Development of high-pressure plasma source through PIC and global simulations based on time-resolved diagnostics

Abstract

High-pressure plasmas with large Coulomb coupling constant are an interesting state of matter where transport phenomena occur among multiple species (electrons, ions, neutrals, and cluster ions) with a wide range of space and time scales. The transport phenomena in high-pressure plasmas can be utilized to make an efficient source of chemically reactive species. Generation of high-pressure plasma requires understanding of the energy transport process from external power source to electrons as well as internal transport energy processes among different species. In particular, confinement of electrons and energy back-transport to electrons from excited heavy species are critical for sustainment of the plasma state. Motivated by experimental findings of higher densities excited species in microwave driven plasmas, recent particle-in-cell/Monte Carlo simulations on micro-sized (0.1—1 mm) high-pressure gas discharge between planar electrodes driven by microwave (~1 GHz) showed that the electron confinement is substantially enhanced above a critical frequency, maintaining the electron transport in the diffusion-dominant regime. Better confinement results in higher electron heating rate and thereby higher population of energetic electrons ( > 4 eV) with overall Maxwellian distribution. This work led to the extension of the Paschen’s law to include the frequency as another key parameter in the gas breakdown process. In addition, time-resolved measurements showed that reactive species generation is further enhanced in plasmas driven by pulsed microwave. Global simulations taking into account the plasma volume change during the pulse operation have been developed, revealing the role of heavy species (in particular, dimer ions) as energy storage and the enhancement of power coupling efficiency via better impedance matching. These results provide general rule of thumb design principles for the optimization of high pressure plasma sources.