Session KW73: Plasma Diagnostics: Spectroscopy I

Non-invasive optical emission diagnostic to determine plasma parameters in oxygen discharges

An emission model has been developed to exploit the relative spectral intensities of oxygen discharge emissions to non-invasively determine plasma properties. The emission model employs electron-temperature dependent rates for electron-impact excitation from the ground states of O, O₂ and O₂⁺. For O₂⁺ ions, excitation rates and the effect of quenching of excited states via electron collisions have been characterized by fitting relative emission intensities over a range of discharge pressures and powers for which electron density and temperature were independently measured. Outputs of the emission model are the electron temperature and the oxygen dissociation fraction. Implementation and validation of the emission model is in progress using an extensive set of measurements taken, including O and O₂⁺ relative emission intensities (optical emission spectroscopy (OES)), gas temperature (OES), electron density (multipole resonance probe), dissociation fraction (actinometry) and electron temperature (Langmuir probe). Results will be presented for an oxygen inductively coupled plasma for pressures of 1 to 30 mTorr and discharge powers in the range of 250 to 2000 watts.    

Electric field and temperature patterns in lower-pressure streamer discharges

The electric field is a fundamental plasma physical parameter. Determination of the electric field of streamers can become quite challenging as their velocity can reach significant values (generally ~10⁶ m/s). To investigate these highly transient electric fields, we deploy the line ratio technique by measuring the ratio of luminous intensities emitted by N₂(C-B) and N₂⁺(B-X) bands with a resolution of several hundreds of picoseconds. In our case, we investigated low-pressure discharges in nitrogen-containing gas mixtures operated at repetition frequencies of hundreds of Hz in a point to plate arrangement. The radiative transitions N₂(C³∏_u,ν=2)→N₂(B³∏_g,ν=5) and N₂⁺(B²∑_g⁺,ν=0)→N₂⁺(X²∑_g⁺,ν=0) bands were measured by ICCD camera to obtain a reduced electric field. Simultaneously, the whole recorded optical emission spectra were fitted with massiveOES software to obtain 2D maps of rotational/vibrational temperatures, giving a more detailed insight into the kinetic processes in streamer discharges.   

Arbitrary electron energy distribution determination from the continuum emission spectrum for atmospheric-pressure plasma electron diagnostics

In several fields, novel low-temperature non-equilibrium atmospheric-pressure plasma applications are being developed. In this type of plasma, mainly electron collisions are causing the desired reactions. Therefore, the shape of the electron energy distribution function (EEDF) is essential for understanding and predicting the plasma behavior. The EEDF shape generally does not resemble a Maxwellian or Druyvesteynian. Instead, determination of arbitrary EEDF should be considered. Existing methods to determine the arbitrary EEDF can either not yet be applied to atmospheric-pressure plasma (probe measurement) or are not very accessible (Thomson scattering). In this work, a novel method to determine arbitrary EEDF from optical emission spectroscopy is presented. By using the continuum emission spectrum, dominated by neutral bremsstrahlung, a machine learning scheme can obtain the arbitrary EEDF in the low energy region through reinforcement learning. Further analysis of the machine learning output allows further extraction of electron energy distribution information. The current capabilities of this method will be discussed together with expected limitations and future improvements.   

Electric field strengths within a micro cavity plasma array measured by Stark shift and splitting of helium

Over the last years micro-structured plasma devices have received increased attention for decomposition and reformation of volatile organic compounds (VOC) [1]. Here, we present a metal-based microcavity reactor which is a convenient alternative to silicon-based devices. This layer-structured reactor consists of a nickel foil operating as an electrode and an electrically grounded magnet. Both electrodes are separated from each other by a 40 µm thick dielectric ceramic foil. The nickel foil consists of four sub-arrays where hundreds to thousands of cavities in the 100 µm range are arranged equally. To obtain more control over charged particles, the electric field is of high importance. Here, the Stark splitting and shifting of the allowed 492.19 nm Helium line and its forbidden 492.06 nm counterpart are exploited to investigate the electric field strengths. By using a combination of a plane grating spectrometer and an attached ICCD camera the typical displacement of about 0.2 nm between both transitions can be resolved. With this technique spatial integrated and phase-resolved electric field strengths with a time resolution up to 1 µs can be measured for this reactor depending on typical operation and geometric parameters. Depending on the polarity of the applied voltage, the electric field strengths increase with smaller cavity diameters up to 60 kV cm^-1.

[1] A. Koutsospyros et al. Int. J. Mass Spectrom. 233 305-315   

Diagnostics of laser-produced Mg plasma through a detailed collisional radiative model

A detailed fine-structure resolved collisional radiative model is developed to investigate the laser-produced Mg plasma. The dominant electron impact excitation and de-excitation have been considered in the model. The required electron impact excitation cross-sections for the transitions from the ground state 3s² (J=0) to the 3s3p, 3s4s, 3s3d, 3s4p, 3s5s, 3s4d, 3s5p, 3s6s, 3s5d, and 3s6p excited states and from 3s3p manifolds to other fine structure levels of 3s4s, 3s3d, 3s5s, 3s4d, 3s6s, and 3s5d configurations are calculated by relativistic distorted wave approximation theory. The plasma diagnostics is carried out by coupling the present collisional radiative model with the laser-induced breakdown spectroscopy measurements reported by Delserieys et al. [J. Appl. Phys., 106, 083304, 2009]. Five measured intense Mg-I emission line intensities viz. 383.3, 470.3, 517.8, 552.8, and 571.1 nm are used and corrected through the self-absorption to extract the plasma parameters i.e. electron temperature and electron density. The obtained plasma parameters at different delay time ranging from 100-700 ns are compared with the results of Delserieys et al. that were estimated using the Thomson scattering and Boltzmann plot approaches