are rich scientific activities in their own right that contribute to advancing the study of LTP physics. The four enabling activities described below will enhance our core validation, scaling, and measurement missions.
FR2.4(a) Controlling particle charge in magnetized dusty plasmas:
Background: The key property of a dusty plasma is the coupling of the particles to the surrounding plasma via the charge on the “dust.” The controlled modulation of an electric field at frequencies above the dust plasma frequency, pulsed plasma excitation to control the electron temperature, or UV illumination can provide pathways for “user-control” of the dust grain charge in unmagnetized plasmas.32,33,34,35
Proposed Research: We will install a pulsed UV light to reproducibly control the charge on both polydisperse and monodisperse suspensions of micrometer to nanometer sized grains in first unmagnetized and later, magnetized, plasmas. The primary research tasks will be to:
- Modify the interaction between the plasma and dust grains for an extended population of particles, e.g., to actively modify the Coulomb coupling parameter through charge modulation at B=0.
- Use active charge control to modify imposed pattern formation and discrete particle motion in magnetized plasmas at B ≥ 1 T.
Impact: If charge control can be scaled to magnetized LTPs, it provides a potential new tool for nanoparticle control in processing plasmas (TT1, TT2, FR2.2).
FR2.4(b) Probing the plasma/materials interface in laser-generated LTPs
Background: The novel Nanomechanical Resonator Diagnostic (NaRD) technique harnesses the technology of 2D materials nanoresonators36. Laser interferometry is used to measure changes in resonant frequency and mechanical dissipation in optical cavities whose “nanodrums” undergo phase transitions37,38,39. The motion of 2D nanomembranes is measured while they experience the effects of lasergenerated plasmas. Since the laser-plasma pulse acts as an impulsive force on the membranes, and its ions/electrons induce currents and materials transformations on them, the nanoresonators are a sensitive detector of plasma processes and provides information on materials transformations induced by the plasma.
Proposed Research: A new NaRD tool will be developed and incorporated into a pulsed laser deposition system to complement existing diagnostics, that include Langmuir probes, optical emission spectroscopy (OES), gated intensified CCD imaging, and laser reflectivity to monitor the material surfaces in support of quantum materials development for TT1.
Impact: Laser-generated LTPs can be tuned to create extreme non-equilibrium environments combining reactivity, temperature, and stress profiles to produce materials transformations that are otherwise inaccessible. This requires interrogating a plasma, and a plasma-irradiated material, at the location where materials chemistry and physics take place.
FR2.4(c) Pulsed laser for non-invasive high-pressure measurements
Background: Laser-induced fluorescence (LIF) is a well-developed diagnostic technique that is used over a broad range of gas and plasma conditions in LTP experiments.40,41,42 However, traditional continuous wave LIF methods cease to work under conditions relevant to many LTP systems, particularly those at high neutral pressures due to the collisional de-excitation of metastable states. To study the effect of ionization, such as in FR2.1, it is important to develop new diagnostics to measure both plasma and neutral densities.
Proposed Research: To use a recently acquired nanosecond pulsed laser system to develop a LIF, TALIF (Two-Photon Absorption LIF) or CRDS (Cavity Ring Down Spectroscopy) system that can be used for diagnosing high pressure (P ≥ 1 Pa) plasmas that extend from lab regimes (TT1, FR2.1, FR2.2) to atmospheric plasma regimes (TT3 and TT4).
Impact: Development of non-invasive plasma diagnostics is critical for probing collisional LTP. Optical emission spectroscopy can be used in conjunction with collisional radiative models to infer neutral densities. However, LIF or TALIF techniques allow spatially resolved neutral and ion densities measurement of LTP. The same tunable pulsed laser used for LIF and TALIF can also be used for cavity enhanced absorption techniques such as CRDS which give chord averaged measurements that can be Abelinverted for radial profiles of both ion and neutral densities in regimes where 3-level energy levels for LIF
are unavailable or systems are experimentally unresolvable due to low signal-to-noise ratios or collisionally quenched. CRDS can also be applied to the APP jet devices used for the food safety (TT3) programs.
FR2.4(d) Low power rf plasma generation mixed with rf attenuation and reflection measurement for plasma control
Background: Making high quality non-invasive measurements of plasma parameters is challenging. A carefully developed rf plasma generation system combined with a rf attenuation and reflection measurement system can allow the rf electrodes to be used simultaneously as rf power emitters as well as plasma densitymeasurement electrodes. In principle, using a vector network analyzer (VNA) circuit operated at frequencies far above the rf power frequency and coupled into the rf power line can independently measure the reflection as well as transmission of signals in the higher frequency range.
Proposed Research: To develop a theoretical plasma circuit model to derive the plasma density from the measured plasma impedance using a VNA with the goal of establishing an LTP generation system that uses real-time impedance measurements to control the average plasma density under varying pressures and/or potential geometry changes.
Impact: RF reflection measurements of plasma density can potentially avoid the disruptive properties of a standard Langmuir probes. This system will be calibrated against in-situ Langmuir probes and used to support plasma measurements made in FR2.2, FR2.3, TT1, and TT2.
FR2.4(e) Development of an experimental platform to evaluate misty plasmas and plasma-liquid interactions.
Background: Following the theoretical developments in FR1.4, “misty plasmas” that describe a gaseous plasma within which liquid micro-droplets levitate and can absorb component plasma species, evaporate, and break up electrostatically, drastically changing plasma properties. This is an emerging area of low temperature plasma research that offers new opportunities for experimental and theoretical development.
Proposed Research: To develop DC plasma source and droplet delivery system that can be used to study both the formation of charged water droplets in the form of a misty plasma as well as the formation of plasmas using liquid cathodes and/or anodes.
Impact: The interaction of plasmas with liquids is of fundamental importance for understand the application of LTP to biological and agricultural systems. The development of an experimental platform for studying these systems will be used to validate the theoretical and computational studies of plasma-liquid interactions to be performed in FR1.4 and used to support applied studies in TT3.