Soft X-Ray Laser Interferometry of Colliding Laser-Created Plasmas in Semicylindrical Cavities

Abstract

Abstract-A tabletop capillary discharge soft X-ray laser was used to acquire high-contrast interferograms that map the evolution of dense aluminum plasmas created by the laser irradiation of a 500-μm-diameter semicylindrical cavity with 120-ps optical laser pulses of ∼1.1 × 10 12 − W · cm −2 peak intensity. The measured electron density maps, which were compared with simulations, show that the plasma converges on axis where it collides to form a localized region with density exceeding 1 × 10 20 cm −3 .Index Terms-Colliding plasmas, plasma diagnostics, soft X-ray laser, soft X-ray laser interferometry. P LASMAS created by intense laser pulses irradiating solid targets usually expand in directions that are perpendicular to the target surface, along the strong pressure gradients that are generated by the high temperatures and densities near the wall. For concave target geometries, this direction of expansion can force the laser-created plasma to converge into a focal region where it can collide with itself. For example, the laser heating of a semicylindrical cavity (half hohlraum) accelerates plasma toward the axis of the cavity where plasma interactions can range from stagnation to extended interpenetration depending on the degree of collisionality of the plasma [1]. Detailed electron density maps are very useful for understanding the dynamics of these colliding plasmas and for benchmarking plasma simulation codes. However, they are difficult to obtain by using optical laser probes due to the presence of strong density gradients that deflect the probe beam. Soft X-ray laser interferometry is a powerful plasma diagnostic tool that has the ability to access very high densities and large scale length plasmas with a reduced refraction and free-free absorption of the probe beam, allowing for the generation of detailed maps of the electron density in high-density plasmas [2], [3].Interferograms were obtained by using a Ne-like Ar 46.9-nm tabletop capillary discharge soft X-ray laser probe beam [4] and an amplitude division soft X-ray interferometer [3]. The probe beam was configured to deliver pulses of ∼1-ns duration and ∼0.15 mJ of energy. The good spatial coherence of this laser assists the generation of interferograms with high fringe visibility. The skewed Mach-Zehnder interferometer uses goldcoated diffraction gratings as beam splitters and grazing incidence gold-coated mirrors to achieve a high throughput of ∼6% per arm. The dense colliding plasma was created by focusing 600-mJ pulses of 120-ps duration from a λ = 800 − nm Ti: Sapphire laser onto the surface of a semicylindrical target. The laser beam was smoothed by a vacuum spatial filter, and it was shaped into a 310-μm-wide 1.5-mm-FWHM-long line focus by a pair of cylindrical lenses. The targets consisted of 500-μm-diameter semicylindrical cavities machined onto the edge of a 1-mm-thick aluminum slab. The targets were placed along the zero order arm of the interferometer. The plasmas were imaged with 25 times magnification onto a microchannel-plate/CCD detec...

Design of a compact device to generate and test beams with orbital angular momentum in the EUV

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Design of a phase-shifting interferometer in the extreme ultraviolet for high-precision metrology

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