E. C. Hammarsten

Extremely compact soft X-ray lasers based on capillary discharges

Extremely compact high repetition rate soft X-ray lasers based on capillary discharge excitation have demonstrated average powers of a few milliWatt at 46.9 nm, milli-Joule-level pulse energy, peak spectral brightness several orders of magnitude larger than third-generation synchrotron beam lines, and excellent spatial coherence. Examples of the use of a capillary discharge soft X-ray laser in dense plasma diagnostics and laser ablation of materials are summarized.

Application of extremely compact capillary discharge soft x-ray lasers to dense plasma diagnostics

Table-top capillary discharge soft x-ray lasers combine the advantages of a small size and a high repetition rate with an extremely high brightness similar to that of their laboratory-size predecessors. When utilized to probe high density plasmas their short wavelength results in a higher critical density, reduced refraction, decreased free-electron absorption, and higher resolution as compared to optical probes. These characteristics allow the design of experiments capable of measuring the evolution of plasmas with density–scale length products that are outside the reach of optical lasers. This paper reviews the use of a 46.9 nm wavelength Ne-like Ar capillary discharge table-top laser in dense plasma diagnostics, and reports soft x-ray laser interferometry results of spot-focus Nd:YAG laser plasmas created at moderate irradiation intensity (∼7×1012 W cm−2) with ∼13 ns pulse width duration laser pulses. The measurements produced electron density maps with densities up to 0.9×1021 cm−3 that show the devel...

Advances in capillary discharge soft x-ray laser research

This paper gives an overview of recent soft x-ray laser research at Colorado State University. Progress related to capillary discharge source development includes the observation of emission from the 13.2-nm laser line of Nickel-like Cd in a plasma column generated by a high power capillary discharge. This result suggests it might be possible to extend capillary discharge lasers to significantly shorter wavelengths. In another approach to the generation of coherent soft x-ray radiation we analyzed the possibility of amplifying high order harmonic pulses in a discharge-pumped amplifier. The study of the already well- characterized 46.9-nm Ne-like Ar laser was extended with new spatial coherence and laser wavefront measurements, in work conducted in collaboration with U. California Berkeley and U. of Paris-Sud groups. In the field of applications, we have extended our previous results of plasma interferometry with a tabletop laser to plasma densities up to 0.9 x 1021 cm-3. Sequences of soft x-ray laser interferograms of plasmas generated by a Nd-YAG laser at intensities between 1 x 1011 W cm-2 and 7 x 1012 W cm-2 show the development and evolution of a concave electron density profile. The detailed mapping of this phenomenon with soft x-ray interferometry exemplifies the usefulness of compact soft x-ray lasers in increasing the understanding of high density plasmas.

Excitation of the 13.2 nm laser line of Nickel‐like Cd in a capillary discharge plasma column

Emission at 13.2 nm from the 4d 1S0 −4p 1P1 laser transition of Ni‐like Cd was observed in a cadmium vapor plasma column excited by a high‐current capillary discharge. The dynamics of the plasma were studied using time‐resolved soft x‐ray pinhole images and hydrodynamic simulations. The pinhole images show that the plasma column maintains a good symmetry up to the time of maximum compression. Fifty‐five Cdxxi lines were identified in the 12.7–18.4 nm region with the assistance of calculations performed using the Slater‐Condon method with generalized least‐squares fits of the energy parameters.

Soft X‐ray Laser Interferometry/Shadowgraphy of Exploding Wire Plasmas

We present the first results from soft x‐ray laser interferometry measurements of current‐driven thin wire explosions obtained using a capillary discharge pumped 46.9 nm laser and an amplitude division interferometer based on diffraction gratings. We have obtained series of high‐resolution soft x‐ray interferograms/shadowgrams that depict the initial stage of the evolution of exploding Al wires 15 μm and 25 μm in diameter. The images show a dense vapor core that completely absorbs the probe beam during the initial part of the explosion, and a surrounding plasma shell where both a shift of the interference fringes and partial absorption of the soft x‐ray laser probe beam are observed. The excitation of the 25 μm diameter wires at a current rate of 30 A/ns is observed to result in the uniform expansion. However, an increase of the rate of energy deposited per unit mass is observed to give rise to significant instabilities. The expansion velocity of the wire core was determined from the variation of the meas...

Interferometric studies of laser-created plasmas using compact soft x-ray lasers

We summarize results of several successful dense plasma diagnostics experiments realized combining two different kinds of table-top soft x-ray lasers with an amplitude division interferometer based on diffraction grating beam splitters. In the first set of experiments this robust high throughput diffraction grating interferometer (DGI) was used with a 46.9 nm portable capillary discharge laser to study the dynamics of line focus and point focus laser-created plasmas. The measured electron density profiles, which differ significantly from those expected from a classical expansion, unveil important twodimensional effects of the dynamics of these plasmas. A second DGI customized to operate in combination with a 14.7 nm Ni-like Pd transient gain laser was used to perform interferometry of line focus laser-created plasmas with picosecond time resolution. These measurements provide valuable new benchmarks for complex hydrodynamic codes and help bring new understanding of the dynamics of dense plasmas. The instrumentation and methodology we describe is scalable to significantly shorter wavelengths, and constitutes a promising scheme for extending interferometry to the study of very dense plasmas such as those investigated for inertial confinment fusion.

Soft x-ray laser interferometry/shadowgraphy of exploding wire plasmas

We present results from soft x-ray laser interferometry/shadowgraphy of current-driven thin wire plasmas using a capillary discharge pumped 46.9 nm laser and a diffraction grating interferometer. We have obtained series of soft x-ray images of exploding Al wires 15 μm or 25 μm in diameter. The high photo-ionization cross sections of atoms and low charge ions at this wavelength allow the soft x-ray laser probe to detect the early stages of formation of a coronal plasma surrounding the wire core. Wires of 25 μm diameter excited by current pulses with a 78 A/ns increase rate are observed to expand uniformly. However, an increase in the rate of energy deposited per unit of mass is observed to give rise to significant instabilities. The simultaneous analysis of the fringe shift and soft x-ray absorption data suggests the coronal plasma contains a significant concentration of low Z impurities. The results illustrate that table-top soft x-ray lasers are a powerful new tool for the diagnostics of dense plasmas.

Extremely compact capillary discharge‐based soft x‐ray laser development and application to dense plasma diagnostics

We give an overview of recent capillary discharge‐driven soft x‐ray laser development experiments and applications at Colorado State University. We report the demonstration of the first desktop size soft x‐ray laser, a capillary discharge Ne‐like Ar soft x‐ray laser that was measured to emit laser pulses with energy up to 10 μJ at 46.9 nm. In relation to the development of capillary discharge lasers at shorter wavelengths, spectra of the highly ionized cadmium plasmas identified strong emission from the 4d1S0− 4p1P1 laser transition of Ni‐like Cd at 13.16 nm. We have also demonstrated optical guiding of intense laser pulses in the plasmas of an Ar capillary discharge, a result that is of interest for the development of efficient longitudinally pumped collisional soft x ‐ray lasers in a gaseous media. In term of applications, we summarize our continued progress in establishing capillary discharge lasers as a compact soft x‐ray source of coherent radiation for dense plasma diagnostics. The combination of a ...

Density depression in laser‐created plasma unveiled with table‐top soft x‐ray laser interferometry

Soft x‐ray laser interferograms of laser‐created plasmas generated at moderate irradiation intensities (1×1011 − 7×1012 W cm−2) with λ = 1.06 μm light pulses of ∼13 ns FWHM duration and narrow focus (∼30 μm) reveal the unexpected formation of an inverted density profile with a density minimum on axis and distinct plasma sidelobes. Model simulations show that this strong 2‐dimensional hydrodynamic behavior is essentially a universal phenomenon that is the result of plasma radiation induced mass ablation and cooling in the areas surrounding the focal spot. These measurements, which mapped plasma densities up to 0.9×1021 cm−3, demonstrate the use of a portable soft x‐ray laser interferometer as a high resolution tool for the study of high density plasma phenomena and the validation of hydrodynamic codes.

Table-top soft X-ray laser interferograms of dense laser-created plasma

We report soft X-ray interferograms that illustrate the use of a very compact table-top soft X-ray laser in mapping the electron density evolution of a dense plasma. The spatial density distribution of a point-focus laser-created plasma with /spl sim/300 /spl mu/m dimension was measured for densities up to /spl sim/6.5 /spl times/ 10/sup 20/ cm/sup -3/.