Short wavelength x‐ray laser research at the Lawrence Livermore National Laboratory

Two-Color Soft X-Ray Lasing in a High-Density Nickel-like Krypton Plasma

We report evidence of strong lasing on the 4p-4s transition at 62.7 nm in nickel-like krypton occurring simultaneously with the usual 4d-4p lasing at 32.8 nm. The gain dynamics of both transitions were experimentally and numerically investigated and found comparable. The two-color amplifier was seeded by the same harmonic pulse, therefore producing a short-duration coherent two-color soft x-ray laser pulse. Both transitions offer similar prospects of pulse energy and duration and could lead to the delivery of intense and ultrashort two-color coherent soft x-ray pulses with a controllable delay.

Slippage boosted spectral cleaning in a seeded free-electron laser

The realization of fully coherent light sources at extreme ultraviolet to x-ray region has been a long-standing challenge for laser technologies. While modern single pass free-electron lasers (FELs) hold the ability to produce very intense x-ray radiation on few-femtosecond timescale, the output radiation pulses usually have noisy spectra and limited temporal coherence since the amplification starts from electron noise. A promising way for producing stable transform-limited pulses is based on the harmonic up-conversion techniques with a conventional laser as the seed. However, it is found that the insignificant phase error in the seed laser may be eventually multiplied by the harmonic number, leading to a degradation of the output temporal coherence at x-ray wavelength. Here, we report for the first time on the demonstration of a slippage boosted spectral cleaning technique to mitigate the impact of seed laser induced phase errors and to significantly improve the temporal coherence of a seeded FEL with large phase errors in the seed laser. Experimental results indicate the possibility of generating fully coherent x-ray radiation pulses with this technique.

Study of spectral linewidth in Ne-like Se x-ray laser

Based on the modeling of the geometrically dependent gain coefficient (GDGC) and rate equations, this theoretical study explains the spectral linewidth behavior in an Se x-ray laser operating at 206.4 Å. The study used the reported experimental measurements for an Se target of 6.3 cm in length for the analysis. To obtain a finite value for the amplified spontaneous emission (ASE) intrinsic linewidth that initiates at the threshold length, we subtracted the ASE intensity background from the calculated ASE intensity to derive the corresponding FWHM. For homogeneously and Doppler broadened linewidths of 14 mÅ and 36 mÅ, we calculated the ASE Voigt profile linewidth to be 44 mÅ, initiated at the threshold length of 0.13 cm. The interaction is rather insensitive to the collision broadening; for the homogeneously broadened linewidth up to 21 mÅ, we could still explain the linewidth measurements. In this case, we calculated the related Voigt profile linewidth to be 48.5 mÅ. The current GDGC modeling approach greatly simplified the ASE analyses of these lasers.

X-ray optics of gold nanoparticles

Gold nanoparticles have been investigated as contrast agents for traditional x-ray medical procedures, utilizing the strong absorption characteristics of the nanoparticles to enhance the contrast of the detected x-ray image. Here we use the Kramers-Kronig relation for complex atomic scattering factors to find the real and imaginary parts of the index of refraction for the medium composed of single-element materials or compounds in the x-ray range of the spectrum. These complex index of refraction values are then plugged into a Lorenz-Mie theory to calculate the absorption efficiency of various size gold nanoparticles for photon energies in the 1-100 keV range. Since the output from most medical diagnostic x-ray devices follows a wide and filtered spectrum of photon energies, we introduce and compute the effective intensity-absorption-efficiency values for gold nanoparticles of radii varying from 5 to 50 nm, where we use the TASMIP model to integrate over all spectral energies generated by typical tungsten anode x-ray tubes with kilovolt potentials ranging from 50 to 150 kVp.

Neonlike iron x-ray laser: population kinetics and radiative transfer

In this work, we present collisional-radiative calculations for neonlike iron x-ray lasers. These calculations show the importance of the interaction between the x-ray laser beam and the amplifying medium, which is taken into account in the paraxial Maxwell-Bloch approach. Our calculations are in better agreement with a recent experiment (a prepulse plus two main pulses) on the 3p-3s 0-1 line, than the code EHYBRID which ignores the above interaction. Saturation is attained for plasma lengths near 1 cm, and the calculated effective gain agrees with the experimental value, at least for the first main pulse.

Gain saturation regime for laser-driven tabletop, transient Ni-like ion X-Ray lasers

We have demonstrated small signal gain saturation on several transient-gain Ni-like ion x-ray lasers by using a high-power, chirped-pulse amplification, tabletop laser. These results have been achieved at wavelengths from 139-203 A using a total of 5-7 J energy in a traveling-wave excitation scheme. Strong amplification is also observed for Ni-like Sn at 119 A. Gain of 62 cm(-1) and gL product of 18 are determined on the 4d-->4p transition for Ni-like Pd at 147 A with an output energy of 12 &mgr;J. A systematic evaluation of the laser driver parameters yields optimum beam divergence and small deflection angles of 2-5 mrads, in good agreement with simulations.

Demonstration of transient gain x-ray lasers near 20nm for nickellike yttrium, zirconium, niobium, and molybdenum

We demonstrate strong lasing on the Ni-like 4d(1)S(0)?4p(1)P(1) transition at 18.9, 20.3, 22.0, and 24.0 nm for Mo, Nb, Zr, and Y ions, respectively, using the transient collisional excitation scheme. Approximately 5 J of laser energy in a combination of a 600-ps pulse and a 1-ps pulse from the Compact Multipulse Terawatt (COMET) tabletop laser system is used to irradiate slab targets of these materials. Small-signal gains of 17-26cm (-1) are determined on the 4d?4p transition, with overall gain-length products gL of 11-12. Lasing is observed and gain is measured on the 4f(1)P(1)?4d(1)P(1) transition, which is pumped by collisional excitation combined with self-photopumping, for what is to our knowledge the first time.

High-resolution tunable spectrograph for x-ray laser linewidth measurements with a plane varied-line-spacing grating

We describe a spectrograph for x-ray laser linewidth measurements in the range 100-220 A. The design employs a plane varied-line-spacing grating operating in the convergent light produced by imaging of the entrance slit with a concave spherical mirror. By the appropriate choice of the linear term in the grating-spacing variation, two separate wavelengths can be focused at the same image distance. As a result all wavelengths within the range of interest are focused at or near the same distance. The spectrograph can be tuned by rotation of the grating to bring any wavelength within the range to the center of the focal plane, and the spectra are dispersed on a surface that is erect or practically flat and perpendicular to the principal ray. This allows the use of a planar detector. With a streak camera used as a detector, the instrument obtained time-resolved linewidth data on x-ray lasers with a resolving power of 1 x 10(4) to 2 x 10(4). This paper presents the design methods used to optimize the varied-line-spacing grating, the design of the tunable spectrograph, and the results from the instrument in operation.

Simultaneous Measurement of Local Gain and Electron Density in X-ray Lasers

X-ray lasers (XRLs) have experimental average gains that are significantly less than calculated values and a persistently low level of spatial coherence. An XRL has been used both as an injected signal to a short XRL amplifier and as an interferometer beam to measure two-dimensional local gain and density profiles of the XRL plasma with a resolution near 1 micrometer. The measured local gain is in agreement with atomic models but is unexpectedly spatially inhomogeneous. This inhomogeneity is responsible for the low level of spatial coherence observed and helps explain the disparity between observed and simulated gains.

Nickellike soft-x-ray lasing at the wavelengths between 14 and 7.9 nm

We report what we believe is the first observation of clear soft-x-ray lasing in Ni-like Ag, Te, La, Ce, and Pr and also in Nd covering the spectral range 14.3-7.9 nm. A curved slab target was irradiated with quadruple 1.053-microm laser pulses. The pulse-to-pulse separation for the first three pulses was 400 ps, and that between the third and the fourth pulses was 1.6 ns. The pulse duration and irradiance on the target were 100 ps and ~7 x 10(13) W/cm(2), respectively. For all the targets the most intense lasing was observed at the fourth pump pulse.

Compact high-resolution soft-x-ray spectrograph design using two matched grazing-incidence gratings

A novel and simple soft-x-ray grating-spectrograph design for high-resolution plasma-spectroscopy applications is presented and analyzed. This design uses dual, matched, concave varied-groove-density gratings to provide high dispersion and slit magnification and can achieve a very high spectral resolution (λ/Δλ > 25,000) in an instrument that is less than 1 m in length while using detectors, such as microchannel plates, with relatively coarse spatial resolutions. The advantages of this design over other designs with a comparable spectral resolution include its simplicity and small size, whereas the disadvantages include a narrow useful spectral range (λ/Δλ ≈ 50). Potential applications include x-ray laser linewidth measurements.

Lasing at 7.9 nm in nickellike neodymium

Using a series of 150-ps pulses from the Nova laser to illuminate neodymium slab targets, we observe a 38-psduration lasing pulse at 7.906 nm in nickellike neodymium on the J = 0 ? 1, 4d ? 4p transition. To overcome the low gain in this system, we used a traveling-wave geometry in conjunction with two slab targets that were coupled lengthwise to increase the effective length and whose surfaces were curved to compensate partially for refraction effects.

Significant improvement in the efficiency and brightness of the J = 0-1 19.6-nm line of the germanium laser by use of double-pulse pumping

We describe significant improvement in the efficiency and brightness of the J = 0-1 19.6-nm line of the neonlike germanium laser. A curved slab target was irradiated by two laser pulses of 100-ps duration separated by 300 ps with a total energy of 200-300 J on target. Compared with that obtained by 1-ns single-pulse pumping with 1.1 kJ of energy, the J = 0-1 line has a 12 times higher peak intensity, with a smaller beam divergence of 2 mrad, corresponding to a factor-of-25 improvement in brightness.

Use of the prepulse technique to enhance the weak 18.2-nm laser line in neonlike selenium

We report strong lasing in selenium on the neonlike 3p ? 3s, J = 0 ? 1 transition at 18.2 mn when the prepulse technique of using a low-intensity prepulse before the main optical drive pulse is used to illuminate the selenium target. Without the use of the prepulse, this line is 2 orders of magnitude weaker. The usual J = 2 ? 1 lines at 20.6 and 20.9 nm are observed to dominate the spectra both with and without the use of the prepulse. These experiments may provide important clues to help our understanding of why the 18.2-nm line, which was predicted to dominate the laser output, was always observed weakly in previous experiments.