Measurements of reflection and transmission in a laser-produced plasma
Butler, Thomas Harold
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A high energy laser with a pulse width of 30 nsecs (FWHM) irradiated an aluminum target at an angle 30° from the target's normal. The power density at the target's surface was varied between 10 8 and 10 10 W/cm2 . Measurements were made of the reflective and transmissive characteristics of the ablated surface particles, and from the measured data, an absorption coefficient was inferred. At intensities greater than 10 9 W/cm2 the transmitted beam showed a cutoff approximately 25 nsecs after the start of the laser pulse, indicative of the formation of an overdense plasma. All reflection measurements showed that the scattering was primarily specular. At intensities between 10 8 and 10 9W/cm2 , the reflected beam exhibited a narrow pulse attributable to thermal damage at the target surface. At intensities of 10 9W/cm2 or greater, the reflected beam also indicated a sharp decrease in amplitude attributed to absorption in the newly- formed plasma. The value of the absorption coefficient was calculated to be 0.545.
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