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UCRL-JC-123154PREPRINT

Electron Density Measurement of a CollidingPlasma Using Soft X-Ray Laser Interferometry

A. S. Wan, C. A. Back, T. W . Barb ee, Jr., R. Cauble P. CelliersL. B. DaSilva, S. Glenzer, J.C. Moreno, P. W. Rambo, G. F. Stone,J. E. Trebes and F. Weber

This paper was prepared for submittal to the5th International Con ferenc e on X-ray LasersLund, Sw eden, June 10-14,1996

M ay 1996

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DThis docum ent was prepared as an account of wo rk sponsored by an agency ofthe United States Government. Neither the United States.Govemm ent nor theUniversity of California nor a ny of their employees, makes any w arranty, expressor im plied, or assumes a ny legal liability or responsibility for the accuracy,completeness, or usefu lness of an y information, apparatu s, prod uct, o r processdisclosed, or represents that its use would n ot infringe privately owned rights.Reference herein to an y specific commercial product, process, o r service by tradename, trad emark, man ufacturer, or otherwise, does no t necessarily.constitute orimply its endorsement, recommendation, or favoring by the United StatesGov ernm ent or the University of California. The views and op inions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernmen t or the University of California, and sh all not be used for advertisingor product endorsemen tpurposes.

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ELECTRON DENSITY MEASUREMENT OF A COLLIDINGPLASMA USING SOFT X-RAY LASER INTERFEROMETRYA. S.WAN, C. A. BACK, T. W. BARBEE, JR, R. CAUBLE, P. CELLIERS. L. B. DASILVA.S.GLENZER. J. C. MORENO, P. W. RAMBO, G. E STONE, . E.TREBES, F. WEBER

Lawrence Livermore National Laboratory, P. 0 .Box 808, Livennore CA 94550The understanding of the collision and subsequent interaction of counter-streaming high-densityplasmas is important for the design of indirectly-driven inertial confinement fusion (ICF)hohlraums. We have employed a soft x-ray Mach-Zehnder interferometer. using a Ne-like Y x-ray laser at 155 A as the probe source, to study interpenetration and stagnation of two co llidingplasmas. We observed a peaked density profile at the symmetry axis with a wide stagnationregion with width of order 100 pm. We compare the measured density profile with densityprofiles calculated by the radiation hydrodynamic code LASNEX nd a multi-specie fluid codewhich allows for interpenetration. The measured density profile falls in between the calculatedprofiles using collisionless and fluid approximations. By using different target materials andirndiation configurations, we can vary the collisionality of the plasma. We hope to use the softx-ray laser interferometry as a mechanism to validate and benchmark our numerical codes usedfor the design and analysis of high-energy-densityphysics experiments.

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1. IntroductionThe understanding of the collision and subsequent interaction of counter-streaming high-density plasmas is important fo r the design of IC F hohlraums [I]. In a typical indirectly-driven vacuum hohlraum, the interaction of the optical laser drive with high-Z innersurfaces generates counter-streaming plasmas which flow unimpeded and collide on theaxis of cylindrically-shaped ho hlraums. Single-fluid radiation hydrod ynam ics cod es thatwe typically use to design IC F and other laser-plasma experiments, such as LASNEX [2].do not allow for plasm a interpenetration. Without interpenetration, as the plasmascollide and stagnate, their kinetic energy converts to internal energy, resulting in

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fCollimatedx-ray laser V

Fig. 1 The colliding plasmaexperimental setup. (a) 3-Dv iew of N o v a b ea milluminating the 2 Au slabs.(b) side view showing thewindow of collimated x-raylaser beam which defines theview of the gated detector.

vant regime. Compare to conventional optical interferometers [7], we operate at 155 A,using a collisionally pumped Ne-like Y x-ray laser as the probe source, which allows us toobtain a two order of magnitude enhancement in spatial resolution due to reduced lightrefraction and more than a three order of magnitude enhancement in signal strengthbecause of reduced absorption. The short pulse and high brightness of thkx-ray laserallowed us to obtain an interferogram in a single 350 ps exposure thereby reducing theeffects of vibrations and motion blumng. The timing between the two Nova lasers, one togenerate the x-ray laser and one to produce the target laser plasma, was defined by thetime-of-flight path of our interferometer setup and the desired probe time.Th e setup of our first colliding plasma experiment is shown in Fig. 1. Two Au slabsare al igned at 45 deg with respect to the symmetry axis. The minimum gap between thetips of the two slabs is 500 pm. W e generate a 500 pm full-width line-focused laser beamwhich incidents the slabs, as shown in Fig. l(a ), and generates lasmas blowing towardtemporal pulse shape. At late time the two plasma streams collide at the symm etry axis.By varying the geometry, the slab materials, and the intensity of the incident laser, wecan change the collisionality of the plasma. At high density and low temperatures, theplasma behaves like a fluid wh ere codes like LASNEX should be able to m odel accurately.The plasma shifts into a collisionless region with increasing temperature and reducingdensity, where w e expect to obs erve signific ant plasma interpenetration.The target was backlit edge-on by the x-ray laser beam 1.2 ns after the start of the

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each other. Th e laser has an in te ns ity o n ta rg et o f 3 ~ 1 0 ' ~lc mr and has a 1-ns squared

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Fig. 2. Measured interferogram of thecolliding plasma with excellent fringevisibility and strong self emission nearthe slab surface. Large fringe shifts on-axis is evident due to plasma stagnation.

Fig. 3 A snapshot of a LASNEX-calculated2-D e profileat a time of 150 ps after optical laser pulse. This timecorresponds to the peak of the x-ray laser pulse that served asthe gate for our imaging detector used for thecxperiment.

the radiative effect. Neglecting the radiation opacity results in significantly lower plasmatemperatures. In the blowoff plasma, T, s as high as 3 keV. Using the three temperature(T,,i.an d T,, he radiation temperature) approximation where the radiation is assumed tobe optically thin, LASNEX predicts a T, of order 0.5 keV. The change in the plasmaparameters significantly impact the ionization balance and collisionality of the plasma.Th e LASNEX symmetry axis has a mirror reflectivity boundary condition. Th isgeom etry simulates the lower half of the experiment. As the blowoff plasma reaches thesymmetry axis, the velocity of the zone boundary for a Lagrangian code is set equal tozero, and the slowing and stagnation of the counter-streaming single-fluid plasmas resultsin the conversion of kinetic to internal energy. In this case Ti can reach a unphysicallylarge values exceeding IO 3 keV. Th e resulting shock waves, whose intensity depen ds onthe collisionality of the plasma, propagate away from the symmetry axis.

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Fig. 4 Comparisons of measured (solid Line) and calculatedI-D e profiles: LASNEX-ca lculated single-fluid profile(dashed-dotted line) and profile with a collisionless

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~~~~ c( approximation (dashed line).2reo

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v - - - - high-density. high-temperature plasmas that isof interest to the design of ICF hohlraums.The measured ne profile peaks at the symmetry0 41 8 ) e-~ la, plane between the two slabs with a widestagnation region. Th e peaked ne values are afactor of 3-4 larger than the values produced by LASNEX in a collisionless approximationwhich assum es completely interpenetrating plasmas. Sin gle fluid radiation Lagrangianhydrodyna mics codes, .such as LASNEX. do not allow for plasma interpenetration andpredicts a unphysically large ion temperature and strong shocks propagating from the Isymm etry plane. Th e LASNEX-calculated ne profile, in the signal fluid approximation, .shows co mparab le stagnation width but with ne profiles peaking off the symmetry plane,which is characteristic of strongly shock-heated, outward propagating plasmas.The ultimate motivation of the development of soft x-ray laser interferometry is toprovide a mechanism to probe the deficiencies of our numei-ical model in areas such aslaser deposition by both resonance and inverse bremsstrahlung absorption, flux-limitedheat conduction, hydrodynamics, and non-local thermodynamics equilibrium aromickinetics. Th e validation and benchmarking of the codes will allow us to gain betterunderstanding of the physics of high-density laser-produced plasmas as we design moreand more complex laser experiments for studying high-energy-density physics. and more

specifically for hohlraum and capsule designs for ICF applications.

031do .lldo --------uUiionless

x (microns)

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AcknowledgmentsWork performed under the auspices of the U. S. DOE by L LNL under the contract number W-

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Technical Illforination Department Lawrence Livermore National LaboratoryUniversity of California Livermore, California 94551