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Execution and modeling of drive symmetry experiments in gas-filled hohlraums have been pursued to provide both a better understanding of radiation symmetry in such hohlraums and to verify the accuracy of the design tools which are used to predict target performance for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)]. In this paper, the results of a series of drive symmetry experiments using gas-filled hohlraums at the Nova laser facility [C. Bibeau et al., Appl. Opt. 31, 5799 (1992)] at Lawrence Livermore National Laboratory are presented. A very important element of these experiments was the use of kineform phase plates (KPP) to smooth the Nova beams. The effect of smoothing the ten Nova beams with KPP phase plates is to remove most of the beam bending which had been observed previously, leaving a residual bending of only 1.5°, equivalent to a 35 μm pointing offset at the hohlraum wall. The results show that the symmetry variation with pointing of implosions in gas-filled hohlraums is consistent with time integrated modeling.

We have designed and produced hot, millimeter-scale, high-{ital Z} plasmas of interest for National Ignition Facility hohlraum target design. Using a high-{ital Z} gas fill produces electron temperatures in the 3.5{endash}6-keV range, the highest temperatures measured to date for high-density (10{sup 21} e/cm{sup 3}) laser-heated plasmas, and much higher than the 3 keV found for low-{ital Z} (neopentane) fills. These measurements are in good agreement with the target design calculations, and the {ital L}-shell spectroscopic approach used to estimate the electron temperature has certain advantages over traditional {ital K}-shell approaches. {copyright} {ital 1999} {ital The American Physical Society}

The potential for laser-plasma instability is a serious concern for indirect-drive inertial confinement fusion (ICF), where laser beams illuminate the interior of a cavity (called a hohlraum) to produce x-rays for imploding a fusion capsule symmetrically. The speckled nature of laser beams used in ICF is an important factor in laser-plasma instability processes. For example, models which calculate the spatial growth of convective instability by properly accounting for the laser speckles successfully predict the observed onsets of backscattering due to stimulated Brillouin and Raman scattering instabilities (SBS and SRS). Assuming pump depletion as the only saturation mechanism in these models results in very large predicted levels of SBS and SRS backscattering from the long-scale plasmas expected in ignition hohlraums. However, in the long-scale plasmas studied in the Nova and Trident lasers [E. M. Campbell, Rev. Sci. Instrum. 57, 2101 (1986) and N. K. Moncur et al., Appl. Opt. 34, 4274 (1995)], SRS and S...

A review of recent progress on the design of a diagnostic system proposed for ignition target experiments on the National Ignition Facility (NIF) will be presented. This diagnostic package contains an extensive suite of optical, x ray, gamma ray, and neutron diagnostics that enable measurements of the performance of both direct and indirect driven NIF targets. The philosophy used in designing all of the diagnostics in the set has emphasized redundant and independent measurement of fundamental physical quantities relevant to the operation of the NIF target. A unique feature of these diagnostics is that they are being designed to be capable of operating in the high radiation, electromagnetic pulse, and debris backgrounds expected on the NIF facility. The diagnostic system proposed can be categorized into three broad areas: laser characterization, hohlraum characterization, and capsule performance diagnostics. The operating principles of a representative instrument from each class of diagnostic employed in this package will be summarized and illustrated with data obtained in recent prototype diagnostic tests.

Los Alamos National Laboratory is contributing to the core science and technology of the inertial confinement fusion program leading to the National Ignition Facility. Short summaries of a sample o...

We have successfully employed isoelectronic line ratios to measure the electron temperature in gas-filled Hohlraum targets and gas bags shot with the Nova laser. These targets produce millimeter-scale-length plasmas with electron density ${\mathit{N}}_{\mathit{e}}$\ensuremath{\sim}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and electron temperature ${\mathit{T}}_{\mathit{e}}$\ensuremath{\sim}3 keV. The Hohlraum targets can also produce radiation temperature exceeding 200 eV. Isoelectronic line ratios are well suited to this measurement because they are relatively insensitive to radiation field effects in Hohlraum targets, opacity, transients, and variations in electron density compared to conventional line ratios. We survey the properties of isoelectronic line ratios formed from ratios of n-to-1 resonance transitions in heliumlike Cr to the same transitions in Ti and compare with conventional ratios of n-to-1 transitions in hydrogenlike Ti to the corresponding transitions in heliumlike Ti, concentrating on plasma parameter ranges of interest to the Nova experiments. We also consider the same ratios using K and Cl. Atomic kinetics are treated using collisional-radiative models and experimental data are analyzed with the aid of radiation-hydrodynamics calculations. When we apply isoelectronic techniques to the Nova experimental data, we find that the targets have electron temperatures of at least 3 keV. \textcopyright{} 1996 The American Physical Society.

Z-pinches can produce intense fluxes of argon K-shell (3 keV) radiation, but typically only a fraction of the load mass near the axis of the pinch radiates in this spectral range. The majority of the mass does not get hot or dense enough to radiate efficiently in the K-shell. We have designed, built, and tested an instrument to image pinch emission, specifically the radial emission profile, at energies below the K-shell in order to track the location of the cooler mass. A gold mirror provides a high-energy cut-off at 2 keV while a transmission grating disperses the incoming radiation and provides a low-energy cutoff at 0.1 keV. A vertical slit images the pinch radiation in the radial direction and the emission profile is recorded with either an extreme ultraviolet-sensitive charge-coupled device camera (time-integrated) or a linear photodiode array (∼1 ns time resolution). We present results for the mirror, grating, and system characterization obtained at the Advanced Light Source synchrotron located at L...

Large plasmas are created by illuminating gas‐filled thin‐walled balloon‐like targets using the Nova laser [E. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)]. The targets consist of a 5000–6000 A skin surrounding 1 atm of neopentane, which, when ionized, becomes a plasma with an electron density of 1021 electrons/cm3. X‐ray images of the gas bag target are used to evaluate the size and uniformity of the plasma by comparison with LASNEX [R. M. More, J. Quant. Spectrosc. Radiat. Transfer 27, 345 (1982)] simulations. The gas bags are heated with converging and diverging beam spots. The most uniform plasmas are created by illuminating the target with large converging beam spots that overlap to cover most of the surface of the gas bag. The gas bag plasma is heated to a peak temperature of approximately 3.5 keV, with 25 kJ of 3ω laser light in a 1 ns square pulse.

Apart from their intrinsic interest, plasma physics processes are important because they affect the coupling of the laser energy into laser-irradiated targets. Recently, new gas-filled targets have been developed to create large mm-size plasmas for the study of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). We present x-ray images and x-ray spectra to characterize these targets, which show that the plasmas are homogeneous, have electron densities of ~10 21 cm −3 , and attain electron temperatures of ~3 keV. We also give SBS measurements to demonstrate how systematic studies of physical phenomena can be performed using these targets.

Stimulated Brillouin backscatter from large scale‐length gas‐filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te≳3 keV), and gradient scale lengths (Ln∼2 mm, Lv≳6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas‐filled balloons with polyimide walls (gasbags) and gas‐filled hohlraums. Detailed characterization using x‐ray imaging and x‐ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time‐resolved SBS backscatter from these targets is

A tentative schedule of experiments for the ignition campaign on the National Ignition Facility (NIF) has been developed. These experiments will be used to validate beam pointing and balance, to tune time history and symmetry of drive of NIF hohlraums, and to implode subignition and igniting targets. The initial target diagnostics are designed to validate beam pointing and to demonstrate the properties of the hohlraums.

The National Ignition Facility (NIF) will have a large suite of sophisticated target diagnostics. This will allow thoroughly diagnosed experiments to be performed both at the ignition and pre-ignition levels. As part of the national effort Los Alamos National Laboratory will design, construct and implement a number of diagnostics for the NIF. This paper describes Los Alamos contributions to the {open_quotes}phase I{close_quotes} diagnostics. Phase I represents the most fundamental and basic measurement systems that will form the core for most work on the NIF. The Los Alamos effort falls into four categories: moderate to hard X-ray time-resolved imaging; neutron spectroscopy-primarily with neutron time of flight devices; burn diagnostics utilizing gamma ray measurements; and testing measurement concepts (e.g., some soft X-ray ideas) on the TRIDENT laser system at Los Alamos. Because of the large blast, debris and radiation environment, the design of high resolution X-ray imaging systems present significant challenges. Systems with close target proximity require special protection and methods for such protection are described. The system design specifications based on expected target performance parameters are also described. Diagnosis of nuclear yield and burn will be essential to the NIF operation. Nuclear reaction diagnosis utilizing both neutron and gamma ray detectionmore » is discussed. The Los Alamos TRIDENT laser system will be used extensively for the development of new measurement concepts and diagnostic instrumentation. Some of its potential roles in the development of diagnostics for NIF are given.« less

Supersonic, counterstreaming plasmas were produced by ablating plasma from the inside surfaces of two parallel disks made of aluminum and magnesium, respectively, with a 0.53 μm laser at an intensity of 1014 W/cm2 for 1.3 nsec. Diagnostics included holographic interferometry, a time‐integrated x‐ray pinhole camera and a gated x‐ray crystal spectrograph with imaging slits. The plasmas interpenetrate for the first half of the laser pulse but stagnate once the electron density exceeds 5×1020 cm−3. Spectroscopic measurements suggest a coronal electron temperature of ∼800 eV and an ion temperature of ∼15 keV in the stagnated plasma. The observations are in good agreement with a two ion fluid model of interpenetrating plasmas in which the dominant slowing down process is ion–ion collisions.

This paper reviews the motivation for, results from, and analyses of 12‐cm‐diameter argon gas‐puff experiments carried out over the last four years on three generators at 3.2‐ to 6.5‐MA peak currents, all with implosion times ⩾200 ns. Using the argon K‐shell yield as a metric of implosion quality, high‐quality implosions are obtained for an appropriate initial radial mass distribution, i.e., a distribution that is peaked on axis. Higher compressed densities and smaller final radii are observed compared to shell‐like initial mass distributions. Theory and data suggest that these distributions mitigate the magnetic Rayleigh‐Taylor instability. An energy analysis shows that (1) significant electrical energy is directly coupled to the pinch during the K‐shell radiation pulse and (2) conversion of radially‐directed kinetic energy into thermal energy is not the dominant mechanism responsible for the pinch K‐shell radiation.

Z Pinches formed from Ar gas puffs have been investigated for more than two decades. Experiments have been performed on many generators; a frequent objective has been maximization of the yield in the K‐shell lines. The increase in available current during that time, from 2 to 15 MA, has resulted in a remarkable enhancement in yield from a few kJ on PITHON to ∼300 kJ on Sandia’s Z generator. We have analyzed spectroscopic and other radiation data from seven Z‐pinch drivers, some dating back to 1991, in an effort to determine what properties of the pinches correlate with high K‐shell yield. The strongest correlation is with the amount of mass that is heated to K‐shell emitting temperatures. Those temperatures, effective at emitting Ar K‐shell x rays, exhibit a range of ∼1 to 2.4 keV.

The overall performance of both a single‐ and a double‐microchannel plate framing camera were measured. In the case of the double‐plated device, only the front plate was gated. A short‐pulse, 20‐ps full width half‐maximum (FWHM), uv laser was used to create photoelectrons on a time scale rapid compared to the temporal variation of the high‐voltage gate pulse (965 V peak amplitude and 270 ps FWHM). Measurements were made of the variation in the light output pulse amplitude (8%–24%), width (85–115 ps), and peak location (±22 ps), the latter corresponding to a temporal jitter. It was found that when a dc bias was applied to the plate (in addition to the high‐voltage pulse) the output signal increased as the dc bias to the 13 and 5.2 powers, below and above a bias of 450 V, respectively, and a factor of 20 difference in signal between the single‐ and double‐plate cameras. It was also determined that the rejection ratio drops from 500 to 1300 at a bias value of 450 V to 125–190 at 650 V.

Gated-microchannel-plate cameras have primarily been used for time-resolved x-ray measurements on high current z pinches because they are compact and unaffected by magnetic fields and hard x rays. X-ray streak cameras are relatively large, and sensitive to magnetic fields, but they provide continuous time records with subnanosecond resolutions. We have demonstrated that an x-ray streak camera can be fielded successfully at the multimegampere Double EAGLE facility. We describe initial spectroscopy and imaging measurements and the vacuum housing and rf shielding developed to field the camera.

Intense laser light (10 12 I 17 W / cm 2 ) has been used at KMS to produce high-electron-temperature and -density plasmas which efficiently convert the incident light energy to X-radiation. Experiments investigated the use of laser plasmas for both ICF backlighting and X-ray lithography source applications. The most recent ICF application produced a small (15–20 μm), spectrally broad (2.6–3 keV) source, with a pulse FWHM of 110–180 ps and an average power of 0.7–2 GW. The source investigated for lithography applications had a peak conversion efficiency (from laser energy to X-rays) of 3.5–5.5%/sr in the energy band 0.8–1.2 keV, and the data suggested ways in which the efficiency can be maintained at lower peak power levels. The incident laser wavelength was 526 nm for both investigations.

The lasnex computer code [Comments Plasma Phys. Controlled Fusion 2, 51 (1975)] was used in the design and analysis of an experimental study of laser‐driven plasma blowoff from gold disks. In the study, several temporal profiles of 0.53 mm laser illumination were used, including square pulses, picket pulse trains, and pulses with graduated leading edges. Preliminary modeling suggested diagnostic techniques [time‐ and space‐resolved imaging of M‐band x‐ray emission and time‐ and space‐averaged measurements of high‐energy (3.5–20 keV) x‐ray spectra] that complemented diagnostics already used in such experiments (four‐frame holographic imaging to determine the electron‐density profile in the underdense corona plasma). In this article, the lasnex results are analyzed and are compared with the measured plasma electron‐density profiles and with time‐ and space‐averaged measurements of the corona temperature. The simulation tracks the observed electron‐density profiles fairly well during the early portions of the laser drive, during which the spatial profiles are approximately self‐similar, but overestimates the electron density in the later, steady‐state segment of the profile. For the corona electron temperature, simulation and experiment agree to within the experimental accuracy of ±20%.

Gold disk targets were irradiated with green (λ=0.53 μm) laser light at intensities between 0.5 and 5×1014 W/cm2 using shaped laser pulses. Plasma conditions near and below critical density (nc≂4×1021/cm3) were determined from three diagnostics. Streaked one‐dimensional images of the M‐band emission (2–3.5 keV) along the laser axis yield the peak emission trajectory. Temporally and spatially averaged measurements of the hard x‐ray spectrum (4–25 keV) were used to determine the average coronal electron temperature. Holographic interferometry was used to determine the electron density profiles and scale lengths along the laser axis. The peak emission trajectory, electron temperature, and scale lengths are all in good agreement with simulations, but the magnitudes of the electron density profiles are not, as discussed in an accompanying paper [Phys. Fluids B 2, 2448 (1990)].

Over the last several years, the Z Accelerator has been engaged in research on near-Planckian x-ray sources for inertial confinement fusion and on K-shell emitting sources for radiation-material interaction studies. These radiating z pinches exhibit complex dynamics that have been, and continue to be, studied over a wide range of configurations. In this paper, the progress to date for the production of tens to hundreds of kilojoules of K-shell emission from 8 keV to 3 keV will be presented. Nested wire arrays and multi-shell gas puffs have been employed to help mitigate implosion instabilities and asymmetries to produce x-ray powers of tens of terawatts with emitted x-ray risetimes of a few nanoseconds. Spectroscopy and modeling of these pinches are providing insight into the role of temperature and density gradients and other plasma phenomena in the production of the radiation. Future directions will also be discussed.

We have proposed and demonstrated successfully a new approach for generating high-yield K-shell radiation with large-diameter gas-puff Z pinches. The novel load design consists of an outer region plasma that carries the current and couples energy from the driver, an inner region plasma that stabilizes the implosion, and a high-density center jet plasma that radiates. It increased the Ar K-shell yield at 3.46 MA in 200 ns implosions from 12 cm initial diameter by a factor of 2, to 21 kJ, matching the yields obtained earlier on the same accelerator with 100 ns implosions. A new "pusher-stabilizer-radiator" physical model is advanced to explain this result.

We have developed spectrographs to measure the electron temperature in gas‐filled targets and low‐density foams, and find it to nominally fall in the 2–4‐keV range. The instrument we designed, built, and fielded can simultaneously record the Ti Heα and Cr Heα line emission. After compensating for the instrumental response, we can estimate the electron temperature from this line ratio to within ±15%.

An X-ray drive has been developed to shock compress metal foils in the solid state using an internally shielded hohlraum with a high contrast shaped pulse from the Nova laser. The drive has been characterized, and hydrodynamics experiments designed to study the growth of the Rayleigh-Taylor (R-T) instability in Cu foils at 3 Mbar peak pressures in the plastic how regime have been started. Preimposed modulations with an initial wavelength of 20-50 mu m and amplitudes of 1.0-2.5 mu m show growth consistent with simulations. In the Nova experiments, the fluid and solid states are expected to behave similarly for Cu. An analytic stability analysis is used to motivate an experimental design with an Al foil where the effects of material strength of the R-T growth are significantly enhanced. The conditions reached in the metal foils at peak compression are similar to those predicted at the core of Earth.

Frequently, the most energetic radiation emitted in abundance from a Z pinch is photons from the H- and He-like stages of the load element(s), i.e., the K shell. Consequently, the K-shell spectroscopy of Z pinches is well established and often used for diagnostics. Four recent advances are described in this paper that have been made in the diagnostic use and interpretation of K-shell radiation emitted from Z pinches. First, use of a trace element in double shell gas puff loads on Double EAGLE (at Maxwell Physics International) has demonstrated that it is the subsequently heated inner puff gas that radiates most of the K-shell photons from the assembled pinch. This finding is consistent with previous data analyses and theory which showed that the hot pinch interior radiates most of the K-shell energy, and shows that little or no mixing of the outer and inner puffs occurs during the implosion and assembly. Second, the use of the intercombination to resonance line ratio to infer density has been reconciled w...

The development of intense x‐ray sources and sensitive gated detectors is needed for the diagnosis of ICF implosions. A laser plasma x‐ray source, with an instantaneous power of 200–500 MW in the range 2.7–3 keV, was used to evaluate a specific detector configuration. The detector, consisting of a PET crystal and a MCP framing camera, was used to obtain radiographs of NaCl‐doped targets. Applications are currently signal limited, indicating the need for brighter x‐ray sources and/or more sensitive detection systems.

The reflectivity of a laser due to stimulated Raman scattering (SRS) from long scale-length hohlraum plasmas is shown to depend on the damping of ion-acoustic waves. This dependence is observed in plasmas with either low or high ionization states. Since the SRS process itself is unrelated to acoustic waves, these data are evidence of a nonlinear coupling of SRS to other parametric processes involving daughter acoustic waves. {copyright} {ital 1996 The American Physical Society.}

Special Nova hohlraums have been designed to simulate the plasma conditions calculated for various NIF hohlraum point designs. These hohlraums attempt to maximize the laser pathlength for parametric instability measurements. A toroidal‐shaped hohlraum with a diameter of 3200 microns and a length of 1600 microns allows a laser pathlength of about 2 mm. Filling the hohlraum with 1 atmosphere of neopentane gas gives an electron temperature of 3 keV and electron density near 0.1 of critical. Detailed LASNEX calculations for these hohlraums and comparisons to the NIF point design will be presented. Comparisons between data and calculations that characterize the plasma conditions (electron, radiation, and ion temperatures, electron density, etc.) in these Nova hohlraums will also be shown.

In a proof-of-principle test, we measured how the L-3 Pulse Sciences MBS (~200 keV peak voltage) Bremsstrahlung spectrum changed in time using a pair of filter-fluorescer channels, one at 41 ± 18 keV and another at 87 ± 27 keV. This demonstrates an approach for measuring a time-resolved bremsstrahlung spectrum for SGEMP applications, albeit with coarse energy resolution (30–45%). Using filter fluorescers and detecting the resultant X-rays with plastic scintillator PMTs, there is adequate sensitivity and dynamic range to correlate spectra from two sources, MBS and PITHON, which differ by a factor of up to ~100× in brightness (from 1 to 100 rad CaF2 at the spectrometer entrance). For such a correlation, no hardware changes are required—only the PMT gains need to be changed. The time resolution of the measurement is expected to be 2 ns.

Gold disk targets were irradiated with and without induced spatial incoherence (ISI) at {lambda}=0.526 {mu}m and intensities of (1--5){times}10{sup 14} W/cm{sup 2}. The distribution of laser energy on target and {ital M}-band x-ray (2--4 keV) pinhole images of the target were smoothed with ISI. Absorption of laser energy was increased by 5--10 % with ISI. Soft-x-ray conversion efficiency with and without ISI were equal within the data scatter and uncertainty of {similar to}10%.

High power x-ray sources built for nuclear weapons effects testing are evolving toward larger overall diameters and smaller anode cathode gaps. We describe a framing camera developed to measure the time-evolution of these 20-50 ns pulsed x-ray sources produced by currents in the 1.5-2.5 MA range and endpoint voltages between 0.2 and 1.5 MV. The camera has up to 4 frames with 5 ns gate widths; the frames are separated by 5 ns. The image data are recorded electronically with a gated intensified CCD camera and the data are available immediately following a shot. A fast plastic scintillator (2.1 ns decay time) converts the x-rays to visible light and, for high sensitivity, a fiber optic imaging bundle carries the light to the CCD input. Examples of image data are shown.

We found that a CCD compared favorably with the film currently used to record gated XUV camera images from plasma physics experiments. The CCD was 20 times as sensitive as the film, indicating that the CCD has an effective ISO of 40 000. The camera was successfully shielded from electromagnetic pulses with electric field strengths of 0.7–1 kV/m and pulse widths of 30–65 ns. The CCD also survived the γ‐ray pulse, and no degradation in performance was seen after an accumulated dose of 10–20 rad. We also successfully acquired optical light images of a Z pinch using a commercial camcorder.