Streamer discharges and their interaction with dielectrics
Xiaoran Li defended her PhD thesis at the Department of Applied Physics and Science Education on October 30th.
Streamer discharges are a common initial stage of electrical discharges. They create the first ionized paths for later heat-dominated spark discharges, playing an important role in electric breakdown in nature and in high voltage devices. Simulations, which provide a full temporal and spatial evolution of fields and plasma species, are a powerful tool to study the physics of streamer discharges. For their PhD research, Xiaoran Li explored the computational study of streamer discharges by considering a number of open questions on discharges in high-voltage devices.
Xiaoran Li looked at four open questions in relation to discharges in high-voltage devices; namely how accurate are current commonly used simulation models, how streamer properties depend on the background electric field, how streamers interact with dielectrics, and how positive streamers propagate in gases different from air.
Positive streamer discharges
First, Li compared simulations and experiments of positive streamer discharges in air at 100 mbar for model validation. Good qualitative agreement was achieved between the experimental and simulated optical emission profiles, as well as the streamer velocity and radius throughout the evolution.
Quantitatively, the simulated streamer velocity and radius were approximately 20% to 30% smaller. The study examined the impact of various parameters, such as transport data, background ionization levels, photoionization rates, gas temperatures, voltage rise time, and voltage boundary conditions on the agreement between the simulations and experiments. The observed discrepancies could potentially be attributed to an increase in gas temperature caused by the experimental discharge repetition frequency of 50 Hz.
Second, Li investigated streamer propagation in air gaps through two aspects: steady propagation of positive streamers and streamer deceleration in fields below the steady propagation field. An important finding is that faster streamers are able to propagate in significantly lower background fields than slower ones, indicating that there is no unique stability field.
The background electric fields for steady streamers are from 4.1 kV/cm to 5.4 kV/cm, which corresponds well with experimentally determined stability fields. Additionally, the deceleration of streamers was investigated, and a phenomenological model was presented to describe the evolution of velocity and radius for stagnating streamers, which provides a theoretical basis for predicting breakdown voltages for air gaps.
Third, Li explored streamers that propagate towards a dielectric surface, attach to it, and propagate along it with a 2D fluid model. It was found that streamers are attracted to dielectrics via electrostatic forces.
Compared to streamers in bulk gas, surface streamers have smaller radii, higher electric fields, and greater electron densities, resulting in faster propagation. Positive surface streamers have a high electric field and low electron density area between the streamer head and the dielectric, while negative ones may touch the surface, creating a high field area inside the dielectric.
Effects of various parameters on surface streamers were also investigated, including applied voltage, dielectric permittivity, secondary electron emission, positive ion mobility, and preset surface charges.
Streamers in CO2
Finally, we focused on streamers in CO2, which is increasingly used as an insulating gas in high voltage devices. Photoionization in CO2 is significantly weaker compared to air because fewer ionizing photons are produced and because their absorption distance is much shorter.
To investigate the effect of photoionization in CO2, a Monte Carlo photoionization model was developed. By conducting PIC-MCC simulations incorporating this novel photoionization model, Li and her colleagues found even a small amount of photoionization was able to sustain positive streamer growth in CO2, but this required a higher electric field around the streamer head than in air.
Title of PhD thesis: Streamer discharges and their interaction with dielectrics – a computational study. Supervisors: Ute Ebert, Anbang Sun, and Jannis Tenuissen.
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