Particle-in-Cell Simulations With Fluid Metastable Atoms in Capacitive Argon Discharges: Electron Elastic Scattering and Plasma Density Profile Transition

Particle-in-cell/Monte Carlo collision (PIC/MCC) simulations are an important tool for understanding low-temperature plasma dynamics, and benchmark work is needed to build a solid base for the correctness of PIC/MCC codes. In our recent publication (Wen et al., 2021), benchmarking of the object-oriented PIC/MCC oopd1 code was performed against the well-established xpdp1 code for a simplified argon reaction set. Furthermore, oopd1 was upgraded to incorporate the excited state atoms as space- and time-varying fluids. Here, we show more details and perform further analysis of the benchmark work. The plasma density profile transition is further explored; the 'passively' flat plasma density profile in the absence of metastables is found to be parabolic at low pressure and flat at 1.6 and 5 Torr. In the presence of metastable atoms, the 'parabolic' profile at 5 Torr becomes 'flat' at 15 Torr due to the reduced excited state atom density in the discharge center, which decreases the step-wise ionization rates. In addition, the effects of electron elastic scattering, i.e., Coulomb-screening-based non-isotropic scattering, total elastic (and momentum transfer) cross-section-dependent non-isotropic scattering, and momentum transfer isotropic scattering on capacitive discharges, are examined, showing that at a low pressure of 50 mTorr Coulomb-screening-based scattering underestimates the plasma density and electron power absorption by around 15%. However, isotropic scattering and cross-section-dependent non-isotropic scattering give almost the same plasma density and electron power absorption. At a higher pressure of 1.6 Torr, the plasma properties are independent of electron scattering in the presence of metastable atoms. In the absence of metastable atoms, different electron scattering treatments bring a few percent difference for plasma density and electron power absorption.