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Analysis of long pulse physics experiments at LLL: Late 1977-Early 1979

Description: In the two years since the last Gordon Conference, we have shifted our interest in laser-plasma interactions to the long pulse (1 ns) regime on the Argus facility. A variable Z disk series (at 3 x 10/sup 14/ W/cm/sup 2/) has been carried out and the Z dependence of line emission, backscatter, /sub h/ (hot electron temperature) and transport inhibition will be described. A variable thickness Au disk series that studied preheat and shocks will be analyzed. A simple theory for why T/sub h/ scales as Z/sup 1/4/ is presented, and a description of an electron transport experiment through layered disk targets is given.
Date: July 18, 1979
Creator: Rosen, M.D.
Partner: UNT Libraries Government Documents Department

Fundamentals of ICF Hohlraums

Description: On the Nova Laser at LLNL, we demonstrated many of the key elements required for assuring that the next laser, the National Ignition Facility (NIF) will drive an Inertial Confinement Fusion (ICF) target to ignition. The indirect drive (sometimes referred to as ''radiation drive'') approach converts laser light to x-rays inside a gold cylinder, which then acts as an x-ray ''oven'' (called a hohlraum) to drive the fusion capsule in its center. On Nova we've demonstrated good understanding of the temperatures reached in hohlraums and of the ways to control the uniformity with which the x-rays drive the spherical fusion capsules. In these lectures we will be reviewing the physics of these laser heated hohlraums, recent attempts at optimizing their performance, and then return to the ICF problem in particular to discuss scaling of ICF gain with scale size, and to compare indirect vs. direct drive gains. In ICF, spherical capsules containing Deuterium and Tritium (DT)--the heavy isotopes of hydrogen--are imploded, creating conditions of high temperature and density similar to those in the cores of stars required for initiating the fusion reaction. When DT fuses an alpha particle (the nucleus of a helium atom) and a neutron are created releasing large amount amounts of energy. If the surrounding fuel is sufficiently dense, the alpha particles are stopped and can heat it, allowing a self-sustaining fusion burn to propagate radially outward and a high gain fusion micro-explosion ensues. To create those conditions the outer surface of the capsule is heated (either directly by a laser or indirectly by laser produced x-rays) to cause rapid ablation and outward expansion of the capsule material. A rocket-like reaction to that outward flowing heated material leads to an inward implosion of the remaining part of the capsule shell. The pressure generated on the outside of ...
Date: September 30, 2005
Creator: Rosen, M D
Partner: UNT Libraries Government Documents Department

Analytic Expressions for Minimizing Hohlraum Wall Losses

Description: We apply our recent analytic solutions to the radiation diffusion equation to problems of interest for ICF hohlraums. The solutions provide quantitative values for absorbed energy, which are of use for generating a desired radiation temperature vs. time within the hohlraum. In particular we use analytic fits to the Rosseland mean opacity and to the specific heat of combinations of materials (''cocktails'') designed to maximize the former while minimizing the latter. By doing so we find good agreement with numerical simulations and with experimental results. In particular we find that the wall loss savings of cocktails vs. the standard gold walled hohlraums have both pulse-length and temperature dependencies. Due to those dependencies we predict that NIF cocktail hohlraums will perform better than present day cocktail experiments. In addition, we apply our solutions to finding that density of foam hohlraum walls which minimizes wall loss by being of sufficiently low density to be supersonic, thus reducing kinetic energy losses, yet high enough density to not unduly suffer from enhanced specific heat capacity.
Date: May 31, 2006
Creator: Rosen, M. D.
Partner: UNT Libraries Government Documents Department

Analytic Methods for Minimizing Hohlraum Wall Losses and the Problem of Oxygen Contamination

Description: The authors wish to minimize ICF hohlraum wall loss, E, in order to operate the National Ignition Facility (NIF) far from its damage threshold and still provide the capsule with the drive it requires to reach ignition. In this paper they consider cocktail walls--a mixture of materials that can improve on the conventional pure Au walls. they use Hammer and Rosen (HR) (2003) which solved the radiation diffusion/hydrodynamics problem analytically. They take Au's T, {rho} dependencies of {kappa} (opacity) and e (specific heat) to be: {kappa} = {kappa}{sub 0} {rho}{sup 0.2}/T{sup 1.5} and e = e{sub 0}T{sup 1.6}/{rho}{sup 0.14}.
Date: May 31, 2006
Creator: Rosen, M. D.
Partner: UNT Libraries Government Documents Department

Scaling laws for specialized hohlraums

Description: The author presents scaling laws for the behavior of hohlraums that are somewhat more complex than a simple sphere or cylinder. In particular the author considers hohlraums that are in what has become known as a {open_quotes}primary{close_quotes} {open_quotes}secondary{close_quotes} configuration, namely geometries in which the laser is absorbed in a primary region of a hohlraum, and only radiation energy is transported to a secondary part of the hohlraum that is shielded from seeing the laser light directly. Such hohlraums have been in use of late for doing LTE opacity experiments on a sample in the secondary and in recently proposed {open_quotes}shimmed{close_quotes} hohlraums that use gold disks on axis to block a capsule`s view of the cold laser entrance hole. The temperature/drive of the secondary, derived herein, scales somewhat differently than the drive in simple hohlraums.
Date: September 28, 1993
Creator: Rosen, M.D.
Partner: UNT Libraries Government Documents Department

Marshak waves: Constant flux vs constant T-a (slight) paradigm shift

Description: We review the basic scaling laws for Marshak waves and point out the differences in results for wall loss, albedo, and Marshak depth when a constant absorbed flux is considered as opposed to a constant absorbed temperature. Comparisons with LASNEX simulations and with data are presented that imply that a constant absorbed flux is a more appropriate boundary condition.
Date: December 22, 1994
Creator: Rosen, M.D.
Partner: UNT Libraries Government Documents Department

The physics of radiation driven ICF hohlraums

Description: On the Nova Laser at LLNL, we have recently demonstrated many of the key elements required for assuring that the next proposed laser, the National Ignition Facility (NIF) will drive an Inertial Confinement Fusion (ICF) target to ignition. The target uses the recently declassified indirect drive (sometimes referred to as {open_quotes}radiation drive{close_quotes}) approach which converts laser light to x-rays inside a gold cylinder, which then acts as an x-ray {open_quotes}oven{close_quotes} (called a hohlraum) to drive the fusion capsule in its center. On Nova we`ve demonstrated good understanding of the temperatures reached in hohlraums and of the ways to control the uniformity with which the x-rays drive the spherical fusion capsules. In this lecture we briefly review the fundamentals of ICF, and describe the capsule implosion symmetry advantages of the hohlraum approach. We then concentrate on a quantitative understanding of the scaling of radiation drive with hohlraum size and wall material, and with laser pulse length and power. We demonstrate that coupling efficiency of x-ray drive to the capsule increases as we proceed from Nova to the NIF and eventually to a reactor, thus increasing the gain of the system.
Date: August 7, 1995
Creator: Rosen, M.D.
Partner: UNT Libraries Government Documents Department

Analysis of electron heat transport in femtosecond laser-plasma interactions with layered solid targets and with thin foils

Description: We analyze two types of experiments that enhance our knowledge of the electron heat transport in femtosecond laser produced plasmas. The first involves layered targets and the fall off of substrate signal with increasing overlayer thickness. The experiment uncertainties are too large at present to definitively quantify the heat transport. The second involves the radiative output from thin foils. The behavior of the spectral response to foil thickness implies that our understanding of the transport within the foils is quite good. 12 refs., 11 figs.
Date: January 10, 1991
Creator: Rosen, M.D.
Partner: UNT Libraries Government Documents Department

Progress in the analysis of selenium x-ray laser targets

Description: We review progress in the modeling of Ne-like-Se XRLs. Dielectronic recombination plays an important role in the level kinetics as well as in ionization balance. Refraction becomes important at target lengths greater than 2 cm by reducing signal at 0/sup 0/ view, and by having much larger signals emitted at a 10 to 20 mrad view. We predict success in scaling these systems to lower lambda with higher Z targets, but at great cost in required driver power.
Date: March 1, 1986
Creator: Rosen, M.D. & Hagelstein, P.L.
Partner: UNT Libraries Government Documents Department

Analysis of the Z dependence of laser generated suprathermal electron temperature

Description: Recent experiments with high Z disks on the Argus facility have exteanded our knowledge of the temperature (inferred from the slope of the hard x-ray spectrum) of laser generated suprathermal electrons, from the previous regime of 1 less than or equal to Z less than or equal to 30 to the Z approx. = 80 regime. The systematic rise of temperature with Z is theoretically analyzed. We believe that material albedoes (electron reflectivity) which increase with Z can account for this behavior, since electrons can make multiple passes through the region of resonant electric fields and are thus reheated. We treat this effect quantitatively and obtain reasonable agreement with experiment. The effects of magnetic fields and filamentation are also examined.
Date: April 17, 1979
Creator: Rosen, M.D. & Estabrook, K.G.
Partner: UNT Libraries Government Documents Department

Time-resolved suprathermal x-rays

Description: Temporally resolved x-ray spectra in the range of 1 to 20 keV have been obtained from gold disk targets irradiated by 1.06 ..mu..m laser pulses from the Argus facility. The x-ray streak camera used for the measurement has been calibrated for streak speed and dynamic range by using an air-gap Fabry-Perot etalon, and the instrument response has been calibrated using a multi-range monoenergetic x-ray source. The experimental results indicate that we are able to observe the ''hot'' x-ray temperature evolve in time and that the experimentally observed values can be qualitatively predicted by LASNEX code computations when the inhibited transport model is used.
Date: August 30, 1978
Creator: Lee, P.H.Y. & Rosen, M.D.
Partner: UNT Libraries Government Documents Department

Analytic Expressions for Optimal ICF Hohlraum Wall Density and Wall Loss

Description: Solutions to the radiation diffusion equation predict the absorbed energy (''wall loss'') within an inertial confinement fusion (ICF) hohlraum. Comparing supersonic vs. subsonic solutions suggests that a high Z metallic foam as hohlraum wall material will reduce hydrodynamic losses, and hence, net absorbed energy by {approx}20%. We derive an analytic expression for the optimal density (for any given drive temperature and pulse-length) that will achieve this reduction factor and which agrees well with numerical simulations. This approach can reduce the cost of a reactor driver. Radiation heat waves, or Marshak waves, play an important role in energy transport and in the energy balance of laser, z-pinch and heavy ion beam hohlraums for ICF and high energy density physics experiments. In these experiments, a power source, e.g. a laser, delivers energy to the interior of a high Z cavity that is converted to x-rays. Typically, most of the energy is absorbed in a thin, diffusively heated layer on the hohlraum interior surface, and re-emission from the heated layer sets the radiation temperature T achieved in the hohlraum. In our recent paper, (henceforward referred to as HR) we developed an analytic theory of Marshak waves via a perturbation theory using a small parameter {var_epsilon} = {beta}/(4 + {alpha}) where the internal energy varies as T{sup {beta}} and the opacity varies as T{sup -{alpha}}. A consistent theory was built up order-by-order in {var_epsilon}, with the benefits of good accuracy and order-by-order energy conservation. We first derived analytic solutions for supersonic Marshak waves, which remarkably allowed for arbitrary time variation of the surface temperature. We then solved the full set of subsonic equations, though specialized to the case that the surface temperature varies as t{sup k}, where self-similar solutions can be found. Our solutions compared very well with exact analytic solutions (for the specialized ...
Date: May 25, 2004
Creator: Rosen, M D & Hammer, J H
Partner: UNT Libraries Government Documents Department

Exploding pusher targets for the SHIVA laser system

Description: The first targets for the 20 TW SHIVA laser system were designed. They are simple glass micro-balloons, approximately 300 ..mu..m in diameter and 2 ..mu..m thick, filled with D-T gas. Using LASNEX, whose model physics was utilized successfully for ARGUS targets, we optimize for both gain and yield. The target behaves as an exploding pusher. Different simple analytic models for the physics of this mode are presented, and are tested by comparing their scaling predictions, at constant absorbed power, with those demonstrated by LASNEX. Emphasis is placed on successful prediction of the basic quantities of peak ion temperature and compression, rather than neutron yield or n tau.
Date: September 26, 1977
Creator: Rosen, M.D.; Larsen, J.T. & Nuckolls, J.H.
Partner: UNT Libraries Government Documents Department

Laser produced plasma soft x-ray generation

Description: The efficiency of soft x-ray production from laser-irradiated plasmas is simulated for two different spectral regions. These two regions, 14{Angstrom} {plus minus} 15% and 130{Angstrom} {plus minus} 1%, were chosen for proximity mask or point-projection technological applications. Relatively large conversion efficiencies were obtained from irradiation of a stainless steel target using the conditions suggested by recent Hampshire Instruments' experiments for proximity masking. Pulse-width and laser frequency parameter studies were performed for point-projection applications which suggest that the conversion applications which suggest that the conversion efficiency is sensitive to pulse-width but not to laser frequency. One of the critical components of any x-ray lithographic scheme is of course the x-ray laser source. There are two primary contenders for a reliable, efficient source currently: synchrotron radiation and spectral emission from laser produced plasma. The dominant issue for laser-plasma emission is the conversion efficiency -- output in the intended operating spectral region relative the required incident laser energy. Simulations are described in the following for both high and low energy spectral regions which have been suggested by either the proximity masking or point-projection technology.
Date: May 20, 1991
Creator: Cerjan, C. & Rosen, M.D.
Partner: UNT Libraries Government Documents Department

Progress and prospects for soft x-ray lasing driven by high power optical lasers

Description: We will describe our optical laser pumped xuv Laser Program. To date, we have concentrated our efforts on exploding foil amplifier designs using Ne-like n=3p to 3s inversion schemes. We will describe our latest modeling results as well as measurements which demonstrate output power near the 1 MW level at 206 and 209 A and lasing at wavelengths as short as 106 A.
Date: July 3, 1986
Creator: Rosen, M.D.; Matthews, D.L. & Hagelstein, P.L.
Partner: UNT Libraries Government Documents Department

Electron thermal transport and short-pulsed laser experiments

Description: The goal of this LDRD project is to provide theory for the LLNL ultra-short pulse laser experiments. The goal includes analysis of the experiments performed and help with planning new experiments. this final report we describe, the technical challenges we faced and he success we had with this project.
Date: March 18, 1996
Creator: More, R.M.; Rosen, M.D. & Langdon, A.B.
Partner: UNT Libraries Government Documents Department

Z dependence of sub-keV x rays emitted by laser-produced plasmas

Description: Recent results obtained by using a 10 channel, filtered XRD detector system to record sub-keV x-ray emission from laser-irradiated targets are reported. Targets materials were Be, Al, Ti, Sn, Au and U, with Z ranging from 4 to 92. Targets were irradiated with 1ns FWHM, 1.06 ..mu..m wavelength pulses at an intensity of 5 x 10/sup 14/ W/cm/sup 2/. Time-resolved x-ray emission pulses show systematic and striking variations with Z. These variations can most probably be attributed to the onset of inhibited electron conduction. Time-integrated x-ray yields are obtained as a function of target Z as well as calculated charge states anti Z, indicating the type of physical processes that give rise to the x-ray emission. Typical sub-keV x-ray spectra are presented; in some cases such as Ti where prominent lines are present, the charge state of the plasma can be directly deduced from line ratios, agreement with numerical simulation is very good.
Date: November 6, 1979
Creator: Lee, P.H.Y.; Tirsell, K.G.; Campbell, E.M.; McClellan, G. & Rosen, M.D.
Partner: UNT Libraries Government Documents Department

Theoretical interpretation of high-Z discs irradiated with 1. 06. mu. laser light

Description: High Z discs have been irradiated with 1.06 ..mu.. laser light at intensities between 7 x 10/sup 13/ and 3 x 10/sup 15/ W/cm, and pulse lengths between 200 and 1000 ps. Due to the high Z, inverse bremsstrahlung becomes an important absorption effect and competes strongly with resonance absorption and stimulated scattering. We find that inhibited electron thermal conduction and non-LTE ionization physics are important. Their inclusion in the LASNEX modeling results in steepened temperature and density profiles near critical, thus producing a several keV underdense corona. These conditions bring what would otherwise be 100% inverse bremsstrahlung absorption down to the experimentally observed values (50% at 10/sup 14/ W/cm). The non-LTE physics is essential to correctly compute the level populations of the high Z atoms moving rapidly through a steep density gradient into the corona. This modeling also shows that x-rays are emitted in a thin overdense region, and on a time scale 50% longer than the laser pulse. Both of these effects are seen in the experiments.
Date: March 31, 1978
Creator: Rosen, M.D.; Mead, W.C.; Thomson, J.J. & Kruer, W.L.
Partner: UNT Libraries Government Documents Department

A Consistent Approach to Solving the Radiation Diffusion Equation

Description: Diffusive x-ray-driven heat waves are found in a variety of astrophysical and laboratory settings, e.g. in the heating of a hohlraum used for ICF, and hence are of intrinsic interest. However, accurate analytic diffusion wave (also called Marshak wave) solutions are difficult to obtain due to the strong non-linearity of the radiation diffusion equation. The typical approach is to solve near the heat front, and by ansatz apply the solution globally. This works fairly well due to ''steepness'' of the heat front, but energy is not conserved and it does not lead to a consistent way of correcting the solution or estimating accuracy. We employ the steepness of the front through a perturbation expansion in {var_epsilon} = {beta}/(4+{alpha}), where the internal energy varies as T{sup {beta}} and the opacity varies as T{sup -{alpha}}. We solve using an iterative approach, equivalent to asymptotic methods that match outer (away from the front) and inner (near the front) solutions. Typically {var_epsilon} < 0.3. Calculations are through first order in {var_epsilon} and are accurate to {approx} 10%, which is comparable to the inaccuracy from assuming power laws for material properties. We solve for supersonic waves with arbitrary drive time history, including the case of a rapidly cooling surface, and generalize the method to arbitrary temperature dependence of opacity and internal energy. We also solve for subsonic waves with drive temperature varying as a power of time. In the subsonic case, the specific heat, (pressure/density) and opacity are each assumed to vary as density to a small power, of order {var_epsilon}. We find the theory compares well with radiation hydrodynamics code calculations of the heat front position, absorbed flux and ablation pressure.
Date: November 6, 2002
Creator: Hammer, J H & Rosen, M D
Partner: UNT Libraries Government Documents Department

Generation of intense ultrashort x-ray pulses

Description: Modeling of x-ray emission from targets heated by an ultrashort-pulse high-intensity optical laser is discussed. One application, using the emitted x rays, is pumping inner-shell photo-ionized x-ray lasers. Short wavelength lasing ({lambda} {le} 15 {Angstrom}) requires fast rise-time 1--3 key x rays to ionize inner K-shell electrons. It has been shown that structured targets, consisting of grooves on a solid material or a composite of clusters, have high absorption. We model grooved targets as an ensemble of exploding foils finding that the rise time of x rays is rapid enough for pumping inner-shell x-ray lasers. We show that simple atomic models can overestimate the energy in x-ray emission bands. High-Z materials are found to have the highest conversion efficiency but mid-Z materials can be used to provide a band of emission at a particular energy. We show that the pondermotive inhibition of expansion has only a small effect on the x-ray emission. The emission of a Au plasma is found to be appropriate for pumping inner-shell lasing at 14.6 {Angstrom} in Ne. The required optical laser intensity is of order 10{sup 17} W/cm{sup 2} using a 100 fsec FWHM duration pulse. To produce a laser with a gain-length product of order 10 requires 5--15 J of optical energy.
Date: August 1, 1993
Creator: Eder, D. C.; London, R. A.; Rosen, M. D. & Strobel, G. L.
Partner: UNT Libraries Government Documents Department

Saturation and kinetic issues for optical-field-ionized plasma x-ray lasers

Description: Lasing between excited states and the ground state following optical-field ionization is studied. Saturation of an x-ray laser when the lower lasing level is a ground state of a H-like or Li-like ion is discussed. Efficiencies of 10{sup {minus}5} to 10{sup {minus}4} are calculated for the 3d{sub 5/2}--2p{sub 3/2} transition at 98 {Angstrom} in Li-like Ne. The assumption that the fine-structure levels are populated according to their statistical weights is shown to be justified through comparisons with calculations using a detailed atomic model. The effect of saturation by a given fine-structure transition on the populations of the fine-structure levels is analyzed. 4 refs., 2 figs.
Date: April 29, 1991
Creator: Eder, D.C.; Amendt, P.; Rosen, M.D.; Nash, J.K. & Wilks, S.C.
Partner: UNT Libraries Government Documents Department