CH Targets Research Articles

In-Flight Measurements of Capsule Shell Adiabats in Laser-Driven Implosions

We present the first x-ray Thomson scattering measurements of temperature and density from spherically imploding matter. The shape of the Compton downscattered spectrum provides a first-principles measurement of the electron velocity distribution function, dependent on T(e) and the Fermi temperature T(F)∼n(e)(2/3). In-flight compressions of Be and CH targets reach 6-13 times solid density, with T(e)/T(F)∼0.4-0.7 and Γ(ii)∼5, resulting in minimum adiabats of ∼1.6-2. These measurements are consistent with low-entropy implosions and predictions by radiation-hydrodynamic modeling.

Compressing magnetic fields with high-energy lasers

Laser-driven magnetic-field compression producing a magnetic field of tens of megaGauss is reported for the first time. A shock wave formed during the implosion of a cylindrical target traps an initial (seed) magnetic field that is amplified via conservation of magnetic flux. Such large fields are expected to magnetize the electrons in the hot, central plasma, leading to a cyclotron frequency exceeding the collision frequency. The Omega Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] was used to implode cylindrical CH targets filled with deuterium gas and seeded with an external field (>50 kG) from a magnetic pulse generator. This seed field is trapped and rapidly compressed by the imploding shell, minimizing the effect of resistive flux diffusion. The compressed field was probed via proton deflectrometry using 14.7 MeV protons from the D+H3e fusion reaction emitted by an imploding glass microballoon. Line-averaged magnetic fields of the imploded core were measured to between 30 and 40 MG. Experimental data were analyzed with both a magnetohydrodynamic version of the one-dimensional hydrocode LILAC [J. Delettrez et al., Phys. Rev. A 36, 3926 (1987); N. W. Jang et al., Bull. Am. Phys. Soc. 51, 144 (2006)] and the particle propagation code GEANT4 [S. Agostinelli et al., Nucl. Instrum. Methods Phys. Res. A 506, 250 (2003)].

Laser-Driven Magnetic-Flux Compression in High-Energy-Density Plasmas

The demonstration of magnetic field compression to many tens of megagauss in cylindrical implosions of inertial confinement fusion targets is reported for the first time. The OMEGA laser [T. R. Boehly, Opt. Commun. 133, 495 (1997)10.1016/S0030-4018(96)00325-2] was used to implode cylindrical CH targets filled with deuterium gas and seeded with a strong external field (>50 kG) from a specially developed magnetic pulse generator. This seed field was trapped (frozen) in the shock-heated gas fill and compressed by the imploding shell at a high implosion velocity, minimizing the effect of resistive flux diffusion. The magnetic fields in the compressed core were probed via proton deflectrometry using the fusion products from an imploding D3He target. Line-averaged magnetic fields between 30 and 40 MG were observed.

Imprinting and consequent Rayleigh-Taylor growth

In direct drive inertial confinement fusion， surface perturbations seeded by imprint from laser intensity variations play an important role in Rayleigh-Taylor（RT） growth. Laser imprinting and consequent RT growth in planar CH targets driven by different pulse shapes have been investigated. The experimental results reveal that the range of density modulation induced by laser imprinting is larger when the driving laser has lower foot intensity and rises more slowly. Enhancing the pre-pulse intensity can restrain the effect of laser imprinting notablely.

Comparison of simulation to absolute X-ray emission of CH plasma created with the Nike laser

The Nike laser group at the Naval Research Laboratory has an ongoing effort to improve and benchmark the radiation hydrodynamic simulations used to develop pellet designs for inertial confinement fusion. A new postprocessor, Virtual Spectro, has been added to the FAST code suite for detailed simulation of non-LTE spectra, including radiation transport effects and Stark line profile. This new combination enhances our ability to predict the absolute emission of soft x-rays. An absolutely calibrated transmission grating spectrometer and a high resolution grazing incidence spectrometer have been used to collect time integrated and time resolved spectra emitted by CH targets irradiated at laser intensities of ∼ 10 TW/cm 2 . Comparison between these observations and simulations using Virtual Spectro demonstrates excellent agreement (within factor of ∼ 1.5) for the absolute emission.

Experimental study on the momentum coupling efficiency of laser plasma

During the interaction of laser with solid targets, the counteracting force is generated by the plasma, which can be used as a new type source of propulsion. Compared with the chemical propulsion, laser plasma has unique advantages such as high specific impulse and large load ratio. In this paper, the momentum generated by the laser plasma is measured during the interaction of nanosecond laser pulses with aluminum, graphite, lead and CH targets. The relation of target momentum with laser focal area in atmosphere and vacuum ambient pressure is obtained,and the comparison is performed between the experimental and analytical results. It is shown that the target momentum in vacuum is different from that in atmosphere. In vacuum, the target momentum shows a dependence on the target property. These results are different from those generated by long width pulses.

Soft x-ray emission from postpulse expanding laser-produced plasmas

A diagnostic spectrometer has been developed at the Naval Research Laboratory to measure the time resolved absolute intensity of radiation emitted from targets irradiated by the Nike laser. The spectrometer consists of a dispersive transmission grating of 2500 lines/mm or 5000 lines/mm and a detection system consisting of an absolutely calibrated Si photodiode array and a charge coupled device camera. In this article, this spectrometer was used to study the spatial distribution of soft x-ray radiation from low Z elements (primarily carbon) that lasted tens of nanoseconds after the main laser illumination was over. We recorded soft x-ray emission as a function of the target material and target orientation with respect to the incoming laser beam and the spectrometer line of sight. While a number of spectral features have been identified in the data, the instrument’s combined temporal and spatial resolution allowed observation of the plasma expansion from CH targets for up to ∼25 ns after the cessation of the main laser pulse. The inferred plasma expansion velocities are slightly higher than those previously reported.

Experimental measurements of deep directional columnar heating by laser-generated relativistic electrons at near-solid density.

In our experiments, we irradiated solid CH targets with a 400 J, 5 ps, 3 x 10(19) W/cm(2) laser, and we used x-ray imaging and spectroscopic diagnostics to monitor the keV x-ray emission from thin Al or Au tracer layers buried within the targets. The experiments were designed to quantify the spatial distribution of the thermal electron temperature and density as a function of buried layer depth; these data provide insights into the behavior of relativistic electron currents which flow within the solid target and are directly and indirectly responsible for the heating. We measured approximately 200-350 eV temperatures and near-solid densities at depths ranging from 5 to 100 microm beneath the target surface. Time-resolved x-ray spectra from Al tracers indicate that the tracers emit thermal x rays and cool slowly compared to the time scale of the laser pulse. Most intriguingly, we consistently observe annular x-ray images in all buried tracer-layer experiments, and these data show that the temperature distribution is columnar, with enhanced heating along the edges of the column. The ring diameters are much greater than the laser focal spot diameter and do not vary significantly with the depth of the tracer layer for depths greater than 30 microm. The local temperatures are 200-350 eV for all tracer depths. We discuss recent simulations of the evolution of electron currents deep within solid targets irradiated by ultra-high-intensity lasers, and we discuss how modeling and analytical results suggest that the annular patterns we observe may be related to locally strong growth of the Weibel instability. We also suggest avenues for future research in order to further illuminate the complex physics of relativistic electron transport and energy deposition inside ultra-high-intensity laser-irradiated solid targets.

Absolutely calibrated, time-resolved measurements of soft x rays using transmission grating spectrometers at the Nike Laser Facility

Accurate simulation of pellet implosions for direct drive inertial confinement fusion requires benchmarking the codes with experimental data. The Naval Research Laboratory (NRL) has begun to measure the absolute intensity of radiation from laser irradiated targets to provide critical information for the radiatively preheated pellet designs developed by the Nike laser group. Two main diagnostics for this effort are two spectrometers incorporating three detection systems. While both spectrometers use 2500 lines/mm transmission gratings, one instrument is coupled to a soft x-ray streak camera and the other is coupled to both an absolutely calibrated Si photodiode array and a charge coupled device (CCD) camera. Absolute calibration of spectrometer components has been undertaken at the National Synchrotron Light Source at Brookhaven National Laboratories. Currently, the system has been used to measure the spatially integrated soft x-ray flux as a function of target material, laser power, and laser spot size. A comparison between measured and calculated flux for Au and CH targets shows reasonable agreement to one-dimensional modeling for two laser power densities.

Measurements of shock heating using Al absorption spectroscopy in planar targets (abstract)

In direct-drive laser fusion, the tradeoff between stability and overall efficiency requires precise control of the implosion isentrope. Most target designs use the temporal shape of the drive pulse to create shocks that slightly preheat the capsule shell and establish the isentrope for the rest of the implosion. Also, the use of foam overcoatings has been proposed as a means to reduce laser imprinting. These foams can alter the structure and intensity of the initial shock. To ensure that our hydrocodes adequately model these effects it is important that shock heating of targets be measured and understood. We report on measurements of shock heating in planar targets irradiated with the OMEGA laser system. Planar 20-μm-thick CH targets were irradiated with six ultraviolet (UV) beams at intensities of ∼2×1014 W/cm2 with temporally square and ramped pulses. Some targets also have low-density foam (30 mg/cc) on the irradiated surface. A thin (0.5 μm) Al layer, imbedded in the target, is probed with x rays from a Sm backlighter. The 1s-2p absorption lines in the Al are observed with a streaked x-ray spectrometer. The absorption lines from the F-like to Ne-like ion populations provide a measure of the temperature of the target as a function of time. We present data on measurements that show the relative shock heating by square and ramp pulses. We also present results of atomic physics calculations1 of the absorption spectra that are used to infer the target temperature and show results from hydrodynamic simulations of the experiments.

Direct-drive high-convergence-ratio implosion studies on the OMEGA laser system

A series of direct-drive implosion experiments, using room-temperature, gas-filled CH targets, are performed on the University of Rochester’s OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The target performance at stagnation and its dependence on beam smoothing and pulse shaping is investigated. Compressed core conditions are diagnosed using x-ray and neutron spectroscopy, and x-ray imaging. The individual beams of OMEGA are smoothed by spectral dispersion in two dimensions (2D SSD) with laser bandwidths up to ∼0.3 THz, with 1 ns square to 2.5 ns shaped pulses. A clear dependence of target performance on pulse shape and beam smoothing is seen, with the target performance (yield, areal density, and shell integrity) improving as SSD bandwidth is applied.

Single-mode, Rayleigh-Taylor growth-rate measurements on the OMEGA laser system

The results from a series of single-mode, Rayleigh–Taylor (RT) instability growth experiments performed on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] using planar targets are reported. Planar targets with imposed mass perturbations were accelerated using five or six 351 nm laser beams overlapped with total intensities up to 2.5×1014 W/cm2. Experiments were performed with both 3 ns ramp and 3 ns flat-topped temporal pulse shapes. The use of distributed phase plates and smoothing by spectral dispersion resulted in a laser-irradiation nonuniformity of 4%–7% over a 600 μm diam region defined by the 90% intensity contour. The temporal growth of the modulation in optical depth was measured using throughfoil radiography and was detected with an x-ray framing camera for CH targets. Two-dimensional (2-D) hydrodynamic simulations (ORCHID) [R. L. McCrory and C. P. Verdon, in Inertial Confinement Fusion (Editrice Compositori, Bologna, 1989), pp. 83–124] of the growth of 20, 31, and 60 μm wavelength perturbations were in good agreement with the experimental data when the experimental details, including noise, were included. The amplitude of the simulation optical depth is in good agreement with the experimental optical depth; therefore, great care must be taken when the growth rates are compared to dispersion formulas. Since the foil’s initial condition just before it is accelerated is not that of a uniformly compressed foil, the optical density measurement does not accurately reflect the amplitude of the ablation surface but is affected by the initial nonuniform density profile.

Nonlinear evolution of broad-bandwidth, laser-imprinted nonuniformities in planar targets accelerated by 351-nm laser light

Planar, 20 and 40 μm thick CH targets have been accelerated by 351 nm laser beams of the OMEGA laser system [Opt. Commun. 133, 495 (1997)]. Different beam-smoothing techniques were employed including distributed phase plates, smoothing by spectral dispersion, and distributed polarization rotators. The Rayleigh–Taylor evolution of three-dimensional (3D) broadband planar-target perturbations seeded by laser nonuniformities was measured using x-ray radiography at ∼1.3 keV. Fourier analysis shows that the perturbations evolve to longer wavelengths and the shorter wavelengths saturate. The saturation amplitudes and rates of growth of these features are consistent with the predictions of Haan [Phys. Rev. A 39, 5812 (1989)].

Rear surface light emission measurements from laser-produced shock waves in clear and Al-coated polystyrene targets

The Nike KrF laser, with its very uniform focal distributions, has been used at intensities near 10 14 W/cm 2 to launch shock waves in polystyrene targets. The rear surface visible light emission differed between clear polystyrene (CH) targets and targets with a thin (125 nm) Al coating on the rear side. The uncoated CH targets showed a relatively slowly rising emission followed by a sudden fall when the shock emerges, while the Al-coated targets showed a rapid rise in emission when the shock emerges followed by a slower fall, allowing an unambiguous determination of the time the shock arrived at the rear surface. A half-aluminized target allowed us to observe this difference in a single shot. The brightness temperature of both the aluminized targets and the non-aluminized targets was slightly below but close to rear surface temperature predictions of a hydrodynamic code. A discussion of preheat effects is given.

Laser-plasma interactions in long-scale-length plasmas under direct-drive National Ignition Facility conditions

Laser-plasma interaction experiments have been carried out on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] under plasma conditions representative of the peak of a 1.5 MJ direct-drive laser pulse proposed for the National Ignition Facility (NIF). Plasmas have been formed by exploding 18–20 μm thick CH foils and by irradiating solid CH targets from one side, using up to 20 kJ of laser energy with phase plates installed on all beams. These plasmas and the NIF plasmas are predicted to have electron temperatures of 4 keV and density scale lengths close to 0.75 mm at the peak of the laser pulse. The electron temperature and density of the exploding-foil plasmas have been diagnosed using time-resolved x-ray spectroscopy and stimulated Raman scattering, respectively, and are consistent with predictions of the two-dimensional Eulerian hydrodynamics code SAGE [R. S. Craxton and R. L. McCrory, J. Appl. Phys. 56, 108 (1984)]. When the solid-target or exploding-foil plasmas were irradiated with an f/6 interaction beam at 1.5×1015 W/cm2, well above the NIF f/8 cluster intensity of ∼2×1014 W/cm2, stimulated Brillouin backscattering (SBS) was found to be completely inhibited. A conservative upper limit of direct-backscattered SRS was found to be ∼5% from the solid targets. SRS and SBS are thus unlikely to have a significant impact on target performance at the peak of the NIF direct-drive laser pulse.

Stimulated Brillouin Scattering in Multispecies Laser-Produced Plasmas.

Growth and saturation of the stimulated Brillouin scattering (SBS) instability has been studied in C, CH, and CH{sub 2} laser-produced plasmas which have nearly identical density profiles. The backscattered reflectivity is highest for the CH{sub 2} plasma ({approximately}15{percent}) and up to {approximately}5 times lower for the pure carbon plasma. This result is explained by the stationary inhomogeneous theory of SBS and confirmed by numerical simulations provided one allows for the nonuniform expansion of hydrogen and carbon ions that leads to an axial asymmetry of the SBS gain and results in the observation of a blueshifted scattered light spectrum from CH targets. {copyright} {ital 1996 The American Physical Society.}

Angularly resolved observations of sidescattered laser light from laser-produced plasmas.

Small scale structure is observed in angularly resolved measurements of scattered laser light from 1.06 \ensuremath{\mu}m laser irradiations of CH targets. The structure size varies as the laser spot diameter is changed. The structure is consistent with light produced by the stimulated Brillouin sidescattering instability. The overall angular dependence of the scattered laser light intensity is compared to the distribution predicted by convective growth of stimulated Brillouin sidescattering. Two-dimensional plasma effects are shown to be important in determining the angular distribution of the scattered light.

Observation of stimulated Compton scattering from resonant electrons in a laser-produced plasma.

An experimental study of stimulated Compton scattering (SCS) from resonant electrons in a laser-produced plasma is reported. Up to 1.9 kJ of 350-nm laser light was used to irradiate 3-\ensuremath{\mu}m-thick CH targets in 1.3 to 3.3-ns pulses at intensities up to 1.3\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$. SCS was detected by making time-resolved measurements of the emission near 420 nm. The measurements are compared to the results of theoretical modeling.

Narrow Raman spectra: The competition between collisional and Landau damping

Narrow Raman spectra can be produced when the collisional damping rate becomes comparable to the homogeneous growth rate for stimulated Raman scattering (SRS). Landau damping limits the SRS spectrum at short wavelength, and collisional damping can limit it at long wavelength through the variation of the damping rate with plasma density. Data are shown to demonstrate this effect for the first time, from experiments in which constant-intensity pulses of 0.35 μm light irradiated 3 μm thick CH targets at intensities up to 2×1014 W/cm2. The observed spectra have a peak wavelength, spectral width, spectral shape, and shift of the peak in time that are consistent with the anticipated effects of damping. Reduction of the emission from the plasma to near thermal levels at heavily damped frequencies is also demonstrated. In addition, the application of two specific models of SRS to these data is discussed.

Hydrodynamic expansion of exploding-foil targets irradiated by 0.53 μm laser light

An experiment is reported in which several diagnostics were used to study the hydrodynamic expansion of exploding-foil targets. The CH targets, 1–3 μm thick, were irradiated with 3–4 kJ of 0.53 μm laser light in 1 nsec pulses. The electron density was diagnosed using the transmission and scattering of light at the laser frequency, the scattering of light at 3/2 of the laser frequency, the spectrum of Raman-scattered light, the x-ray spectrum from doped targets, and the image of the ultraviolet bremsstrahlung emission from the plasma. The measurements were consistent with one another, but generally were not consistent with two-dimensional computer simulations. The targets burned through later and took longer after burnthrough to reach quarter-critical density than predicted. This discrepancy may be a consequence of excessive profile steepening caused by the local, heat-transport model used in the simulation.