We report the demonstration of an amplitude-division soft-x-ray interferometer that can be used to generate high-contrast interferograms at the wavelength of any of the saturated soft-x-ray lasers (5.6 -46.9 nm) that are available at present. The interferometer, which utilizes grazing-incidence diffraction gratings as beam splitters in a modif ied Mach -Zehnder conf iguration, was used in combination with a tabletop 46.9-nm laser to probe a large-scale ͑ϳ2.7-mm-long͒ laser-created plasma.

We report what is believed to be the f irst demonstration of soft-x-ray interferometry of a plasma with a tabletop soft-x-ray laser. A Lloyd's mirror interferometer was used in combination with a very compact l 46.9 nm capillary-discharge-pumped laser to map the electron density in the cathode region of a pinch plasma. © 1999 Optical Society of America OCIS codes: 140.7240, 170.7440, 120.3180. Interferometry using optical lasers is a powerful technique for the diagnostics of plasmas that provides time-resolved two-dimensional maps of the electron density. However, free -free absorption and refraction of the probe beam limit the maximum electron density, plasma size, and plasma density gradients that can be probed with optical wavelengths. The development of soft-x-ray lasers has opened the possibility of extending plasma interferometry to shorter wavelengths, significantly expanding the range of plasma parameters that can be probed. Recently, a 15.5-nm Ne-like Y laser pumped by the Nova laser at Lawrence Livermore National Laboratories was used in combination with a skewed Mach -Zehnder interferometer to probe very high-density laser-created plasmas.1,2The amplitude-division interferometer used in those pioneering experiments was implemented by use of thin multilayer beam splitters and multilayer-coated mirrors developed for that wavelength. The advent of gain-saturated discharge-pumped tabletop soft-x-ray lasers 3,4 and the development of several saturated 5 or nearly saturated 6,7 soft-x-ray lasers pumped by relatively compact optical lasers will allow the probing of a great variety of dense plasmas. However, multilayer beam splitters cannot be developed for the wavelengths corresponding to some of these lasers at present, owing to the lack of materials with adequate optical constants. Alternatively, amplitude-division interferometers based on diffraction gratings 8 and wavefront-division interferometers 9,10 have been discussed. Recently, a Fresnel bimirror wave-front-division softx-ray interferometer was demonstrated 10 and was used in combination with a 21.2-nm Ne-like Zn laser to probe surface defects induced by an intense electric field. 11In this Letter we report what we believe to be the first use of a tabletop soft-x-ray laser in plasma interferometry. We have utilized a very compact l 46.9 nm capillary-discharge-pumped tabletop soft-xray laser in combination with a Lloyd's mirror 12 to probe the cathode region of a pinch plasma with subnanosecond time resolution. The maximum plasma electron density n e and size L that can be probed with this laser are significantly larger than those accessible with the fourth harmonic of a Nd : YAG laser at l 265 nm (free -free absorption,~n e 2 L, is significantly smaller at 46.9 nm, e.g., ഠ36 times smaller for a plasma with an electron temperature T e 100 eV). Moreover, larger plasma gradients can be probed at this short wavelength owing to the reduction of ഠ30 times in the refraction angle. A Lloyd's mirror is the simplest possible wave-front-division interfero...

Soft-x-ray laser interferograms of laser-created plasmas generated at moderate irradiation intensities (1 ϫ10 11 -7ϫ10 12 W cm Ϫ2 ) with ϭ1.06 m light pulses of ϳ13-ns-FWHM ͑full width at half maximum͒ duration and narrow focus ͑ϳ30 m͒ 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 twodimensional hydrodynamic behavior is essentially a universal phenomena that is the result of plasma radiation induced mass ablation and cooling in the areas surrounding the focal spot. The understanding of the dynamics of laser-created plasmas is of fundamental and practical interest. The hydrodynamic motion of plasmas created by laser irradiation of solid targets is conventionally known to result in electron density distributions with maximum density along the axis of the irradiation beam. However, in plasmas generated with high irradiation intensities the ponderomotive force has been observed to cause a density depression or cavity in the electron density profile. Early interferometry experiments of lasercreated plasmas at irradiation intensities of 3ϫ10 14 W cm Ϫ2 showed a flattening of the interfering fringes in the subcritical region that, for an axis-symmetric plasma, is indicative of a density depression ͓1͔. Density depressions induced by the ponderomotive force have also been observed in numerous other high-intensity laser experiments, in agreement with simulations. Some of the most recent studies include the formation of plasma channels and laser-hole boring in underdense and overdense plasmas motivated by the fast ignitor concept in inertial confinement fusion ͓2,3͔. These experiments involved laser intensities of 1.7ϫ10 15 and 2 ϫ10 17 W cm Ϫ2 , respectively. In addition, for several cases involving short laser pulses the saturation of the heat flux, refraction, or channeling of the laser radiation due to relativistic self-focusing at ultrahigh fluxes are found to be responsible for density suppression ͓4,5͔.In this paper, we report the observation of a pronounced density minimum on axis in both line-focus and spot-focus plasmas, generated at intensities as low as 10 11 W cm Ϫ2 , which cannot be explained by the ponderomotive force, the effects of laser radiation refraction, electron heat saturation, nor the influence of plasma instabilities. Our case is different, yet universal enough to exist in a wide parameter space.In fact, in retrospective, evidence of these effects may be inferred from published visible laser plasma interferograms mapping much smaller electron densities of the order of ϳ10 18 cm Ϫ3 ͓6,7͔. However, this kind of two-dimensional ͑2D͒ plasma behavior was neither clearly revealed from the data nor understood. In our measurements, the 2D effect is distinctively uncovered by the use of soft-x-ray laser ͑SXRL͒ interferometry, which for the case of the spot-focus plasma experiment allowed us to map the density profile up to ϳ10 21 cm Ϫ3 , close to the critical density. Simulations show tha...

We have used a tabletop soft-x-ray laser and a wave-front division interferometer to probe the plasma of a pinch discharge. A very compact capillary discharge-pumped Ne-like Ar laser emitting at 46.9 nm was combined with a wave division interferometer based on Lloyd's mirror and Sc-Si multilayer-coated optics to map the electron density in the cathode region of the discharge. This demonstration of the use of tabletop soft-x-ray laser in plasma interferometry could lead to the widespread use of these lasers in the diagnostics of dense plasmas.

We report the demonstration of an amplitude-division soft-x-ray interferometer that can be used to generate high-contrast interferograms at the wavelength of any of the saturated soft-x-ray lasers (5.6 -46.9 nm) that are available at present. The interferometer, which utilizes grazing-incidence diffraction gratings as beam splitters in a modif ied Mach -Zehnder conf iguration, was used in combination with a tabletop 46.9-nm laser to probe a large-scale ͑ϳ2.7-mm-long͒ laser-created plasma.