X-ray Laser Line Research Articles

The effect of process parameters on residual stress evolution and distortion in the laser powder bed fusion of Ti-6Al-4V

Laser powder bed fusion (LPBF) is a popular additive manufacturing (AM) process that has shown promise in fabricating novel components that can be utilized for a wide variety of applications. However, one of the main drawbacks of LPBF is that it produces large thermal gradients and fast cooling rates during the solidification of each layer, which can lead to large levels of residual stress/distortion, sometimes resulting in build failure/rejection. In the present work, several experimental techniques (x-ray diffraction, hole drilling, contour method, and laser line profilometry) were utilized to establish the effect of LPBF process parameters (scan speed, laser power, build height, build plan area, and substrate condition) on residual stress evolution and distortion. X-ray diffraction and hole-drilling measurements were performed on the surfaces of the LPBF deposits and substrates, while bulk residual stresses were measured using the contour method. In addition, a laser line profilometer was used to measure the distortion after fabrication. The results obtained by the non-destructive and destructive measurement techniques suggested that process parameters greatly influence the development of residual stress and distortion throughout the LPBF deposit and the substrate. Furthermore, the experimental results in this work provide a valuable foundation for future modeling and simulation of the evolution of residual stress and distortion.

Gain Coefficients for Possible 4d-4p X-ray Lasers of Ni-like Nd, Sm, Eu, Gd, Ta and W Ions

Populations of the 249 levels belonging to the ground configuration 3s23p63d10 and those of single excitations of a 3d, 3p and 3s electron to the orbitals of n=4 and 5 have been calculated for Nickel-like Nd, Sm, Eu, Gd, Ta and W ions for temperatures from 300 to 2000 eV and densities of 1.0  1021 and 1.0  1022cm-3. The gain coefficients for possible x-ray laser lines have also been calculated at these plasma conditions. The possible laser lines are transitions from levels 1 3/2 3/2 0 (3d 4d ) and 1 5/2 5/2 0 (3d 4d ) to levels of 1 3/2 1/2 1 (3d 4p ) , 1 5/2 3/2 1 (3d 4p ) and 1 3/2 3/2 1 (3d 4p ) . Estimation is made on the optimum plasma condition for x-ray laser gain.

X-ray laser near 13.5 and 11.3 nm in Xe26+ driven by an intense pump laser interacting with xenon cluster jet as a promising radiation source for nanolithography

The model of highly efficient (yield ~0.1–1% of pumping energy) monochromatic radiation sources near λ = 13.5 and 11.3 nm, intended for commercial nanolithography is presented. The sources are based on x-ray lasers on four transitions: 3p53d104l–3p53d104l′ (λ near 13.5 nm) and self photo-pumped 3d 94f (J = 1)–3d 94d (J = 1) (λ ~ 11.3 nm) transition of Ni-like xenon (Xe26+). New inversion mechanisms are discussed. A sufficiently intensive pumping pulse interacting with xenon cluster jet forms plasma. Optimal plasma parameters and quantum yields are predicted for each x-ray laser line. At pump intensities ⩾1018 W cm−2, immediately after the interaction, plasma is formed in the state with dominating Xe26+ ions. This condition is absolutely necessary for the short-lived lasing at 11.3 nm and for the wavelengths of laser transition measurements.

X-ray laser lines in nickel-like erbium

Energy levels of 249 excited levels in nickel-like erbium are calculated using the 3s23p63d10 as a ground state and the single electron excited states from n = 3 to n = 4, 5 orbitals, calculations have been performed using FAC code (Gu. Astrophys. J. 582, 1241 (2003). doi:10.1086/344745 ). The populations are calculated over electron densities from 1020 to 1023 cm−3 and electron temperatures 1/2, 3/4 of the ionization potential of Ni-like Er. The gain coefficients of the transitions are calculated.

Zeeman splitting of laser-driven soft x-ray laser line by the enhancement of magnetic field in plasma

We observed a laser-driven plasma soft x-ray laser (SXRL) line at the wavelength of 18.895 nm from the nickel-like molybdenum ions under the existence of external magnetic field. In this experiment, 4 mm-diameter magnetic coil driven by an electrical pulsed power supply provided the external magnetic field of 15 T along the direction of the plasma column, and we could obtain the left-handed circular and right-handed circular polarization components separately. The experimental Zeeman splitting indicated that the strength of the magnetic field was enhanced by a factor of more than 10 than that of the applied field, which implied that the compression of magnetic field occurred in the dynamics of intense laser-plasma interaction.

High-resolution Spectroscopy of the Nickel-like Molybdenum X-ray Laser Toward the Generation of Circularly Polarized X-ray Laser

We attempted the first measurement of the spectral width of the nickel-like molybdenum x-ray laser (<TEX>${\lambda}\;=\;18.895\;nm$</TEX>) by use of a high-resolution spectrometer in order to determine the strength of the magnetic field required for the generation of a circularly polarized x-ray laser. The spectral width was measured to be <TEX>${\Delta}{\lambda}\;=\;18\;m{\AA}$</TEX> under the substantial lasing condition. The magnetic field required for the generation of a circularly polarized x-ray laser was 40 T. The splitting of the x-ray laser line was clearly obtained under 15 T external magnetic field. The strength of the magnetic field estimated from the splitting of the x-ray laser line was large compared with the external magnetic field. It implies that there might be an alternative mechanism for enhancement of the magnetic field in the gain medium plasma.

A coupled gain calculation and output characteristics of the two Ne-like 2s and 2p hole X-ray laser lines

In most collisionally pumped X-ray lasers the lasing transitions considered are a result of collisional excitation of a Ne-like ground state 2p electron into a 3p excited level. However, there are suggestions of producing plasma conditions for collisional pumping of the inner-shell 2s electrons into highly excited 3s levels, with lasing arising as a 2p electron fills the 2s hole. Simulations on various Ne-like ions such as germanium, krypton, and yttrium, using collisional pumping, to get gain on the inner-shell transitions at 62, 50.2, and 42 Å , respectively, have shown gains on the inner-shell transitions up to 30 cm - 1 . It has been suggested that the large Doppler linewidth associated with shorter wavelengths is responsible for the smallness of the small-signal gain in the inner-shell transitions relative to than that predicted for the more usual 2p-hole transitions. However, experimental investigations of this 2s-hole inner-shell laser line, using collisional pumping technique, were unable to register any output. In this paper we report the result of calculations of the gain and the total spontaneous emission rate for the 2s-hole and the 2p-hole X-ray laser lines using a coupled four level model. It is shown that the small-signal gain of the 2s-hole inner-shell transition decreases with increasing pumping rates of the 2p-hole upper and lower levels. The output characteristics of the Ne-like inner-shell transition is simulated using this four-level model and the effects of saturation of the 2p-hole line on the 2s-hole transition is studied showing that the saturation of the former may have a severe effect on the output of the later.

Line shape and ray trace calculations in saturated X-ray lasers: Application to Ni-like silver

Longitudinal coherence length in X-ray lasers depends strongly on the shape of the amplified line. We have modelled an experiment performed at the LULI facility of Ecole Polytechnique. The experiment was devoted to the study of the temporal (longitudinal) coherence of the transient Ni-like silver 4d–4p transition X-ray laser at 13.9 nm. Accurate line shape calculations using PPP, a spectral line shape code, confirm that the Voigt profile is a good approximation for this X-ray laser line. This allows us to extensively use the Voigt shape in conditions where the amplifier, i.e. the plasma produced by the interaction of a high intensity laser with a slab target, is neither stationary nor homogeneous. Our calculations involve a ray trace code which is a post-processor to the hydrodynamic simulation EHYBRID. As the effect of saturation is important for the level populations and gains we include the interaction between the amplified beam and the medium using the Maxwell-Bloch formalism. While the FWHM of the spontaneous emission profile is ∼10 mÅ, the amplified X-ray line exhibits gain narrowing leading to the smaller width ∼3 mÅ. Comparison with experiment is discussed.

Generation of Attosecond X-Ray Laser Pulses

We propose to generate as-X-ray laser pulses by beating of two or more X-ray laser lines with a frequency separation in the range of 10/sup 15/ Hz. We focus on nickel-like X-ray lasers, some of which have a few almost equidistant laser gain lines with an appropriate difference frequency. It is shown that in the case of three or more lines, these can be phase-locked by means of a Langmuir wave generated in the gain medium at a suitable electron density.

Nickel-Like Samarium 4d−4p X-Ray Laser Gain inLaser Plasmas

Populations of the 249 levels belonging to the groundconfiguration 3s23p63d10 and those of single excitations ofa 3d,3p or 3s electron to the orbitals of n = 4 and 5 have beencalculated for nickel-like samarium at temperatures from 300 to 1600 eV andat densities from 1020 to 1022 cm−3. The gaincoefficients for possible x-ray laser lines are also determined underthese plasma conditions. The possible laser lines are transitions from levels(3d3/2−14d3/2)0 and (3d5/2−14d5/2)0 tolevels (3d3/2−14p1/2)1, (3d5/2−14p3/2)1and (3d3/2−14p3/2)1. The favourable plasma condition forx-ray lasers is at the temperature of about 800 eV and at the densityof approximately 5.0×1021 cm−3.

Study of XUV beam splitter flatness for use on a Michelson interferometer

A XUV Michelson interferometer has been developed by LIXAM/CEA/LCFIO and has been tested as a Fourier-transform spectrometer for measurement of X-ray laser line shape. The observed strong deformation of the interference fringes limited the interest of such an interferometer for plasma probing. Because the fringe deformation was coming from a distortion of the beam splitter (5 × 5 mm2open aperture, about 150 nm thick), several parameters of the multilayer deposition used for the beam splitter fabrication have been recently optimized. The flatness has been improved from 80 nm rms obtained by using the ion beam sputtering technique, to 20 nm rms by using the magnetron sputtering technique. Over 3 × 3 mm2, the beam splitter has a flatness better than 4 nm rms.

High-Precision Measurement of the Wavelength of a Nickel-like Silver X-ray Laser

We conducted high-precision measurements of the wavelength of a 4d1S0→4p1P1 line of a nickel-like silver X-ray laser. The Lyman series lines of hydrogen-like helium ions emitted from low-density plasmas were used as wavelength references, and the wavelength of the X-ray laser line was determined to be 13.887 nm (±0.002 nm). The experimental results were compared with Multiconfiguration Dirac–Fock calculations and were found to agree with theoretical wavelengths.

Two-dimensional imaging and modeling of the 147 nm Ni-like Pd x-ray laser

At the Compact Multipulse Terawatt laser facility we produced saturated output on the Ni-like Pd x-ray laser line at 14.7 nm, using a 2-J, 600-ps prepulse followed 700 ps later by a 6-J, 6-ps drive pulse to illuminate a 1.25-cm-long Pd slab target. We used the LASNEX code to calculate the hydrodynamic evolution of the plasma and provide the temperatures and densities to the CRETIN code, which then made the kinetics calculations to determine the gain. Experimental measurements of the two-dimensional near-field and far-field images are presented, and these are compared with the simulations.

Energy levels and transition probabilities for possible X-ray laser lines of highly charged Ni-like ions

The 45 low-lying energy levels and the transition probabilities among these levels connected with possible X-ray laser lines are calculated for Ni-like ions from Z=74 to 84. In the calculations, main correlation, relaxation and relativistic effects are considered. The calculated energy levels and transition probabilities are compared with some experimental and theoretical results. Finally, the contributions of lots of weak correlations on the transition wavelengths and probabilities are discussed by including partly single and double excitations from the 3 l inner-shells into the 4 l and 5 l orbital layers.

X-ray laser line narrowing: new developments

The idea of using an optical laser to decrease the Stark width of hydrogenic spectral lines from a plasma and hence increase X-ray laser gain, proposed some 8 years ago, is first applied in a full calculation of the narrowing of the LiIII L- α line, which is of interest to recombination optical field ionization X-ray lasers. Next, the concept is analysed and the mechanism is explained in detail. Finally, it is demonstrated that the idea is also applicable to nonhydrogenic lines and hence of interest to current X-ray laser work. The main point of this idea is that for any line it is possible to reduce its Stark width by using an optical laser.

Theoretical predictions for the polarization of X-ray laser lines in the presence of a directed beam of hot electrons

The 19.6 nm laser line due to a 3p - 3s, J=0-1 transition in Ne-like Ge ions was found to be polarized parallel to the target plane with a degree of polarization of -53 % by Kawachi et al. (1995). They intepret it as a consequence of a an unbalanced population of the 3s magnetic sublevels. Kieffer et al. (1993) have shown that, during plasma heating, some electrons could be accelerated preferentially in the same direction as the pump laser. In the present work, we shall show that such anisotropic electrons can give a small difference in populations of the 3s sublevels. The 3s level spontaneous emission to the ground level is linearly polarized and can be reabsorbed by the Ne-like ion ground state which amplifies the population of the 3s levels, as well as it increases the different sublevel populations. The 3p-3s X-ray laser amplification depending on the 3s level population becomes polarized but with an opposite sign to the absorbed radiation from the ground level.

Gauss broadening of X-ray laser lines by electric potential fluctuations inside atomic nuclei

The broadening of X-ray laser lines by fluctuations of electric potential inside corresponding nuclei is studied. The fluctuations are produced by neutron-proton and proton-neutron transformations when localization of electric charge inside heavy nuclei changes. Atomic (and ionic) bound electrons feel such fluctuation as non-central Coulomb potential. It is shown that the potential fluctuations have the Gauss distribution while nuclei are considered as drops both of normal and Fermi-liquids. Finally, an electron level asquires the Gauss broadening. Estimations show that the linewidth can be of the same order as the Doppler width for highly-ionized plasmas ions.

Reduced absorption of neon-like bromine X-ray laser radiation in helium

We have measured the absorption of the 19.47-nm neon like bromine (J=2–1) X-ray laser line in low-pressure helium. The experiment was motivated by the coincidence of this line with the low-absorption wing of an autoionizing transition in helium. We observe that, with 1 mbar of helium, the continuum background and another bromine X-ray laser line at 19.82 nm are strongly reduced, enhancing the relative strength of the 19.47-nm laser line. Increasing the helium pressure to 1.5 mbar makes the continuum virtually disappear, resulting in an almost monochromatic emission of the X-ray laser line. An estimate of the absorption cross section for the 19.47-nm line is given as ≈3.9×10-19 cm2 and for the nearby continuum as 0.9–1.3×10-18 cm2.

Lasing on the self-photopumped nickel-like4f1P1→4d1P1x-ray transition

We demonstrate lasing on the nickel-like ${3d}^{9}4f{}^{1}{P}_{1}\ensuremath{\rightarrow}{3d}^{9}4d{}^{1}{P}_{1}$ x-ray laser line in Zr $(Z=40),$ Nb $(Z=41),$ and Mo $(Z=42)$ and present measured wavelengths for these ions as well as predicted values for ions from $Z=36$ to 54. Lasing on this line was first predicted two years ago and has now been achieved. Unlike the usual collisional excitation laser, this line lases because radiation trapping allows a large radiation field to build up on the ${3d}^{10}{}^{1}{S}_{0}\ensuremath{\rightarrow}{3d}^{9}4f{}^{1}{P}_{1}$ resonance line and populate the $4f$ upper laser state by the self-photopumping process. For Ni-like Mo a gain of 13 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is measured on this transition at 226 \AA{} for targets up to 1 cm long.

Wavelengths of the Ni-like4d1S0–4p1P1x-ray laser line

We measure the wavelengths of the Ni-like ${3d}^{9}4d{}^{1}{S}_{0}--{3d}^{9}4p{}^{1}{P}_{1}$ x-ray laser line in several low-Z Ni-like ions ranging from Y $(Z=39)$ to Cd $(Z=48).$ These wavelengths are compared with optimized level calculations using a multiconfiguration Dirac-Fock code. With the help of these results, we identify this line to very high accuracy in nonlasing plasmas from As $(Z=33)$ to Mo $(Z=42).$ Accurate values of these wavelengths are essential for performing plasma imaging and interferometry experiments with multilayer optics that use the x-ray laser to backlight other plasmas. These results also provide important atomic data that are currently missing about the energy of the $4d{}^{1}{S}_{0}$ level in the Ni I sequence.