Structure of the Plasma Fireball Produced by a CO2 Laser

The space and time resolved structure of a helium plasma produced with a repetitive CO2 laser is studied during the first 15 μsec of the afterglow period. The spectra of several neutral and ionized helium lines are used in the determination of the density and temperature profiles of the luminous fireball. It is found that the plasma is comprised of a dense, hot core which emits primarily ionic lines, and a well-defined tenuous outer shell which is primarily the source of neutral emission lines. This “two-component” plasma structure develops at ∼0.4 μsec after breakdown, at about the time when the luminous fireball dissipates its expansion energy and comes to a virtual standstill.