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.