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Lawrence Livermore Laboratory LASER FUSION IMPLOSION AM)! PLASMA INTERACTION EXPERIMENTS lr* PREPRINT UCRL- 79819 [ Lawrence Livermore Laboratory LASER FUSION IMPLOSION AM)! PLASMA INTERACTION EXPERIMENTS HARLOW G.^AHLSTROM •August 1977 ' ' • - This paper was prepared for submission to the CECAM Organized meeting on "Models for Transport Phenomena in Laser Fusion Plasmas," Menton, France, September 5-9, 1977 "ALSO- the 11th European Conferenca on Laser Interaction with Matter, Oxford England, September 19-23, 1977. .This is a preprint of a paper intended for publication in a journal or proceedings. Since .changes may be made 'before publication, this preprint is made available with the understanding that it will not'be cited or reproduced without the permission of the author. ^^^^ LASER FUSION IMPLOSION AND PLASMA INTERACTION EXPERIMENTS* Harlow G. Ahlstrom University of California, Lawrence Livermore Laboratory Livermore, California 94550 ABSTRACT Laser fusion experiments at the Lawrence Livermore Laboratory have' provided basic data' concerning: laser beam propagation and absorption in high temperature plasmas, profile modification due to the inc.ident laser radiation pressure, electron transport processes which transfer the absorbed laser energy to' the high density ablation region, th2 general fluid dynamic expansion and compression of the heated plasma -id the processes responsible for the production of 14 MeV. neutrons during the implosion experiments. Irradiation experiments were performed with three neodymium glass laper systems: The two beam Janus (<£ 40 J/100 ps,> 0.4 Ttf) and Argus (£ 140 J, 35 ps,^ 4 TW) and the single beam Cyclops (£.70 J/100 ps,i> 0.7 TW). Two. classes of targets have been used: glass microshells (* 40 to 120 ym in diameter with n. 0.75 ym thick walls) filled with an equiraolar deuterium-tritium gas mixture and disks (<v> 160 to 600 Mm in diameter and ^ 10 um thick) of several compositions. The targets were supported in vacuum (pressure £ 10-5 Torr) by thin glass stalks. The first portion of this paper reports on results related to the propagation, absorption and scattering of laser light by both spherical and planar, targets. Our absorption measurements indicate that for intensities of interest, inverse bremsstrahlung is not. the. dominant . absorption mechanism. The laser light scattered by the 'plasma is polarization dependent and provides evidence that Brillouin scattering • and resonance absorption are operative. Special diagnostics have been designed and experiments have been performed to elucidate the, nature of •^rRlBUTIONOFTH/SDOCU^riSUNUMiTE^ ' 1 "Mt r*fwt «M ftfumt at an WCMM of «wk m—an* kr *• IMM SUM COWMAM. tkkkK *» Wfi SUM* wt. ikt twud SUM Ea«pr *"* tmfkjttt, Mr w «f tMr ctttfnctwi. MMMMclon, « tMr t^tof**, mkM *ay mumr, ***** •> imfUt, M MMMM* i«r '^i KMW JiwfaM, M nr <«•« Ihtt Mi «• iwU not these two processes. .Measurements of the absorption as a function of angle and polarization haye given direct evidence that resonance, absorption is responsible for a significant fraction of the laser light absorption. Other experiments have demonstrated the significance of Brillouin scattering in laser irradiated targets. Both indirect and direct measurements of the plasma distribution demonstrate the effect of radiation pressure on the steepening of the plasma density distribution in the region of the critical density. This, data has shown that the scale.length of the plasma, at the critical density, is of the order of 1 w for our 1.06 ym:laser irradiated targets. Since most, of the laser energy that is absorbed is due to col!active plasma processes suprath?riaai electrons 'are generated. One signature of the suprathermal electrons is a so called two temperature "distribution of the x-rays emitted by the target.. We present data on both the thermal distribution of x-rays as a,function of intensity on target and also the suprathermal x-rays. We.have data Ion the suprathermal temperature in time integrated measurements and we have obtained recent data on the temporal and spacial distribution of these supratherraal x-rays. In this paper we also summarize our implosion results on glass microshell targets filled with DT gas. These experiments are for targets intentionally operated in the portion of parameter space characteristic of exploding pusher events. Experiments have ,,been performed over s yield range from 0 to 10' neutrons per event, we show how this data can be normalized with a simple scaling law. We give a- prescription for determining the fraction of specific energy which is useful in exploding the .shell and.compressing and heating the fuel. Spatially and temporally resolved x-ray measurements have provided data on the implosion history. We have previously demonstrated that the reactions from these targets are thermonuclear. Here we present a. summary of data from neutron time of flight, alpha time to flight and reaction ratios which are used to determine the average fuel temperature of the' reacting DT ions.':-'Further we have imaged the burning fuel of our target using both pinhole imaging and zone -plate coded aperture imaging of the alpha particles. These 3 results show the size of.the reacting region, however, the resolution not yet sufficient to provide detailed structure information on the compressed, burning fuel. *Work performed under the auspices of the U.S. Energy Research and .. Development Administration under contract No. W-7405-Eng-48. Acknowledgments This paper is a compilation of the results of the experimental program in l.iser fusion at the Lawrence Livermore Laboratory. The groups which have contributed vital efforts towards these results are the Fusion Experiments Program which includes the Laser' Plasma Interaction Croup under the direction of Erik Storm and the Diagnostic Development Group under the direction of i.amar Coleman. The Targets Program which is directed by John Nuckolls has groups which are responsible for Target Design, Plasma Theory under the direction of Bill Kruer, Code' Development under the direction of George Zimmerman and Target Fabrication under the direction of Chuck Hendricks. The Solid State Laser Systems which we use for our experiments have been developed by the Solid State Laser Program which is directed by John Holzrichter and consists of a number of groups. For the purposes of this paper where Argus has been one of the chief instruments'in the performance of these experiments we must acknowledge Bill Simmons and the Argus Project Group for the development of the Argus Laser Fusion facility. In addition we acknowledge the farsighted,direction by Carl Haussmann in the period of 1972 to 1975 in the direction of the laser program at Livermore and the direction of John Emmett and his staff during the period of 1972 to the present. Without the support of all of these groups these experiments would not have been possible nor would they have achieved'the level of accuracy and relevance to the Rational Laser Fusion objectives that we expect of our Program. Ken Manes and Peter Lee helped with- the editing of the paper, Shirley Sanford, Gail Anderson, Ollie Parker, Doris Hine and the LLL Art Department are specially acknowledged far their work in the 'preparation of this paper-. I. INTRODUCTION The primary goal of the inertial confinement fusion program in the United States is to produce a net energy yield irradiating small pellets of thermonuclear fuel with intense particle or photon beams. The - hydrogen isotope fuel which is heated and compressed, undergoes ', thermonuclear reactions and efficient burn-up. In order to achieve this goal we must efficiently absorb and convert the energy of the driving source in such a fashion that it drives thermal wave into the medium and thereby produces ablation driven compression waves. These waves are programmed to efficiently compress the fuel to values of pr 4 3 gm/cm^ and to achieve ignition temperatures of 5 - 10 keV for the deuterium tritium fuel.l>2 The Livermore Program goal is to demonstrate that inertial confinement scientific feasibility can be demonstrated using the neodymium glass laser as a driving source. The eventual practical driving source for a commercial power reactor may be an. electron accelerator, ion accelerator or a laser system. In this paper we discuss facilities which have been used for laser' plasma interaction and laser implosion experiments. We also discuss a number of new diagnostic aspects associated with laser driven, fusion'. The primary subjects of the paper are the laser plasma interaction studies and the implosion experiments. We begin with a discussion of the absorption and scattering of. the laser beam incident on the target. Since the plasmas associated with laser fusion are typically at ' temperatures of 0.10 - 50 keV, the most intense spectral emission from these plasmas is in the soft to hard x-ray regime. Therefore, we spend a considerable fraction of the paper in discussing the x-ray emission fro* these targets. In the past, much data has been presented which' are both temporally and spatially integrated.. In this paper we concentrate on data which show.spatial, temporal or spectral resolution. We also discuss the asymptotic plasma distribution and its implications. In the area of implosion experiments, we discuss the maximum gain or yield presently achievable from' a-> laser driven implosion experiment, • we also discuss the temporal behavior of the implosion. Over the last ' year a number of methods have been used to diagnose the temperature of .the fuel ions in the compressed- core. Data from three such techniques are presented in _ this paper. We show that it is possible to image the reaction products from the compressed burning fuel-core of the laser imploded targets. Finally we show that it is possible to diagnose the density of the fuel, by adding a high Z to the fuel that provides line x-ray emission which only occurs during peak compression and the imploded core of the pellet. In the summary and future directions, we discuss the possibilities for exploding pusher targets, the directions of the Program to achieve higher densities and higher gains and finally scientific feasibility.
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