Modeling RF Molecular Hydrogen Plasmas With Coupled EEDF and Plasma Kinetics

Electric propulsion systems are increasingly considering molecular gases due to advantages such as storability and in-situ availability. Energy losses from vibrational excitation and non-equilibrium effects in molecular plasmas can reduce thrust efficiency and strongly influence discharge behavior, making accurate modeling essential. This work presents a zero-dimensional, detailed model for RF-driven hydrogen discharges that incorporates detailed multi-species chemistry, state-to-state vibrational excitation, and electronic excitation of atomic hydrogen, coupled with a time-resolved electron Boltzmann solver. The model is validated against experimental and computational results, including BOLSIG+ calculations. Pulsed radio-frequency (RF) operation is investigated at 100 mTorr for pulsing frequencies between 10 and 100 kHz, revealing strongly non-Maxwellian electron energy distribution functions (EEDF) and vibrational distribution functions (VDF). Using a Maxwellian EEDF leads to higher predicted electron densities. The EEDF and VDF are largely insensitive to pulsing frequency, enabling reduced-order and parameterized descriptions for multi-dimensional simulations. Pulsed operation increases average electron density for the same absorbed power, offering potential benefits for fuel efficiency and low-power operation. These results provide critical insights for the predictive modeling and design of molecular plasma-based propulsion systems and other gas-discharge technologies.