Early free-fall afterglow of a cylindrical mercury vapor plasma

The free-fall theory for the decay of average electron density and electron temperature in a mercury vapor discharge afterglow presented recently in a letter (Gregory) is improved by the addition of a pressure term for the ions in the second moment equation. The neglect of charge-transfer collisions is justified. The development of the theory is reviewed in detail and a complete series of comparison curves for the measured and calculated electron density decay is presented for four tubes of different diameter.