On the similarity laws for microwave discharges in helium at medium and high pressures

Temporally-resolved two-dimensional numerical simulations of microwave discharge dynamics in helium at medium and high pressures are carried out using an extended fluid model. The range of applicability for the similarity laws is analyzed. It is shown that, at the end of the breakdown phase at the center of the plasmoid, with pressure changes from 47.5 Torr to 95 Torr and then to 190 Torr, the maximum of electron density changes by a factor of 3.4 and 3.2, which yields a deviation from the similarity laws within 15%. With an increase in pressure to 285 Torr and 380 Torr, variations in the electron density at the end of the breakdown phase already lead to a 20% error, and with an increase in pressure to 760 Torr, an error of 40% is observed. It has been shown that with increasing pressure, various scenarios of self-organization of plasma structures in the discharge region are observed: diffuse, diffuse with filaments, strictly filamented, and diffuse forms with multiple filaments. This indicates a violation of the similarity laws, which is associated with both the kinetics of elementary processes and thermophysical and gas-dynamic processes.