We have directly probed the conditions in which the Ni-like Pd transient collisional x-ray laser is generated and propagates by measuring the near-field image and by utilizing picosecond resolution soft x-ray laser interferometry of the preformed Pd plasma gain medium. The electron density and gain region of the plasma have been determined experimentally and are found to be in good agreement with simulations. We observe a strong dependence of the laser pump-gain medium coupling on the laser pump parameters. The most efficient coupling occurs with the formation of lower density gradients in the preformed plasma and when the duration of the main heating pulse is comparable to the gain lifetime ͑ϳ10 ps for mid-Z Ni-like schemes͒. This increases the output intensity by more than an order of magnitude relative to the commonly utilized case where the same pumping energy is delivered within a shorter heating pulse duration ͑Ͻ3 ps͒. In contrast, the higher intensity heating pulses are observed to be absorbed at higher electron densities and in regions where steep density gradients limit the effective length of the gain medium. A detailed understanding of the plasma that constitutes the gain medium is crucial for the development of efficient x-ray lasers. Use of the prepulse technique has allowed x-ray lasers to achieve saturated output using many different elements for the lasing media ͓1͔. However, even the best laser-pumped x-ray lasers typically have an efficiency of 10 −6 . In the transient collisional excitation ͑TCE͒ scheme a low intensity long pulse preforms a plasma, which is allowed to expand and cool before being heated by a high intensity short pulse ͓2͔. This short pulse, in some cases with subpicosecond duration, rapidly heats the plasma to generate a high gain coefficient, saturated x-ray laser output ͓3͔, and x-ray laser pulses as short as 2 ps ͓4͔. In experiments reported on high power laser drivers the pulse duration of the short pulse generated by chirped pulse amplification ͑CPA͒ is in the range of 0.3-3 ps ͓3-7͔. It has been assumed to some extent that by maximizing the intensity of the main heating pulse the temperature, collisional pumping, and local gain coefficient will also be maximized. Under these conditions the lowest saturated wavelength currently demonstrated is 7.3 nm for Nilike Sm ͓5͔.To improve the efficiency we need to better understand the laser-plasma coupling and plasma characteristics of the x-ray laser media. In this paper we combine the techniques of near-field imaging with recently developed picosecond x-ray laser interferometry ͓8͔ to characterize the lasing medium for a Ni-like Pd x-ray laser. It is observed that a combination of controlling and reducing the plasma density gradients while matching the duration of the main pumping pulse to the gain lifetime at a specific density optimizes the coupling efficiency. This increases the x-ray laser output by an order of magnitude over the case where the same pumping energy is delivered into a higher intensity, shorter pulse. In contr...
We have reported the first observation of large soft-x-ray amplification in a discharge-created plasma. A gain coefficient of 0.6 cm " at 46.9 nrn was measured in a Ar-H 2 mixture, while higher laser intensities were reported in pure argon. It was later realized that the fraction of H 2 in the gas mixture experiments was, due to incomplete mixing of the gases, smaller than the 1:2 ratio reported, and amounted to less than 10%. Subsequent experiments have confirmed that larger amplification occurs in pure argon discharges, resulting in gain coefficients of up to 1.1 em -1.
We have reported the first observation of large soft-x-ray amplification in a discharge-created plasma. A gain coefficient of 0.6 cm " at 46.9 nrn was measured in a Ar-H 2 mixture, while higher laser intensities were reported in pure argon. It was later realized that the fraction of H 2 in the gas mixture experiments was, due to incomplete mixing of the gases, smaller than the 1:2 ratio reported, and amounted to less than 10%. Subsequent experiments have confirmed that larger amplification occurs in pure argon discharges, resulting in gain coefficients of up to 1.1 em -1.
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...
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