We have demonstrated small signal gain saturation on several transient-gain Ni-like ion x-ray lasers by using a high-power, chirped-pulse amplification, tabletop laser. These results have been achieved at wavelengths from 139-203 A using a total of 5-7 J energy in a traveling-wave excitation scheme. Strong amplification is also observed for Ni-like Sn at 119 A. Gain of 62 cm(-1) and gL product of 18 are determined on the 4d-->4p transition for Ni-like Pd at 147 A with an output energy of 12 &mgr;J. A systematic evaluation of the laser driver parameters yields optimum beam divergence and small deflection angles of 2-5 mrads, in good agreement with simulations.

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 report an application of the prepulse technique which uses a low-intensity prepulse before the main optical drive pulse to prepare the plasma prior to lasing in low-Z, ¹likeions. ¹likex-ray lasers are now available over a previously inaccessible range of wavelengths. As an illustration of this technique we report an observation of lasing in Cr' + and Fe' + on the ¹likeJ=0 -+1, 3p -+3s transitions 0 at 285 and 255 A as well as gain measurements for Ti' +.PACS number(s): 42.60.By, 32.30.Rj

We report a theoretical equation of state (EOS) table for boron across a wide range of temperatures (5.1×10^{4}-5.2×10^{8} K) and densities (0.25-49 g/cm^{3}) and experimental shock Hugoniot data at unprecedented high pressures (5608±118 GPa). The calculations are performed with first-principles methods combining path-integral Monte Carlo (PIMC) at high temperatures and density-functional-theory molecular-dynamics (DFT-MD) methods at lower temperatures. PIMC and DFT-MD cross-validate each other by providing coherent EOS (difference <1.5 Hartree/boron in energy and <5% in pressure) at 5.1×10^{5} K. The Hugoniot measurement is conducted at the National Ignition Facility using a planar shock platform. The pressure-density relation found in our shock experiment is on top of the shock Hugoniot profile predicted with our first-principles EOS and a semiempirical EOS table (LEOS 50). We investigate the self-diffusivity and the effect of thermal and pressure-driven ionization on the EOS and shock compression behavior in high-pressure and -temperature conditions. We also study the sensitivity of a polar direct-drive exploding pusher platform to pressure variations based on applying pressure multipliers to LEOS 50 and by utilizing a new EOS model based on our ab initio simulations via one-dimensional radiation-hydrodynamic calculations. The results are valuable for future theoretical and experimental studies and engineering design in high-energy density research.

The average-atom model is applied to study Thomson scattering of x-rays from warm dense matter with emphasis on scattering by bound electrons. Parameters needed to evaluate the dynamic structure function (chemical potential, average ionic charge, free electron density, bound and continuum wave functions, and occupation numbers) are obtained from the average-atom model. The resulting analysis provides a relatively simple diagnostic for use in connection with x-ray scattering measurements. Applications are given to dense hydrogen, beryllium, aluminum, and titanium plasmas. In the case of titanium, bound states are predicted to modify the spectrum significantly.

No abstract

kt. LVe denote by Dfc the (complex) Fourier coefficient of frequency f (f = 1, . . . , Nj) for trace t, t = 1, . . . , N, 5 NrNS. Ideally, after they have been corrected for normal move-out (18), all traces corresponding to the same midpoint carry coherent information. If there were no need for statics corrections, all signals, stacked by their common midpoint, should be in phase and yield a maximum for the total powerE(S, R) In Eq. 7, the statics corrections S = (S,, . . . , S,?) and R = (R, . . . , Rhrr) are now considered independent variables. Their ootimum values are found bv maximizing ;he power E. Equation 7 highlights the multimodal nature of E, which, even for relatively lorn-dimensional s and R, exhibits a very large number of local minima. This is illustrated in Fig. 1. To assess the performance of TRUST, we considered a problem Involving N5 = 77 shots and N, = 77 receivers. A data set consisting of N, = 1462 synthetic se~smic traces folded over Ni, = 133 common midpoint gathers was obtained from CognlSeis Corporation (20). The data set uses Nf = 49Fourier comwonents for data reoresentation. Even though this set is somewhat smaller than typical collections obtained during seismic surveys by the oil industry, it is representative of the extreme comwlexitv underlying residual statics problems. T o herive a auantitative estimate of TRUST'S oerformance, let E, denote the total contribution to the stack power arising from midpoint It, and let Bk refer to the upper bound of Ei, in terms of S and R. Using. a oolar coordinates representation for the trace data Djc, that is, Dfc = aft exp(ieufc), we can prove (21 ) thatThe TRUST results, illustrated in Fig. 2, show the significant improvement in the coherence factor of each common gather.This factor is the ratio EJB, and characterizes the overall quality of the seismic image.In conclusion, the TRUST methodology for solving unconstrained global function optimization problems proves to be a powerful tool not only for academic problems; it has the robustness and consistency required by large-scale, real-life applications.

The equation of state (EOS) of materials at warm dense conditions poses significant challenges to both theory and experiment. We report a combined computational, modeling, and experimental investigation leveraging new theoretical and experimental capabilities to investigate warm-dense boron nitride (BN). The simulation methodologies include path integral Monte Carlo (PIMC), several density functional theory (DFT) molecular dynamics methods [plane-wave pseudopotential, Fermi operator expansion (FOE), and spectral quadrature (SQ)], activity expansion (ACTEX), and all-electron Green's function Korringa-Kohn-Rostoker (MECCA), and compute the pressure and internal energy of BN over a broad range of densities and temperatures. Our experiments were conducted at the Omega laser facility and the Hugoniot response of BN to unprecedented pressures (1200-2650 GPa). The EOSs computed using different methods cross validate one another in the warm-dense matter regime, and the experimental Hugoniot data are in good agreement with our theoretical predictions. By comparing the EOS results from different methods, we assess that the largest discrepancies between theoretical predictions are 4% in pressure and 3% in energy and occur at 10 6 K, slightly below the peak compression that corresponds to the K-shell ionization regime. At these conditions, we find remarkable consistency between the EOS from DFT calculations performed on different platforms and using different exchange-correlation functionals and those from PIMC using free-particle nodes. This provides strong evidence for the accuracy of both PIMC and DFT in the high-pressure, high-temperature regime. Moreover, the recently developed SQ and FOE methods produce EOS data that have significantly smaller statistical error bars than PIMC, and so represent significant advances for efficient computation at high temperatures. The shock Hugoniot predicted by PIMC, ACTEX, and MECCA shows a maximum compression ratio of 4.55±0.05 for an initial density of 2.26 g/cm 3 , higher than the Thomas-Fermi predictions by about 5%. In addition, we construct new tabular EOS models that are consistent with the first-principles simulations and the experimental data. Our findings clarify the ionic and electronic structure of BN over a broad range of temperatures and densities and quantify their roles in the EOS and properties of this material. The tabular models may be utilized for future simulations of laser-driven experiments that include BN as a candidate ablator material. (LLNL-JRNL-767019-DRAFT)