Ignition and maintenance of a cw plasma in atmospheric-pressure air with CO2 laser radiation

The focused output from a high-power cw CO2 laser is used to study the maintenance of a plasma in atmospheric-pressure air. The plasma is initiated externally by an electric spark and maintained by the absorbed laser beam energy. Ignition has also been observed when the laser was focused on a solid target. The maintenance threshold is examined as a function of beam diameter; for beams smaller than 1 mm the threshold power is ∼2 kW. When focused with lenses of 14-cm focal length and shorter, the plasma ball is stationary, situated slightly ahead of the focal point. For this focusing system, the transmission through the plasma was a function of incident power and increased with decreasing laser power. The value of the measured plasma absorption coefficient corresponds to an electron temperature of 14 000°K. When the laser beam was focused with 25-cm—focal-length mirrors or larger, the plasma was not stationary but propagated up the beam at a velocity of ∼10 m/sec toward the laser before extinguishing. The maintenance threshold of the plasma is discussed in terms of the loss channels from the plasma.