Atmospheric Pressure Plasmas

Coherent anti-Stokes Raman scattering (CARS)

Atmospheric pressure plasmas - especially those intended for industrial application - are often operated in molecular gases. Compared to atomic gases, these have additional degrees of freedom to store energy in. How exactly the energy of the electrons - coupled into the plasma by the electric field - is distributed over the different degrees of freedom, can vary strongly and depends on multiple parameters, like the used gas mixture, the electric field profile and the gas density. Therefore, by controlling the plasma parameters it is possible to control the energy distribution, for example to favor certain chemical reactions - in the volume of the reactor as well as on catalytic surfaces. But to do this in an efficient and reliable way, it is crucial to measure the number densities of excited molecules. One way to detect rotational and vibrational excited states is by coherent anti-Stokes Raman scattering (CARS). While this technique might seem overly complicated, compared to laser absorption measurements, it has the compelling advantage, that it can probe vibrational transitions which are dipole forbidden (for example due to a lack of permanent dipole moment as it is the case in homonuclear molecules like oxygen and nitrogen). The setup consists of three ns-pulsed lasers two narrowband and one broadband laser, which together with the molecular gas in the plasma produce a fourth coherent (i.e. laser like) beam. While this fourth beam is too weak to be seen by the human eye, it can be detected by a sensitive spectrometer. The measured spectrum is then compared to simulated ones to finally obtain the rotational and vibrational distribution functions of the molecules.

Electric field induced second harmonic generation (E-FISH)

One of the most important parameters determining features of gas discharge plasma is the value of the reduced electric field, E/N, since it controls electron energy distribution function (EEDF) and, consequently, electron energy losses pathways, which have strong influence on active species yield of the discharge. Thus, measurements of E/N in the discharge is a subjects of a prior interest. To measure an electric field in nanosecond discharges, in particular, Atmospheric Pressure Plasma Jet (APPJ), Electric Field Induced Second Harmonic Generation (E-FISH) is used.

The idea of the technique is rather straightforward. The laser emission at frequency ω is focused inside the discharge gap and in a presence of the external electric field (field in the discharge) the light at frequency 2ω (second harmonic) is generated. Intensity of the second harmonic is proportional to a square of the applied electric field strength. Using the calibration curve obtained by measuring the second harmonic intensity at known electric field value on can reconstruct the electric field in the discharge. Tightly focused laser beam allows to obtain high spatial resolution (few tens of micrometers), which is extremely important for studies of APPJ or other microplasmas. Temporal resolution is limited by duration of the laser pulse. A picosecond laser used in our laboratory for EFISH provides the temporal resolution of 100~ps, which is high enough to resolve such transient events as a breakdown in the nanosecond discharge. Another great advantage of this technique is that the same laser and the same optical setup can be used for any gas mixture, no changes are required. For example, the electric field has been successfully measured in pure nitrogen and helium:nitrogen mixture. The electric field combined with other electric measurements (electric current and voltage) provided an additional information about the discharge: electron density and voltage drop over the sheath layer.

Besides the electric measurements an Optical Emission Spectroscopy (OES) is used in our laboratory to study nanosecond discharges. Spectra taken during the discharge as well as streak camera measurements of selected optical transitions allow to investigate development of the discharge with sub-nanosecond time resolution. Spatially resolved OES provides information about the discharge structure and its dynamics.