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The temporal evolution of electric breakdown in air at atmospheric pressure by Nd:yttrium–aluminum–garnet Q-switched nanosecond laser pulses was studied from the nanosecond to the millisecond time scale by shadowgraphy and interferometry techniques. The results were modeled with a gasdynamic code with good agreement. It was possible to simultaneously model the whole evolution of the plasma, the shock wave, and the hot core air. The shock wave velocity was determined to be ⩾60 km s−1 at 20 ns. The plasma temperature was found to reach about 1.7×104 K at 1 μs and the hot core air temperature was determined to be <103 K at 100 μs. This letter presents an experimental work that extends the study of laser induced plasmas to millisecond time scales.

This paper is part o f a more general study aimed to the determination o f the best experimental procedures for reliable quantitative measurements o f F e-M n alloys by LIBS. In this work, attention is pointed on the self-absorption processes, whose effect deeply influences the LIBS measurements, reflecting in non-

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Abstract.We report an experimental assessment of the contributions of the shockwave and the hot channel to the production of nitric oxide by simulated lightning. Lightning in the laboratory was simulated by a hot plasma generated with a pulsed Nd-YAG laser. The temporal evolution of electric breakdown in air at atmospheric pressure was studied from the nanosecond to the millisecond time scale by shadowgraphy and interferometry techniques. The shockwave front velocity was determined to be about 60 km s -x at 20 ns and the temperature behind the shock front was estimated to be about 105 K. The production yield of nitric oxide by shock heating is estimated to be: P(NO) (3 4-2) x 10 TM molecule J-• In contrast it was calculated that the production yield of NO by the hot channel is as much as P(NO) = (1.5 4-0.5) x 10 x7 molecule J-X To the extent our simulation is an accurate representation of natural lightning, the hot channel is the dominant region for nitrogen fixation.

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A plasma induced by focusing a 7 ns Nd:YAG laser at 1.06 µm in air at atmospheric pressure was studied by the probe beam deflection method. The evolution of the shock wave and the thermal expansion of the hot air in the core of the decaying plasma were monitored by using this technique. The shapes of the shock and thermal waves were mapped and the velocities were determined as being as high as 528 and 40 m s −1 at 1 µs. In addition, it was also possible to measure the relaxation time required to bring the hot air to room temperature: it was measured to be 5 ms. This technique is versatile, economic, and simple to implement, giving essential information from processes that occur from the microsecond to the millisecond timescale.

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