E. Jankowska

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.

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...

Thin film absorption characterization by focus error thermal lensing

A simple, highly sensitive technique for measuring absorbed power in thin film dielectrics based on thermal lensing is demonstrated. Absorption of an amplitude modulated or pulsed incident pump beam by a thin film acts as a heat source that induces thermal lensing in the substrate. A second continuous wave collimated probe beam defocuses after passing through the sample. Determination of absorption is achieved by quantifying the change of the probe beam profile at the focal plane using a four-quadrant detector and cylindrical lenses to generate a focus error signal. This signal is inherently insensitive to deflection, which removes noise contribution from point beam stability. A linear dependence of the focus error signal on the absorbed power is shown for a dynamic range of over 105. This technique was used to measure absorption loss in dielectric thin films deposited on fused silica substrates. In pulsed configuration, a single shot sensitivity of about 20 ppm is demonstrated, providing a unique technique for the characterization of moving targets as found in thin film growth instrumentation.

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.

Progress in dense plasma interferometry with table-top soft X-ray laser

We report progress in soft X-ray interferometry of dense plasmas using a very compact 46.9 nm discharge-pumped laser. Soft X-ray interferograms of /spl sim/300 /spl mu/m dimension plasmas generated by /spl sim/0.6 J pulses from a Nd:YAG laser focused into a <30 /spl mu/m diameter spot were recorded and analyzed. The spatial density distribution of the plasma was measured for densities up to /spl sim/6.5/spl times/10/sup 20/ cm/sup -3/.

Advances in dense-plasma interferometry with a tabletop capillary discharge soft x-ray laser

We report an extension of previous tabletop soft x-ray laser interferometry work to plasma densities approaching the critical density. The evolution of line-focus and spot-focus plasmas created with Nd-YAG laser intensities of 0.1 and 7.0 TW/cm2 respectively were studied utilizing a 46.9-nm capillary discharge laser with a diffraction grating interferometer. In the latter case, the electron density was mapped to values up to 0.9x1021 cm-3 (90% of the critical density for the lambda equals 1.06 micrometers pump laser). The interferograms show the development of concave electron density profiles with a minimum on axis and pronounced side lobes. Hydrodynamic model simulations show that the concave profile is the result of the hydrodynamic and radiation effects that enlarge the ablated target area. The measurements exemplify how soft x-ray lasers can be used to probe high density plasmas for the validation of hydrodynamic codes.

Soft x-ray laser interferometer used for dense plasma diagnostics

Laser interferometry allows the recording of the electron density in a great variety of plasmas. However, the absorption and refraction imposes a limitation to the maximum density, plasma size and plasma gradient that can be measured with this technique. The development of compact soft x-ray laser sources gives the opportunity to extend the limits of plasma interferometry, probing plasmas with high densities and steep gradients. We present results of plasma interferometry using an amplitude division interferometer and a table top soft x-ray laser. The interferometer is a modified Mach-Zehnder configuration with diffraction gratings used as beam splitters. The soft x-ray laser is a 46.9 nm capillary discharge table-top laser. The set up was used to probe a laser-created plasma with a temporal resolution of approximately 1 ns and densities up to 6 1020 cm-3.