Validity of the Quasisteady State and Collisional-Radiative Recombination for Helium Plasmas. I. Pure Afterglows

The set of differential equations describing the time-dependent decay of a singly ionized optically thin monatomic gas has been solved numerically for constant-temperature-constant-pressure helium afterglows. In the range of conditions studied (6000≤Tₑ≤14000^∘{}K, 1.5×10¹²≤nₑ≤5.38×10¹⁵ cm^-3, 1.5×10¹⁴≤n₀≤1.63×10¹⁷ cm^-3) it was found that collisional-radiative recombination can be applied to the electron-density decay before the plasma reaches the quasisteady state. The initial condition for each plasma studied was a Boltzmann distribution from the 2³S state throughout the higher states. The mechanism by which the quasisteady state is obtained is examined and the transient coupling between states is illustrated. Times for the plasma to reach the quasisteady state ranged from t≈10^-8 sec for high-density plasmas to t≈10^-4 sec for low-density plasmas.