Two-dimensional effects in laser-created plasmas measured with soft-x-ray laser interferometry

Abstract

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...

Observation of multiply ionized plasmas with dominant bound electron contribution to the index of refraction

“…At these times the fringes shift away from the target, even in the central region of the plasma where the on-axis depression is observed. A similar twodimensional feature was observed in soft X-ray laser interferograms of laser-created Cu plasmas obtained using a 46.9-nm laser [14,15]. In that case the simulations showed that the electron density minimum in the central region of the plasma is the result of pressure equilibrium between the irradiated plasma region and the low temperature side-lobes, created by plasma-radiation ARTICLE IN PRESS Fig.…”

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

“…Some of us have utilized a 46.9 nm capillary discharge Ne-like Ar laser to realize the first demonstrations of plasma shadowgraphy 7 and interferometry with a table-top soft x-ray laser source. [8][9][10][11][12] More recently, a compact laser-pumped transient collisional x-ray laser operating at 14.7 nm was used to obtain interferograms of a laser-created plasma with picosecond resolution. 13 In Sec.…”

The effect of viewing angle on the spectral behavior of a Gd plasma source near 6.7 nm

“…For normal incidence laser irradiation, the spectrum viewed at 45° shows less evidence of absorption at longer wavelengths than that viewed along the surface of the target. As described by Filevich et al [13], EUV emission from laser-produced plasmas creates a cold plasma on the target surface dominated by low ion stages. As the photoabsorption profile of neutral to 3 times ionized Gd consists of giant resonances in the 7−9 nm range with cross sections up to tens of Mb [14][15][16], radiation trapping by low ion stages may considerably reduce the observed intensity in this region.…”