Laser-induced breakdown in very clean air at 10.6 μm is shown to have a threshold of 3 × 109 W/cm2 for large beams and is described well by microwave breakdown theory. Dust particles are shown to lower the threshold by an amount which is greater for larger particles and higher vapor temperatures. There is a minimum in threshold for large particles, which for 200-nsec pulse length is ≤ 108 W/cm2, corresponding to an energy fluence of 10–20 J/cm2. These results are explained qualitatively by the increase in inverse bremsstrahlung rate caused by exploding particles.
The effects of dust on 1.06-μm laser-induced air breakdown for 100-nsec pulses are reported. The thresholds in clean air were measured as a function of spot size and were found to scale as λ−2 compared to the 10.6-μm results. A single 50-μm carbon particle present in the beam lowered the threshold to ≃5×109 W/cm2 —a factor of approximately 0.02 that of clean air. The fractional reduction in threshold due to dust particles was comparable to that reported for 10.6-μm breakdown.
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