The first harmonics beam generated by an iodine laser system was focused by an f/2 optics on an Al foil target. The X-ray output from the laser plasma both in the line and broad-band spectra was registered over an interval around the “ideal” focus. It was found that the maximum X-ray power is not obtained in the focus itself but for a somewhat larger focal spot outside the focus. To explain this phenomena, temperature and density measurements were in addition made. The plasma temperature evaluated from both the line (He-like Al XII resonant line and j, k, l satellites) and broad-band spectra (two foil method) was also measured and found to be largely constant in the vicinity of the focus. The line and broad-band temperatures differ, the broad-band temperature being about 25% higher. The electron density was equally determined using an intercombination line.
A construction and exploitation of a comparatively large I photodissociation laser system (Perun) is reported. This system was constructed in cooperation between the Institute of Physics of Czechoslovak Academy of Science in Prague and the Lebedev Institute of Physics of Soviet Academy of Science in Moscow. The laser produces subnanosecond pulses of maximum 50 J and 0.5 ns in duration. Although the pumping time by Xe flashlamps is long enough for an acoustic disturbance released in the active medium to introduce an optical inhomogeneity across the whole cross section of the laser tube, the radiation can be focused in a focal spot of a power density exceeding 10 14 W/cm 2 , enough for meaningful laser target experiments both for a laser plasma production or a modification of solid surfaces. The repetition time of the shots is about 10 min. Iodine photodissociation lasersHigh-power iodine photodissociation lasers are based upon the laser transition between the two lowest fine structure levels of I atom with an energy difference 0.94 eV: /*<5 2 Pxn) -1(5 2 P 3/2 ) + M0.94 eV).(1)The wavelength 1.315 /im falls in the near infrared. The transition between the fine structure levels is of magnetic momentum type with a comparatively long radiative life time T S = 0.13 s. An interaction between the electron and nuclear angular momentum of I atom 127 I(5/2) leads to an additional hyperfine splitting of the levels to two groups of sublevels characterized by the quantum number F o f the full angular momentum (see figure 1). As shown in the scheme, the selection rules of the full angular momentum admit at low pressures of the laser mixture six radiative transitions, the strongest of which is the transition from F= 3 upper to F= 4 lower level. Collisional broadening of the hyperfine structure levels causes first an overlap at the lower level (interval of medium pressures). In this case, only two transitions remain. If the pressure is raised even further, the upper hyper-
The second harmonic (SH) signal was monitored from a laser plasma created on an Al foil target by the pulses of an iodine photodisociation laser system. Both the time integrated spectra and the time dependence of the output from a narrow spectral interval in the vicinity of the SH were measured. For a p-polarized wave the spectra indicate regions of resonant absorption and of parametric turbulence. Interpretations of physical mechanisms underlying both the spectral regions are given. The dependence of the SH intensity on the incident laser power density is not clearly understood in all cases, but it seems to indicate a possible role of the plasma density scale length changing with the laser energy. L INTRODUCTIONThe occurrence of SH signal from a laser plasma is an important mdicator of the onset of collisionless absorption mechanisms of the laser energy in the plasma. The emission of the SH occurs in two stages: a narrow peak in the spectrum in the vicinity of the SH frequency, which is more or less mirror reflected from the plasma, indicates an increasingly important role of the resonant absorption; a significant broadening of the SH spectrum at somewhat higher power densities of the laser light on the target is typical of excitation of parametric processes, which contribute to an enhanced SH scattering also in the directions lying outside the mirror reflection angle, . The aim of this contribution is to identify the above mentioned regimes by looking at the SH spectra emittedfrom the plasma generated on an Al foil by 350 ps pulses of the iodine laser system Perun at the Institute of Physics of Czechosl. Acad. Sci. EXPERIMENTALThe iodine system Perun was operated on the first harmonics (1.315 am), the pulses were on average 350 ps long and the energy (W. t) was varied between about 4 and 20 J. The beam was focused by a f/2 optics on an Al foil (30 Mm) target placed inside a vacuum chamber and it was p-polarized. The minimum diameter of the focal spot was about 100 Mm. The light emanating from the plasma was focused by a lens looking at the plasma at an angle of about 50 degrees with respect to the beam axis on the slit of an optical multichannel analyzer OMA 284 consisting of a spectrograph POC4 with a dispersion grating 1302 lines/mm. The field of vision of total spectral width 35 nm is projected on an CCD array of 512 piksels, the resolution being thus 0. 7 A/picksel. Simultaneously, a monochromator SPM2 was used using either an optical 0-8194-1 226-O/93/$4.O0 SPIE Vol. 1980 Iodine Lasers and Applications (1992)! 109 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/30/2015 Terms of Use: http://spiedl.org/terms
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