Plasma-surface interactions in atmospheric pressure plasma jets

Within the thesis of Harry Philpott, the approach to converting CO2 is known as plasma-catalysis, and involves combining a plasma source with a catalyst to produce a synergistic effect.

However, there is much that is still not known about this synergy, but it is believed that one of the key components is the electric field from the charge deposited on the catalyst surface.

Mueller matrix

One of the ways to measure the electric field is by measuring the Mueller matrix of electro-optic crystals undergoing plasma exposure, via an experimental technique known as Mueller polarimetry. This diagnostic technique is not trivial though, and much of Philpott’s thesis looks at ways to improve the quality of the results from this technique.

The specific methods outlined in Philpott’s work included over-determining the Mueller matrix, converting the matrix into a vector, and estimating the noise in the measurements used to calculate the matrix. He went on to show that all of these techniques lead to more accurate, less noisy Mueller matrices, and that these techniques will be of use to experimentalists and theoretician alike.

Plasma jets

Alongside directly investigating the charge deposition via Mueller polarimetry, Philpott also investigated its impact on plasma-surface interactions with a kHz atmospheric pressure plasma jet.

These investigations involved a range of other diagnostic techniques such as optical emission spectroscopy, intensity charge coupled device imaging, Schlieren imaging, and electrical measurements.

The outcome of this investigation showed that plasma-surface interactions can take a long time to stabilise and can exhibit hysteresis effects, which Philpott believes is due to charge accumulation on the dielectric surface.

Memory effects

These points are of interest in the plasma-catalysis community as it firstly shows the difficulty of effectively reproducing certain results. For instance, he shows that his experimental setup can take over 90 minutes to stabilise, so if measurements are being performed before the system has reached a good degree of stasis, the results will not be reproducible.

Secondly, the results add to the growing body of research into so-called “memory effects” in plasma-dielectric interactions. Philpott has shown that these memory effects can be more than a localised phenomenon with a short timescale, but can also continue to have an impact much later than previously thought.

Surface-charge phenomena

In addition to highlighting the long stabilisation and hysteresis effects, Philpott also analysed how these surface-charge-related phenomena are affected by the gas composition used.

This is of importance to the plasma-catalysis community as many different gases are used in the various reactions of interest.

To perform this investigation he added shielding gases to our kHz atmospheric pressure plasma jet. In doing so, he discovered that varying the gas composition can drastically alter the long timescale behaviour and can almost completely prevent the hysteresis effect shown.

Additional information

This thesis was one out of 15 double doctorates from the EU funded Marie Sklodowska Curie project “PIONEER”, a research project on exploring plasma catalysis for carbon dioxide recycling.

Title of PhD thesis: Plasma-surface interactions in atmospheric pressure plasma jets. Supervisors: Ana Sobota, Olivier Guatella, and Enrique Garcia-Caurel (external).