Search

It is shown that the production of X-ray emission spectra in the interaction of high-intensity laser radiation with cluster tar- gets may be affected by the bichromatic oscillating electric field arising from the generation of the second harmonic of laser radia- tion. A technique is proposed for diagnosing harmonic generation in laser – cluster interactions using the spectral line profiles of multi- ply charged helium ions. The efficiency of second harmonic genera- tion at a laser intensity of 3 × 1018 W cm–2 is shown to amount to about 2 %.

The shape of the X-ray pulse generated by picosecond laser plasma is experimentally studied. The unusual phenomenon was experimentally observed for the first time for targets made of moderate-heavy chemical elements, namely, the pulse of hard X-ray radiation generated by laser plasma at the laser radiation flux of ~1018 W cm-2 had a longer duration than the pulse of softer X-ray radiation. A simple kinetic model is suggested for explaining this fact. We have suggested a method for controlling the temporal shape of X-ray pulse emitted by laser plasma by varying the contrast of laser pulse.

The X-ray spectra of multiply charged ions were recorded from planar agar (C12H18O9)n based targets with an average density of 2 mg cm-3 irradiated by high-power laser pulses (λ=1.054 μm, τ=2.5 ns, I ≈ 5 ×1013 W cm-2). The spectra were recorded with a high spectral and spatial resolution employing spherically bent (focusing) crystals of mica and quartz. An analysis of the experimental data obtained by the irradiation of Al2O3-doped agar samples allowed us to determine the main parameters of the plasma produced inside the targets. The ion temperature of plasma in low-density porous targets was estimated for the first time to be 1.5 — 2 times higher than the electron temperature.

The first observation of x-ray emission spectra of multiply charged ions in the plasma produced by a 35-fs laser pulse with an intensity up to 1017 W cm-2 in CO2 and Kr gas jet targets is reported. The emission in the wavelength ranges of the 1snp—1s2(n=3–6) transitions of O VII ions and the Lyα line of O VIII ions, as well as of the (2s1/22p63p3/2)1—2s22p6 1S0 and (2s1/22p63p1/2)1—2s22p6 1S0 lines of Ne-like KrXXVII ions testifies that the highly ionised plasma is formed by collision processes in clusters. Modelling the shape of the spectral lines of oxygen ions by including the principal mechanisms of broadening and absorption in optically dense plasmas reveals that the main contribution to the time-integrated intensity is made by the plasma with the parameters Ne=(2–20)×1020 cm-3 and Te=100 — 115 eV.

Complex spectral structures located between the resonance lines of H- and He-like MgXII and MgXI ions were recorded in experiments on plasma heating by the radiation of a low-power short-wavelength excimer XeCl laser (12-ns pulses with an energy of 2 J). The above spectral structures were shown to arise from transitions in the so-called hollow multicharged ions, i.e., in ions with an empty 1s-shell, which were previously observed in laser produced plasmas only with ultrahigh-power femto- and picosecond laser facilities having extremely high-contrast laser pulses.

The first precision measurements of the wavelengths and identifications were made of the satellites of the Heb line of the Si XIII ion in the emission spectrum of a laser plasma. These satellites were the result of radiation decay of the 1s3l12l2 levels of the Si XII ion. The wavelengths were determined more accurately for the 1s3l13l2 configurations. The experimental results obtained were compared with calculations carried out by various methods. The structure of the relative intensities of the 1s3l13l2 satellites indicated that their emission occurred mainly in a thin overdense plasma region with Ne>Necr.

Observations were made of expansion, in a transverse quasisteady magnetic field (B ≈ 10 T), of a Teflon plasma generated by Nd laser radiation (λ = 1.06 μm, E ≈ 6 J, and τ ≈ 1ns). Time-integrated two-dimensional images of the plasma radiance were obtained in the spectrum of continuous and line x-ray radiation and spectroholiograms were recorded of 21P1—11S0 and 23P1—11S0 lines of the He-like F VIII ion. The plasma radiation was found to be anisotropic relative to the expansion axis and the dependence of the intensity on the distance from the target changed as a result of the influence of the transverse magnetic field on the plasma dynamics.

The first precision (with an error of 0.4–1.6 mA) measurements were made of the wavelengths of x-ray spectral lines emitted as a result of n=4 — n'=2 transitions in the Ne- and Na-like Kr XXVII and Kr XXVI ions. The spectra were recorded by means of a spectrograph with a spherically bent mica crystal, characterised by a spectral resolution λ/Δλ ≈ 5 × 103. The dispersion characteristic of this crystal was determined with the aid of reference lines, which were emitted by the He-like Ar XVII ion. The wavelengths of a series of dielectronic satellites of the resonance line of the Ar XVII ion were identified and measured. The results obtained were compared with calculated data.

A focusing spectrograph with a spherical crystal was used to obtain two-dimensional images of an expanding laser plasma which transmitted monochromatic x-ray radiation from a plasma source. Experiments characterised by a high spectral (λ/Δλ ~ 104) and spatial (Δr < 10 μm) resolution showed that the brightness of the shadow images depended on the x-ray absorption mechanism and on the parameters of the investigated plasma. The method is suitable, in principle, for determination of the plasma parameters.

An investigation was made of the interaction of a nanosecond laser pulse with a 'puff' gas target at gas densities sufficient for the formation of a critical-density plasma. The parameters of the laser plasma formed in the central interaction region and at its periphery were determined by x-ray spectroscopic methods. The formation of a plasma channel was observed and it was attributed to self-focusing of the laser radiation in the ionised gas.

Various x-ray spectroscopic methods were used to investigate a plasma generated by the interaction of short-wavelength excimer laser radiation pulses (12 ns, 0.308 μm, 4×1012 W cm-2) with flat targets. A comparison of the profiles and intensities of a number of observed spectral lines of the H-like, He-like, and Li-like sodium, magnesium, and aluminium ions with the results of the reported calculations made it possible to determine the spatial distributions of the laser plasma parameters up to distances of ~0.4 mm from the target surface. These parameters were compared with the predictions of a simple theoretical model of the absorption of short-wavelength radiation in a plasma. This showed that the absorption of such radiation (with the intensities used in this investigation) involved inverse bremsstrahlung in regions with an electron density much lower than the critical value.

The emission spectra of a high-temperature recombining laser plasma and of an X-pinch plasma were used in high-precision (± 0.03–0.05 pm) measurements of the wavelengths of the Al XII lines resulting from the 1snp1P1—1s2 1S0 (n = 6–12) transitions. The lines due to transitions from the 1s11p and 1s12p levels of the He-like Al XII ion were observed for the first time. The experimental results were compared with calculations carried out by several theoretical methods.

An experimental x-ray spectroscopic investigation was made of the stabilisation of a Z pinch with a composite load in the GAEL (225 kA, 50 ns) accelerator. Kinetic calculations of the intensities of the L lines in the spectrum of copper were used for the first time to determine the parameters of a plasma [Ne ? (1?2)?1020 cm-3, Te ? 300 keV] formed by injection of a cylindrical aluminium-plasma jet imploding onto an axial copper wire. Quantitative and qualitative analyses were made of the stabilisation of the Z pinch with a composite load. Spectroscopic methods were used to demonstrate the stabilising influence of the plasma jet. The results indicated that it should be possible to construct a homogeneous high-temperature plasma column.

X-ray spectroscopic methods were used in an investigation of changes in the ion composition of an aluminium laser plasma in the course of its expansion. Plasma diagnostics was based on the intensities and profiles of the 3p—1s and 1snp—1s2 (n = 6–10) spectral lines of H-like and He-like Al XIII and Al XII ions. The greatest changes in the charge state of the plasma occurred quite close (less than 0.5 mm) to the target.

X-Ray spectroscopy methods were used in an investigation of the spectra of plasmas formed from Mg, Al, Ge, Mo, and Ta targets by heating with iodine laser radiation of power density (3-5)x10{sup 14} W cm{sup {minus}2}. In the case of Ta targets the presence of Ta{sup 45+,46+} ions was detected. This information should help in the development of laser-plasma sources of multiply charged ions. The spectra were identified and the plasma parameters were measured. 18 refs., 4 figs., 3 tabs.

An investigation was made of a plasma formed by heating a gas target with neodymium gas laser radiation of ~15 GW power. The working substance was gaseous SF6. The x-ray emission spectra of sulfur and fluorine ions were investigated with one- and two-dimensional spatial resolution. These spectra were recorded by a high-resolution high-luminosity spectrograph with a mica crystal bent to form a sphere with a radius of R = 10 cm. Laser breakdown of a gas puff produced a sufficiently compact near-spherical plasma formation of ~2.5 mm size and with the electron temperature at least 470 eV.

Precision measurements were made of the wavelengths of the satellites of the resonance line of the H-like Mg XII ion as well as of a number of transitions in the Ne-like Ge XXIII ion and their satellites in a plasma created by radiation generated in an Nd-glass laser. The measurements were carried out with a high-resolution, high-luminosity spectrograph containing a mica crystal bent to a spherical surface with a radius R = 100 mm. The absolute wavelength obtained for the 2s22p6 1So—2s2p63p 3P1 transition in Ge XXIII was λ = 842.08 ± 0.09 pm. The results demonstrated that the short-wavelength component of the Lyman doublet of Mg XII (λ = 841.92 pm) could be used to pump optically a short-wavelength laser by transitions in the Ge XXIII ion.

A study has been made of the x-ray emission spectra of zinc ions (λ ~0.65–1.18 nm) excited in a plasma produced by the beam from a Nd laser. The wavelengths were measured within ±0.3 pm. The observed spectra have been identified. Detailed calculations have made it possible to identify, for the first time, some spectra lines due to the radiative decal of autoionizing states of Na-like zinc ions.

We have investigated the structure of the radiating region of an x-pinch plasma source by means of a new physical apparatus: a glass-capillary converter. We show that this converter unambiguously reproduces an image of the structure of the dense, hot x-pinch plasma in the soft-x-ray region of the spectrum. In comparison with standard pinhole cameras, this device lowers the intensity of hard x-radiation by two orders of magnitude and increases the image contrast. A new method is proposed for investigating the time evolution of the spatial distribution of the soft x-rays intensity of plasma sources.

The x-ray emission spectrum of a magnesium plasma produced by a picosecond laser has been studied. Dielectronic satellites of the resonance line of the He-like Mg XI ion have been identified for the first time. These satellites result from radiative transitions from 1s2l2l'3l'' autoionizing levels of the Be-like Mg IX ion.

The properties of a magnesium plasma heated by picosecond laser pulses have been determined by x-ray spectral methods. Experiments were carried out at a laser power density ~ 1.5 1018 W/cm2. The x-ray spectra were detected by spectrographs with a plane CsAP crystal and a mica crystal bent into part of a spherical surface 10 cm in radius. The experimental data are compared with predictions of a calculation on the time-varying kinetics of multiply charged magnesium ions.

The L spectra of Ni and Cu ions produced in an X-pinch plasma have been recorded. A fast-optics Johann spectrograph and detection with spatial resolution were used to observe these spectra both in the hot, dense plasma and in the relatively cold recombining plasma expanding outward from the "hot spot." The wavelengths of a number of spectral lines of multiply charged nickel and copper ions were measured with relative error Δλ/λ ~ 10−4. A number of new lines of Na-like satellite lines were identified in the spectra of these ions.

It has been shown experimentally that a source based on a plasma produced by a picosecond laser is extremely promising for systematic research on the satellite structures of multiply charged ions which have electrons in L or M shells. The combination of the unique characteristics of this source and the particular measurement apparatus used (with a spectral resolution Δλ/λ~10−4) has made it possible to refine the wavelengths of several transitions of Mg IX and X ions which had been identified previously, to identify for the first time ten spectral lines due to 1s2p4l → 1s24l and 1s2p3l → 1s23l transitions of the Mg X ion, and to measure the wavelengths of 47 spectral lines which have tentatively been attributed to the Be-like ion Mg IX.

A high-transmission spectrograph equipped with a spherical mica crystal was used to study x-ray emission from an X-pinch plasma at high spectral (?/?? ~ 3000) and spatial (~30 ?m) resolutions. H- and He-like spectra of Al and Si ions generated in the X-pinch were detected both in the stage of maximum compression and in the dispersal stage. The electron temperature (up to 950 eV) and density (up to 2 ? 1024 cm?3 ) of the plasma at the hot point and also in the dispersal stage were measured by x-ray spectral methods. Energy estimates show that the radiation output in the K lines can reach 1 J (the corresponding power is 1 GW). The overall energy radiated in the wavelength region 0.5 to 10 nm is ~1 kJ. This result indicates that the radiation from the hot point of an X-pinch plasma can be an effective pumping source for the active medium of an x-ray laser. It also indicates that the recombining plasma dispersing out of the hot region can itself be used as an active medium for short-wavelength lasers.

The expansion of laser plasmas of the elements Be, Al, Cu, and Pb in a static transverse magnetic field B≤2.7 T has been studied experimentally. The plasma was produced by a ruby laser at a power density no greater than 1012 W/cm2. For all the elements, a characteristic "two-petal" expansion pattern is observed. At a certain distance from the target, the two petals coalesce into a narrow jet. The radius of curvature of the petals and the position of the coalescence point depend on the field strength and the atomic weight of the element.

Crystallographic and dispersion characteristics of mica at high (V—XI) reflection orders were investigated for the first time. Even the order-XI integral reflection was sufficiently strong to be used in experiments. High-precision bending of a mica crystal along a spherical surface with a small radius R = 100 mm and the use of such crystal analysers in focusing spectrographs with one-dimensional and two-dimensional spatial resolution made it possible to record low-intensity spectra of multiply charged ions in laser and Z-pinch plasmas. The method provides exceptional spectral resolution λ/Δλ ≈ 6000–10 000 in a wide spectral range (0.15–1.8 nm).

Two-dimensional x-ray images were obtained for a laser-generated plasma jet as it collided with an obstacle and expanded in a static magnetic field. A mica crystal bent to form a spherical surface of 100 mm radius was used. The images were formed by the radiation representing the resonance and inter-combination lines of the Mg XI ion. A shock-compressed zone formed near the obstacle and the expansion cone of the Mg XI ions became narrower in a transverse magnetic field. The spatial resolution in the plane of the image was better than 100 μm for objects of large linear dimensions (~10 mm).

The generation of collimated intense monochromatic beams of soft x radiation in the wavelength interval 0.4–1.0 nm from an X-pinch is reported. This is the first such report. High-quality mica crystals with dimensions of 10 × (30–35) mm were used to form beams with an energy of 2–3.2 μJ, a wavelength spread Δλ/λ=4 10−3, and a divergence of 5 10−4 rad. The mica crystals were bent into spherical surfaces with a radius of curvature of 10 or 25 cm. The characteristics of the resulting beams are compared with those of the beams from Ta lasers, with a wavelength ~4.5 nm, which are the shortest-wavelength x-ray lasers which have been reported to date. This comparison shows that the beams obtained in the present study are better than those from the Ta laser in terms of several characteristics (divergence, wavelength, and efficiency), while they are worse (but not greatly so) in terms of certain other characteristics (wavelength spread and energy in the pulse. It is thus possible today to solve many practical problems involving the use of collimated intense monochromatic beams of soft x radiation in the wavelength interval 0.25–2.0 nm. These problems can be solved with the help of the x radiation from an X-pinch or from plasmas produced by picosecond or femtosecond table-top lasers and short-focal-length, large-aperture crystal mirrors.

A method is proposed for obtaining spectra with a high spatial resolution, as high as 0.5 μm. The method involves fabricating a Bragg–Fresnel lens on the surface of a mica crystal. The lens is formed in the direction perpendicular to the dispersion direction. A structure of this sort has been fabricated and has been used to record spectra of the plasmas of Z- and X-pinches. Bragg–Fresnel lenses on crystal surfaces are of much interest for studying the dimensions of the x-ray emission of multiply charged ions formed in plasmas heated by picosecond- and femtosecond-range lasers.

An x-ray spectroscopic determination was made of the distribution of the plasma electron temperature Te(χ) in a channel formed by the interaction of a nanosecond laser pulse with a dense gas target. The distribution Te(χ), combined with the plasma channel parameters measured earlier, could be used to develop and check the validity of a theoretical model of the observed effect.

A dense jet of a plasma consisting of multiply charged ions was generated in the interaction of a laser plasma with a strong external axial magnetic field. Images were formed by spectral lines and the soft x-ray spectrum range of the plasma jet was obtained with a large-aperture spectrograph containing a mica crystal bent to form a spherical surface with a radius of R = 10 cm. A tenfold increase in the density of the He-like Mg XI plasma, compared with a freely expanding plasma, was observed at a distance of 5 mm from the target.

A description is given of a method in which a point-like transmission diffraction grating is used in the absolute determination of the spectrum of soft x-ray radiation. This method is intended for the investigation of targets with an internal coupling-in of laser radiation in the ISKRA-5 facility. The results are given of the determination and reconstruction of the spectrum of soft x-ray radiation. The calculated and experimentally determined spectra of soft x-ray radiation are compared and the results are shown to be in satisfactory qualitative and quantitative agreement.