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The Effect of Standing Acoustic Waves on the Formation of Laser-Induced Air Plasmas
Volume 67, Number 3 (March 2013) Page 329-334
STEPHANIE M. CRAIG, KARA BROWNELL, BRENDON O'LEARY, CHRISTOPHER MALFITANO, and JUDE A. KELLEY*
The expected location of an air plasma produced by a focused YAG laser pulse has been found to be influenced by the acoustics of the surrounding environment. In open air, the expected location of a laser-induced air plasma is centered close to the focal point of the lens focusing the laser beam. When confining the same beam coaxially along the interior of a quartz tube, the expected location of the air plasma shifts away from the focal point, toward the focusing lens, in a region of less laser fluence. This shift is caused by an interaction between standing acoustic waves (formed from sound waves produced by previous laser-induced plasmas) and the impinging laser pulse. Standing acoustic waves in a tube produce areas (antinodes) of slightly higher and slightly lower pressure than ambient atmospheric conditions, that in turn have a noticeable affect on the probability of creating an air plasma at a given location. This leads to two observed phenomena: Increased probability of air plasma formation before the optical focal point is reached, and the formation of distinct (separate) air plasmas at the antinodes themselves.
Index Headings: Laser-induced breakdown spectroscopy (LIBS); Plasma; Imaging.