0D model of magnetized hydrogen-helium wall conditioning plasmas

In this paper the 0D description of magnetized toroidal hydrogen-helium RF discharges is presented. The model has been developed to obtain insight into the ICRF plasma parameters, particle fluxes to the walls and the main collisional processes, which is especially relevant for the comprehension of RF wall conditioning discharges. The 0D plasma description is based on the energy and particle balance equations for nine principal species: H, H ⁺, H ₂, He, He ⁺, He ²⁺ and e ^-. It takes into account (1) elementary atomic and molecular collision processes, such as excitation/radiation, ionization, dissociation, recombination and charge exchange, and elastic collisions, (2) particle losses due to the finite dimensions of the plasma volume and confinement properties of the magnetic configuration, and particle recycling, (3) active pumping and gas injection, (4) RF heating of electrons (and protons) and (5) a qualitative description of plasma impurities. The model reproduces experimental plasma density dependences on discharge pressure and coupled RF power, both for hydrogen RF discharges (n _e ≈ (1-5) × 10 ¹⁰ cm ^-3) and for helium discharges (n _e ≈ (1-5) × 10 ¹¹ cm ^-3). The modeled wall fluxes of hydrogen discharges are in the range of what is estimated experimentally: ∼10 ¹⁹-10 ²⁰ m ^-2 s ^-1 for H atoms, and ∼10 ¹⁷-10 ¹⁸ m ^-2 s ^-1 for H ⁺ ions. It is found that experimentally evidenced impurity concentrations have an important impact on the plasma parameters, and that wall desorbed particles contribute largely to the total wall flux.