Equilibrium composition and the effective adiabatic exponent of a clustering carbon plasma

The equilibrium composition and the effective adiabatic exponent of a low-temperature carbon plasma containing eight components (C, C$_2$, C$_3$, C$_4$, C$_5$, C$^+$, C$_2^+$, and $e$) are calculated. The temperature and pressure ranges are found in which polyatomic clusters form in a gas flow. It is shown that the effective adiabatic exponent of the carbon plasma is a nonmonotonic function of temperature. In the parameter range where the plasma consists largely of polyatomic clusters, the adiabatic exponent is close to unity. It is noted that anomalously low values of the effective adiabatic exponent indicate a considerable concentration of polyatomic molecules in the plasma jet. Such values can be observed in experiments with a low-temperature carbon plasma flowing about bodies.