Absorption of bound states in hot, dense matter

z z zyxw zyxw Preprint UCRL-JC-143053 Absorption of Bound States in Hot, Dense Matter R. Shepherd, P. Audebert, C. Chenais-Popovics, J.P. Geindre, M. Fajardo, C.Igiesias, S.Moon, F. Rogers, J.C. Gaufhier and P. Springer zyxw zyx zyxwvut This article was submitted to gthInternational Workshopon Radiative Propertiesof Hot Dense Matter, Santa Barbara, California, October29 - November 2 , 2000 March 6,2001 U.S. Department of Energy Laboratory 7 7 Approved for public release; further dissemination unlimited DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. 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Springer a ’?*, zyxwvuts zyxwvu zyxw zyxwvu ’) aPhysics and Advanced Technologies, Lawrence Livernore National Laboratory, P. 0. 808, Livemore Ga., USA,94550. Laborafoire pour 1’Utilisation des Lasers Intenses, UMR7605, CNRS - CEA - Universitd ~ d r i sVI - Ecole plytechnique, 91 128 Palaiseau, France Abstract zyxwvut Preliminary experiments usinga long pulse laser generated X-ray source to backlight a short pulse laser heated thin foil have been performed at the Laboratoire pour l’utilisation des LasersIntenses (LULI) at Ecole Polytechnique in France. In these experiment, a 2 J, 300 ps, 532 nm laser was used to create the X-ray back-lighter. The primary diagnostic was a von Hamiss spectrograph coupled to a 500 fs X-ray streak camera (TREX-VHS) developed at LLNL - This diagnostic combines high collection efficiency(M lov4 steradims) with fast temporal response ( FZ 500 fs), allowing resolution of extremely transient spectral variations. The TFWXVHS was used to determine the time history, intensity,and spectra3 content of the back-lighter. The second diagnostic, Fourier Domain Interferometry (FDI), provides information about the position of the critical densityof the taxget and thus the expansion hydrodynamics, laying the ground work for the plasma characterization. The plasmas were determined to be moderately to strongly coupled, resulting in absorption measurements that provide insight into boundstates under such conditions. Key words: Strongly coupled plasma, opacity, short pulse laser * Corresponding author. [email protected] Also with: Grupo de Lasers e Plasmas, Instituto Superior Tkcnico, 1 Avenida Rovisco Pais, Lisboa, Portugal 1 zyxwvutsrq zyxwvu zyxw Introduction The development of high intensity, short pulse lasers offer a new way to study high energy-density plasmas[11. When short pulse, <1 ps, lasers with intensities in the range of 1015 to 10l6 W/cm2are used to illuminate solid targets impulse heating followed by rapid heat conduction produces a high density, high temperature plasma with little hydrodynamic expansion. Although transient, the plasma parameters generated by heating solids with short pulse lasers can result in physical states that are difficult to achieve using other techniques. In these plasmas, the temperature is relatively low (< 100 eV) while the density remains its initial d u e , in the present case close to solid. The result is a state where the interparticle shielding becomes insufficient to neutralize the charge of the individual ions, a state that can be characterized by the dimensionless parameter rii . It is defined as the ratio of the parttcle potential energy to the kinetic energy, zyxwvut zyxwvu zyxw zyxwvu where 2 is the average charge state, T the electron temperature and N . the ion density. When rii approaches 1the ions become correlated and the bound states begin to overlap in such a way that it is difficult to predict ionization balance. Using absorption spectroscopy, one can infer the ionization balance in the plasma and compare these measurements to code predictions. Further, the detailed shape of the absorption features provide information about the effects of contiuum lowering and collisional broadening of upper states that are partially filled. X-ray and laser heated targets have been used for studying bound state absorption in hot, denseplasmas in the past[4][5][6].Additionally, recent experiments have utilized ultra short pulse lmers for point-projection spectroscopy o€expanding plasmas,providing space and time resolved ionformation on the average ionization[7]. These experiments have shown a need for experimental data to test complex absorption theories used to model radiation transport. Here we report on a new experimental technique to study theionization balance in dense, moderate to strongly coupled plasmas. The technique uses an ultra short pulse laser to create a thin, high-density plasma slab fairly uniform in density and temperature. A temporally long X-ray pulse is used to backlight the slab and the time resolved absorption spectra is gathered with an X-ray streak camera. The plasma characteristics were inferred horn Fourier Domain Interferometry (FDZ) measurements of the expansion velocity of the critical surface. The measured expansion velocity was compared to simulations to extract the plasma parameters. We show results from a proof of principle experiment. The experiment demonstrates the ability to measure absorption zyx zyx zyx zyxwvuts zyxwvuts zyxwvu zyxw zyxwvu potassium hydrogen phthalate (KAP) crystal interfaced to a fast X-ray streak camera arranged in the von Ham& geometry[ll]. The X-rays emitted from the samarium back-lighter were focused into the 25.4 mrn long, 120 p m wide slit of the X-ray streak camera. The spectralcoverage was from 7.4 A to 8.4 A, looking at the K, aluminum absorption features between the He-like and Flike ions. The signal brightness wits increased with an image intensifier and displayed using a fiber optic coupled, 1024 x 1024 CCD camera. Assuming a 150 p rn upper bound on the swept spatial resolution, the spectral resolution was estimated to be 0.006 8, providing a resolving power of E/AE=1200-1400. The streak camera used in the experiment had a 500 fs temporal resolution and was operated with a sweepspeed of 1.42 ps/mm, resulting in 36 ps of temporal data per shot. As indicated in Fig. I, the back-lighter was oriented such tha.t t.he emitted X-rays propagated through the foil and were viewed normal to foil surface. Optical quality smoothness was required on the target surface to perform the FDI measurements. The low cost of silicon wafers and the ability to readily etch silicon in it controlled manner made it an excellent choice for producing thin foil substrates for targets. This was achieved by coating 25081 of Si3hT4 on one side of a 500 p m thick silicon wafer. After coating, a mask containing twelve 2 x 2 mm square openings spaced by 500 p m was placed on the silicon side of the wa€er. Within each 2 x 2 mm area, the exposed silicon was etched away completely, leaving only an optically flat 250A Si3Nd window within each of the twelve 2 x 2 mm square openings. The windows were next coated with 500 of aluminum and 150 A of carbon ontop of the aluminum, sandwiching the aluminum between the carbon and SidN3 to reduce the initial expansion. The back-lighter targets were coated with 1 p m of samarium along the edge of a 50 mm long aluminum wedge. The samarium coated edge was placed 3 mm away from the rear surface of the foil target. A 3 Data and Analysis The phase shift data gathered with the FDI was used to determine the expansion velocity of the reflecting surface. Knowing the expansion velocity one can infer the density, temperture, and thickness of the absorbing layer by matching the experimental phase shifts to phase shifts calculated using hydrodynamic simulations performed using the 1-D hydrodynamic codes FILM[12] and LASKEX[13]. The expansion velocity,the transmitted and incident backlighter intensities, were measyred on seperate shots where the laser intensity was simular. 3.I zyxwvutsrq zyxwvu zyx zyxwvut zyxwvuts zyxw Fourier Domain Interferometry Phase shift data obtained with the FDI diagnostic are displayed in Fig. 3. The time dependent phase of the reflected light was simulated using the hydrodynarnicsxode FILM. The measured phase shift data for three shots axe shown in Fig. 4. The measurements are compared with phase shift values calculated using FILM, showing very good agreement. The good agreement found with FILM deconvulved phase data suggest that the plasma follows the dynamics displayed in Fig. 5, where we show the temperature and density conditions its a function of time at the center of the foil. Further simulations were performed using the hydrodynamics code LASNEX. In Fig. 6 we show the corrsponding NLTE and LTE average ionization and the parameter rii. The simulations clearly show that the plasma departs from LTE at late times. 3.2 X-ray Absorption Data The high sweep-speed and change in X-ray path length with dispersed energy produces a measurable difference in the arrival of the low and high energy Xrays. This effect is illustrated in Fig. 7, where we have illuminated the samarium back-lighter with the 300 fs laser at low irradiance. When corrected, the result is a temporal sweep across the spectral range of interest. This effect limits the temporal window to = 13 ps that simultaneously contains 0-like to Belike charge states. In Fig. 8 we show typical data from the samarium backlighter. The back-lighter was generated using a 532 nm, 500 ps pulse focused to W/cm2. The spectral structure is dominated by "shell unresolved transition array (UTA) emission. By post-processing hydrodynamic data from the code LASNEX with the atomic physics package STA (Super Transition Array) 1141, the UTAs were identified as emission from the 4 3 manifolds of V-like samarium around 8.2 A and the 5-3 manifolds of Ni-like, Cu-like, and Zn-like samarium. The temporal vaiation of the spectral window of strongest emission was found to be 20 % over the duration recorded. The spectral structure was found to be consistant from shot to shot. In Fig. 9, we show dat,a plots at delays estimated as 0, 4,8 and 12 ps after the heating of the foil. For spectral smoothing,the reduced data was averaged over 750 fs. No timing fiducial was used on the streak camera so the heating time was estimated from the absorption features in the data. Due to uncertainty in the thermal background of the CCD, the absolute magnitudeof the measured absorption could not be determined. At t=O, the 0-like absorption feature has been clipped slightly due to the spectralsweep mentioned above. At this early period, the only features that-are destinctive the data are those of the Be-like and Li-like charge states. The data clearly shows a significant amount of Be- 5 zyxwvutsrq zyxwvu Acknowledgment zyxwv This work was performed under the auspices of the U S . Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. One of us (M.F.) was financed through MCT (Portugal) under contract PRAXIS XXI/BBD/13732/97. References zyxwvutsr zyxwvut zyxw Murnane MM, Kapteyn HC, Falcone RW. Phys Rev Lett 1989;62:155. Mancini R.C., et al. Phys Rev E 1996;54:4147. zyxwvuts Eidmann K., et al. Jour. of QSpec Rad Trans 2000;65:1?3. Back C.A., Chenais-Popovics C., Renaudin P., Geindre J.P., Audebert P., Gauthier J.C. Phys Rev A 1992;46:3405. Perry T.S., et al. 1996;54:5617. Merdji H., et al. Phys Rev E 1998;57:1042. Fajardo M, et dl. Phys Rev Lett 2001;86:1231. Strickland D, Mourou G. Opt Cornmun 1985;56:219. Rebibo, S et ul. To be published. [lo] Blanc P., et al. J.O.S.A. B 1996;13:118. [Ill von Hamhs L.: Z. Kristallog. 1939;101:17. [12] Teubner U.? et aE. Phys Plasmas 1996;3:2679. [13] Zimmerman G, Kruer W. Comments Plasma Phys Controlled Fusion 1975;2:5. [14]A. Bar-Shalom?J. Oreg,W.H. Goldstein,D. Shvartqand A.Zigler, Phys. Rev. A 1989;40:3183 [lS] Iglesias C.: Rogers F. Astrophy. J 1996;464:943. zyxwvu zyxwvuts zyxwv Fig. 1. Layout for the experiment Fig- 2. Setup geometry for the Fourier Domain hterferometry Fig. 3. Deconvolved phase (in S and P polarizations) as a function of space along a diameter of the focd spot and time. Fig. 4. Phase-shift data compared to simulation performed with FILM and spatially dependent conditions at 1 ps zyx zyxwvutsrq Fig. 5. Time-dependent hydrodynamics of the central cell of the foil Fig. 6. Comparison of NLTE and LTE average ionization and a plot of the NLTE rii. zyxwv Fig. 7. Streaked samarium spectrum heated with a 300 fs laser pulse at low energy. The high sweepspeed produces it sloped display due to the difference in the transit time of X-rays of different energies. This kind of shot was used to determine the time origin of streaked data. Fig. 8. (Left) Typical data, after time distortion correction, from the time-resolved samarium back-lighter. (Right) Lineout of the spectrum between 7.6 and 8.2 A. Fig. 9. Measured transmission at four times and a comparison with the opacity code OPAL at t=12 ps 0 v) zyxw 0 0 l- zyxw zyxwv zyx Temperature 0 l - M 0 cu 0 cv 0 F 1 0 0 0 E I 1 zyxw zyxw Density (g/cc) F 0 7 l - 0 0 11 zyxwv 36 ps time 8.4 wavelength (A) 7.4 zyxwvu zyxwvutsrqp zyx 36 ps A 0 r zyxw I DS 8.4 8.2 8.0 7 . 8 7.6 7.4 Wavelength (A) 8.4 anq Zn-liye . 8.2 8.0 7 . 8 7.6 7.4 Wavelength (A) zyxwv zyx m co 3 0 4 00 I zyxwvutsrqponmlkj 00 e 0 11 cr I co d m I zyxwvutsr 8 zyxwvutsrqponmlk zyxwv Q) i5 c