Session BM21: Workshop I: Plasma Diagnostics

Principles and some progress in floating probe method (FPM) for process plasma diagnostics and monitoring

Floating probe method (FPM) applicable to process plasma has recently been developed. Recent progress and variant methods based on these FPM are introduced. A method of measuring plasma wirelessly by modifying the FPM has been developed, and plasma densities and electron temperatures were measured considering the deposition of the probe surface. This technique will be applicable to process plasma diagnosis and monitoring.   

Pushing the boundaries of established plasma diagnostics

While low temperature atmospheric pressure plasma diagnostics have been extensively studied in the last decades, there remains a strong need for improved diagnostics to increase our understanding of the underlying plasma processes particularly of emerging applications. The interpretation of plasma diagnostics poses often challenges due the distinctive non-equilibrium properties of atmospheric pressure plasmas in combination with their high collisional nature. This is further enhanced by spatial gradients down to micrometer length scale, transient behavior down to nanosecond timescales and challenging operation conditions required for some applications.

 The presentation will summarize the state of the art of well-established diagnostics for atmospheric pressure plasmas including Thomson scattering, laser induced fluorescence, broad band absorption spectroscopy and mass spectrometry. We will highlight some examples that illustrate the capability to advance these diagnostics by enabling a more convenient implementation approach, a broader detection range of species and the investigation of more challenging plasma conditions.   

Coffee Break

Coffee Break   

Recent progress in infrared laser spectroscopy to characterize low and atmospheric pressure plasmas

Despite the ever-growing applications of low-temperature plasma physics and technology, many plasma processes are far from being completely understood, in particular, the physical and chemical interaction of plasmas with solids and liquids. It is therefore essential to diagnose the fluxes of the generated species, to identify the relevant reaction pathways, to be able to tailor the reaction products for specific applications, and to gain further insight into plasma-induced reactivity in condensed matter systems. This requires high precision measurements of reactive molecular precursors, free radicals and short-lived species. The typical low abundances of the key transient reactive species, nowadays often in combination with small plasma dimensions, make the detection of these species a challenge.

To detect atomic and molecular species, absorption spectroscopy has become the method of choice for characterizing the fluxes of species in plasmas as it has several advantages over other optical diagnostic techniques. Molecular spectroscopy in the near- and mid-infrared regions is highly favourable because of the plethora of molecular fundamental, overtone and combination bands that can be accessed. As a result, selective and very sensitive spectroscopic measurements of a large number of compounds can be performed. I will discuss the recent progress in plasma spectroscopy in the infrared spectral region using various types of lasers.

The sensitivity of laser spectroscopy can be enhanced by combining it with a high finesse optical cavity using cavity-enhanced spectroscopy techniques. I will discuss the application of cavity-enhanced spectroscopy to determine species concentrations in, e.g., atmospheric pressure plasma jets, where we achieved effective absorption path lengths of up to 100 meters in mm sized plasma jets and therewith detection limits of ppb down to ppt levels.   

Studies of O(1D) and HO2 Kinetics in Plasma Assisted Low Temperature Fuel Oxidation Using Mid Infrared Faraday Rotation Spectroscopy

O(¹D) reactions with fuels and HO₂ reactions with fuel radicals are the most important reaction channels of low temperature plasma assisted combustion and fuel oxidation. In this lecture, the in situ diagnostics of O(¹D), O₂(¹Δ), and HO₂ reaction kinetics using mid infrared laser absoprtion method, Integrated Cavity Output Spectroscopy (ICOS), and Faraday Rotational Spectroscopy (FRS) in a flow reactor will be presented. Time dependent in situ measurements of O(¹D) and HO₂ in a photolysis reactor will be introduced. By using this approach, the direct measurements of reaction rates and branching ratios of O(¹D) with methanol and ethanol will be discussed. In addition, by using the experimental data, the development and validation of kinetic models for plasma assisted fuel pyrolysis and oxidation of n-alkanes will be presented.     

Lunch Break

Lunch Break   

Laser diagnostics for ro-vibronic excitations in plasmas at atmospheric pressures

Pulsed plasmas at atmospheric pressure have recently attracted increasing interest for a variety of applications. Although the detailed physics and chemistry can vary considerably between different types of plasmas, a common fundamental aspect is the generation of ro-vibrationally excited states in molecules due to electron collisions. Often this primary process is augmented by collisional excitation transfer between molecules. In either case, these excited states can significantly alter the dissociation rate of molecular species in the bulk and/or transfer internal energy to surfaces. Therefore, the ability to non-invasively study the ro-vibrationally excited states with high temporal (ns) resolution is of paramount importance. Depending on the symmetry properties of the relevant states, these allow either electromagnetic dipole or Raman transitions. For more complicated molecules, such as CO2, coupling between these modes is also possible. The first mode allows absorption measurements, the second Raman scattering. In this talk, tunable diode laser absorption spectroscopy (TDLAS) with quantum cascade lasers and broadband coherent anti-Stokes Raman scattering (CARS) will be presented. In addition to the general aspects of these complementary techniques, applications in CO2 and N2 ns-pulsed plasmas are shown [1-2].  

[1] J. Kuhfeld et al., J. Phys.D: Appl. Phys. 54, 305204 and 54305205 (2021).

[2] Yanjun Du et al., J. Phys. D: Appl. Phys. 54, 34LT02 and 365201 (2021).   

Phase Resolved Optical Emission Spectroscopy (PROES): A powerful diagnostic for non-thermal plasmas

Low and atmospheric pressure non-thermal plasmas are used for a wide range of applications. Electrons energized through electric fields play a vital role in plasma sustainment and plasma-generated chemistry. Therefore, electron heating and dynamics are crucial for understanding plasma behavior. Phase-resolved optical emission spectroscopy (PROES) is a powerful diagnostic technique that allows the non-intrusive measurement of space and time-resolved excitation, which reflects the energetic electron dynamics. In addition to space and time-resolved excitation, PROES can be used to evaluate space and time-resolved plasma parameters such as the electron temperature, electron density etc. Also, it can be utilized to assess quenching coefficients of excited states and the secondary electron emission coefficient of surface materials. In this talk, the experimental setup and method of PROES will be reviewed. Several examples of its application in both low pressure (capacitively and inductively coupled plasmas) and atmospheric pressure (jets and dielectric barrier) discharges will be presented. Finally, the limitations of PROES and future interest areas will be discussed.   

Coffee Break

Coffee Break   

Wave-cutoff probe: overview and achievements

Through the dispersion relation of electromagnetic waves propagating into un-magnetized plasma or ordinary wave in magnetized plasma: ω² = ω_pe² + c²k²  where ω², ω_pe², c, and k are the wave frequency, the plasma frequency, the speed of light, and the wave number, the wave cutoff frequency becomes the plasma frequency as ω_cutoff = ω_pe = (n_ee²/ε₀m)^0.5 when k = zero. Thus, we can obtain the absolute electron density from the measurement of the ω_cutoff. This method is called the wave-cutoff probe [1]. In this talk, we present research overview and achievements on the wave-cutoff probe, as follows. First, theory and apparatus of the wave-cutoff probe method are introduced. Second, various technical methods, such as conventional wave-cutoff, phase resolved wave-cutoff, and Fourier wave-cutoff are presented [2]. Third, a method measuring electron temperature, as well as the electron density is shown [3]. Forth, flat cutoff probe for industrial plasma processing monitoring was developed [4]. Fifth, interesting and pioneering works were done with bar-type flat cutoff probe [5], such as electron density measurement at the sheath-plasma boundary and real-time electron density measurement even though the wafer. Finally, we will show the ion density measurement using the wave-cutoff probe [6]. Thanks to this accurate plasma metrology method that can measure the absolute ion and electron densities at the same time, we demonstrate the verified quasi-neutrality of the plasmas for the first time [6].   

Forward, backward, and sideways lasing in atmospheric air

Remote lasing in atmospheric air allows and improves large distance standoff sensing and diagnostics due to the directional coherent emission. This talk will first introduce the fundamentals of air lasing and demonstrate atmospheric lasing using two-photon excitation of the atomic species resulting from dissociation of the air molecules such as nitrogen, oxygen, and water. Using direct excitation of the atomic species already present in air, we present lasing in air from two- and three-photon excitation of krypton and argon, respectively. The coherent forward and backwards air lasing are studied, and the properties of the coherent emission is analyzed. Using cylindrical focusing air lasing can be achieved sideways, providing emission of coherent beams perpendicular to the pump beam, or at an arbitrary angle. We present results on sideways lasing from both molecular dissociating and from directly exciting atomic species. Air lasing is demonstrated to aid remote sensing of atomic and molecular species. The stimulated emission process following multi-photon excitation which allows for air lasing is prevalent when applying nonlinear optical diagnostics such as two-photon absorption laser induced fluorescence (TALIF) using short pulses and can be used for species detection in gases and plasmas.   

Advanced Diagnostics – Ultrafast, Single-Shot, and Hyperspectral Techniques for Non-equilibrium Plasmas

The development of new diagnostic techniques repeatedly has pushed knowledge beyond the state of the art. Historic examples such as scanning tunneling microscopy, single molecule microscopy, and free electron lasers have each offered insight into new physics or material properties. To determine plasma parameters and particle densities in large-scale low-pressure plasmas, conventional plasma diagnostics may still be sufficient, however, conventional techniques do not yield sufficient information about properties of small-scale non-equilibrium plasmas at atmospheric pressure that exhibit high gradients of species concentrations in space and time.

The reactivity of plasmas interacting with surfaces is determined by processes on timescales spanning multiple orders of magnitude from sub-picoseconds to minutes, hours, and days: Ultrafast processes can define intermediate and long-lasting effects, such as, for example, the dissipation of energy within the plasma volume and to the interface of adjacent surfaces. In non-equilibrium plasmas, the electron energy distribution function exhibits a strong spatial and temporal dynamic and is a crucial factor for the energy dissipation pathway selection. Non-equilibrium plasmas interacting with liquids have high gradients of species’ concentrations, multiphase transport processes, and sometimes turbulent interfaces of plasma-, gas-, and liquid-phase boundaries. A thorough understanding of these properties requires the application of advanced diagnostic techniques.

This talk presents advanced diagnostics approaches that allow to measure transient events, with focus set on ultrafast laser diagnostics, single shot techniques, and hyperspectral methods. These methods have in common that they capture high information depth data within a comparatively short time. Current state of art, challenges, and an outlook for these advanced diagnostics in non-equilibrium plasmas will be discussed.