Hi there, my name is Harry Philpott, I am a PhD student in the Elementary Processes in Gas Discharges group at the Technical University of Eindhoven (TU/e). I come from the UK, and I gained my Master’s degree in Theoretical Physics from the University of York, one of the UK’s leading institutes for plasma research. During my time there I focussed on high-temperature plasma processes, specifically related to fusion and astrophysical plasmas, where the underpinning physics was Magneto Hydro Dynamics. For my research project, I investigated linear and non-linear solutions to the Poisson-Boltzmann equation, which is an important equation in plasma physics as it encapsulates both the electrostatic and thermodynamic effects driving plasma phenomena. I’m excited by the more dynamic, non-equilibrium physics, and the industrial applications associated with low-temperature plasmas. The real world implication of the Pioneer program is also an exhilarating prospect for me, and I can’t wait to work with my fellow researchers to tackle the looming problem of climate change.
My PhD project is aimed at understanding the temporal and spatial evolution of the electric field on the surface of a catalyst under plasma exposure. During my time at TU/e I will investigate the surface charging process using the Pockel’s effect and polarimetry techniques. Then during my secondment at École Polytechnique, I will determine the effects the electric field has on the catalyst surface by measuring the adsorption properties of gasses on the surface.
| Overview |  |
| ESR: | 1 |
| Title: | Plasma-surface interactions in atmospheric pressure plasma jets |
| Home Institution: | Eindhoven University of Technology (TU/e-EPG) |
| 1st Supervisor: | Ana Sobota |
| Host Institution: | Laboratoire de Physique des Plasmas (CNRS-LPP) |
| 2nd Supervisor: | Olivier Guaitella |
| Industrial partner: | AFS |
| Industrial contact: | Florian Brehmer |
| Defence: | March 20 2024 |
Abstract
Global warming, and its ensuing climate change, is one of the biggest issues facing humanity and will most likely be the defining aspect of the 21st century. The biggest driver of global warming is anthropogenic CO2 emissions, as such, there has been much time, money and effort applied to combating this issue. One of those efforts has been the PIONEER project, of which this thesis has been a part of. The specific research goal of the PIONEER project was to progress the field of plasma-catalysis for the purpose of recycling CO2 into value-added products such as methane and ethanol.
Plasma-catalysis is considered to be a promising route to recycle
CO2, firstly because it can initiate chemical reactions outside of thermal equilibrium, meaning that the high temperatures and pressures are not necessary, which is of great importance for highly endothermic reactions such as the dissociation of CO2. Secondly, it can be used to store excess renewable energy in chemical form. As plasma-catalysis is fully powered by electricity, any renewable energy used to power the recycling of CO2 will be converted into green fuels, effectively tackling the issue of intermittency by allowing the energy tobe stored and transported.
Within PIONEER, this thesis was focused on exploring plasma-
surface interactions and the surface electric field of catalysts undergoing plasma exposure. This surface electric field is thought to be one of the known unknowns of the synergy arising between the plasma and the catalyst. However, measuring the surface electric field on a porous catalyst as it is exposed to a short-lived plasma source is a non-trivial undertaking. The complex catalyst surface will scatter light, and therefore precludes any of the simpler optical, surface electric field diagnostic techniques.
Hence, the application of a more powerful method known as Mueller
polarimetry becomes necessary. This diagnostic probes more of the
light-matter interaction, enabling the extraction of the electric field,
even when the signal is convoluted with other properties of the sub-
strate. Nevertheless, Mueller polarimetry proves challenging to com-
prehend and execute as a diagnostic. It entails intricate linear algebra and requires complex integration into physical experiments. Consequently, a significant portion of this thesis concentrates on presenting uncomplicated techniques designed to assist plasma physicists involved in experimental diagnostics employing Mueller polarimetry.
One avenue explored in this thesis entails over-determining the
Mueller matrix as a means to overcome the noise inherent in the
short time frame (≤ 1μs) exposure necessary to capture the plasma
phenomena. We go on to show that over-determining the Mueller
matrix is a simple and effective way of improving the accuracy and
noisiness of the Mueller matrix. Additionally, converting the Mueller
matrix into a vector is also explored. This technique is perhaps the
most simple and effective of all those outlined in this thesis, as it only requires a change in calculation method, rather than simply taking more measurements. One interesting aspect that arose from these investigations was the ∆ matrix, which only occurs once the Mueller matrix is over-determined and can be used to identify noise in the measurements used to calculate the Mueller matrix.
In addition to the work on applying Mueller polarimetry in a plasma-catalyst setting, experimental measurements were taken of plasma-surface interactions involving a mixture of working and shielding gasses. These different combinations of working and shielding gas
introduce new species and charge carriers to the plasma-surface interaction, creating an excellent platform for generating reproducible conditions for plasma-surface interactions. Two results chapters are devoted to measurements taken using this shielded plasma source, where the various chemical properties and kinetic processes such as electronegativity and Penning ionisation are linked to measured phenomena, furthering the fundamental understanding of how the properties of the gasses involved will impact the plasma-surface interaction. Additionally, the temporal stability of these shielded plasma sources was investigated, where the findings show that in many circumstances, the plasma-surface interaction can take a long time to stabilise, or even not stabilise at all. This particular set of results has implications for the reproducibility of experimental plasma-surface measurements, like those performed in the field of plasma-catalysis.
Overall, this thesis brought the possibility of recycling CO2 via
plasma-catalysis closer by providing simple techniques to improve
surface electric field measurements, which is of critical importance
to understanding why certain plasma-catalyst combinations result in
better performance. Additionally, the work on how different charge
carriers affect the plasma-surface interaction has provided useful experimental evidence that furthers the fundamental understanding of plasma-surface interactions.
Links with other ESR
Measurements performed with coating of catalysts
Secondments
- CNRS-LPP: Complement he field measurements under plasma exposure performed at TU/e-EPG under supervision of Ana Sobota by measurements of the chemical species adsorbed on the surface by Infrared absorption technique developed at LPP for plasma/catalyst coupling studies
- AFS: Exploration all the excitation power source useable industrially, and their influence on field produced
- LCS-CAEN, AGH, UoB and CSIC-ICB: Deeper understanding of the materials studied