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

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Abstract
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.