Reflection and Scattering from CO2-Laser-Generated Plasmas

Abstract

The intensity of 10.6-^m radiation scattered from carbon plasmas has been measured as a function of incidence and collection angles (with angular resolution ^1 0 3°), and of laser mode structure. Over an incident intensity range lO^-lO 13 W cm" 2 , total reflectivity was typically ^>8%. At certain angles, reflectivity often showed 100% temporal modulation. The relevance of these results to critical-surface and stimulated-scattering phenomena is discussed.The reflection and scattering of radiation from laser-produced plasmas is a topic of active experimental and theoretical interest. Extensive measurements on a wide variety of targets have been reported, for example, at wavelengths of X = 0.694/ 1.06, 2 " 7 and 10.6 juim. 8 " 15 In most of these experiments the focusing arrangements were such that the target simultaneously encountered a wide range of angles of incidence (0). This Letter describes measurements made with angular resolution higher than those in previous work, 5 * 12 and discusses the exceptionally strong time variation of reflectivity which has been observed with use of multimode lasers and the absence of detectable modulation when a laser is operated on a single transverse axial mode.As shown in Fig. 1, a 75-J, 50-ns pulse from a plane-polarized multimode C0 2 laser of cross section 5 cmx 10 cm was focused by a 7.8-cmdiam (//4.4), 22-cm-focal-length, KC1 lens onto a solid-graphite target. Radiation backscattered into the focusing lens, and the incident radiation, were sampled by a 16% NaCl beam splitter and imaged onto photon-drag detectors PD2 and PDl. Radiation sidescattered into the remaining ~2.07r ster was collected by a spherical copper mirror and focused into a photon-drag detector PD3 lon, Back scattered photon drag PD2 cated behind the target. Variations of absorption with 6 were investigated by placing annular apertures at the laser output window (thus restricting the uncertainty in 101 to 0.65°-1.3°) while the backscattered and sidescattered signals were measured into the collection angles of the lens and mirror, respectively. Similarly, the distribution of radiation scattered into angles cp±Acp was measured (with the full //4.4 cone of radiation incident) by placing a series of annular apertures before the PD2 imaging lens, giving a resolution Acp = 0.4°-0.8°

The heating of polyethylene pellets with kilojoule, microsecond duration, CO2laser pulses

“…(The higher reflectivity of "4% for longer pulses is in agreement with Baldis and Neufeld 1975, who noted greater reflection of the tail of low-energy TEA laser pulses incident on polyethylene targets.) Of published measurements on total scattered and reflected radiation under comparable experimental conditions, Donaldson et al(1976) observed N 8 % losses from a carbon plasma whilst Stenz et aZ(l977) reported 70% losses from a (CHa)n target.…”

Resonance Absorption of 1.06- μm Laser Radiation in Laser-Generated Plasma

“…Measured electron temperature as a function of the angle q>9 between the E vector and the incidence plane.…”