Measurements of local and remote energy deposition by fast electrons have been made in 1.3 mu m and 1.05 mu m laser irradiation experiments with plane targets and various pulse lengths. From optical and X-ray streak photography and spatially resolved K alpha yield measurements it is found that up to 30% of the absorbed laser energy spreads laterally to distances of several millimetres from the focal spot with a spreading velocity in excess of 108 cm s-1.
The total and specular reflectances of 1.06-and 0.53-Min laser pulses were measured on planar copper targets. The measurements were made with 30-ps pulses and an intensity on target of 10^* W/cm^, The direction of polarization and angle of incidence (0°-70°) were varied. Only a weak dependence of reflectance on wavelength is found; at both wavelengths reflectance is governed by the characteristic angle and polarization dependence of resonance absorption.Absorption and reflection of intense laser radiation on solid targets is a basic problem in laserfusion studies. A crucial parameter in this respect is the wavelength of the laser. Up to now, most laser plasma experiments have been made with the CO2 (10.6 /im), Nd (1.06 /im) and iodine lasers (1.315 /im). No systematic studies, however, have been undertaken in the shorter-wavelength region, although considerable effort is being made to develop visible and near-uv lasers. In this Letter we present measurements of the total and specular reflectances at 1.06 and 0.53 Mm and pulse duration of 30 ps. Since recent measurements^"^ suggest that resonance absorption is an important absorption mechanism, we have studied in detail the dependence of reflectance on the angle of incidence and polarization of the incident laser light. Much attention has been paid to ensuring identical experimental conditions at both wavelengths to allow comparison of the results.The laser is a Nd: YAIG (neodymium-doped yttrium aluminum garnet) system delivering 0.7 J in 30 ps. A high-contrast Pockelscell-Glan prism combination (extinction ratio 2x10^) guarantees a clean pulse (prepulses are < 6 /i J). For measurements at 1.06-/im isolation against back-reflected light and rotation of the polarization is achieved by two Faraday rotators; the resulting energy on target is 300 mJ. For 0.53-/im generation we use a potassium dihydrogen phosphate type-II crystal of 51 mm in diameter and 17 mm in length. Its conversion efficienty is 40%, giving 200-300 mJ on target. The polarization here is rotated with a X/2 plate. At both wavelengths the polarization is better than 95%. The laser light is focused on target by an achromatic l:8/400-mm lens. The focal spot was measured by focusing through pinholes.^ It contains 50% of the energy in a spot 40 Mm in diameter, resulting in an averaged intensity on target of 3 x lO^' W/cm^ Our 30-ps pulse, the total angle of the lens of T, and a Rayleigh range of 1 cm will enable us to approach a plane geometry in our experiments. Note that with t = 30 ps and an estimated temperature of T^ = 400 eV the plasma expands over a distance L^ =t{Zk^TjmiY^'^^A Mm during the laser pulse, ice., a distance much smaller than the focal-spot diameter. The experiments were done on Cu targets, made from highly polished hemicylindrical steel rods with 2-Mm-thick Cu coatings.It is known that one obtains in general, upon reflection from a plane target, three components of reflected laser light with characteristic angular distributions^: (i) a well-collimated, specular beam (i^spec) with a di...
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