Distributed phase plates for super-Gaussian focal-plane irradiance profiles

High precision control of laser energy for laser-matter interaction studies

Precise, highly reproducible control of the laser energy is required for high confidence laser-matter interaction research such as in dynamic compression science and high energy density physics. The energy must be adjustable without affecting the pulse shape (time varying intensity) or beam smoothness. We have developed a convenient two-stage energy tuning method for a nominal 100 J, 351 nm (UV) laser. The energy is adjusted in 10 J (10%) increments by operating the laser at full energy and inserting a beam splitter in the laser output. As the splitter is located after the final frequency tripling optics, the UV pulse shape is unchanged. The energy is varied by substituting a splitter of different reflectivity. For finer 3 J (3%) increments, the infrared pulse is attenuated inside the laser before the final amplifier. This requires modest tuning to preserve the pulse shape. The demonstrated variation in shot-to-shot reproducibility is less than +/-2.5 J (5% of the full energy), irrespective of the laser output energy. These approaches can be adapted to most ∼100 J class lasers. We describe these techniques and show two examples where they have elucidated the underlying physics in laser shock compression experiments. One used only the beam splitters to establish the pressure for melting in iron. The other combined both techniques to finely increment the peak stress (∼2 GPa steps) in germanium to precisely determine the onset and completion of melting-including the melting kinetics. These unambiguous results would not be possible without the developments described here.

Automatic generation of a holographically shaped beam in an actual optical system for use in material laser processing

In-system optimization involves designing a computer-generated hologram (CGH) in an actual optical system. An important advantage of this approach is automatic generation of a target shaped beam with compensation for imperfections in the actual optical system that would degrade the reconstruction performance. We developed a novel in-system optimization method for beam shaping based on our previous research where it had been applied only to generate parallel focused beams. The key point in the application to beam shaping is to accurately express the conditions and coordinates of the actual optical system in the CGH calculation.

Beam-pointing verification using x-ray pinhole cameras on the 60-beam OMEGA laser

On the OMEGA laser system, the beam-pointing accuracy is verified by irradiating a 4 mm diameter Au-coated spherical target with ∼23 kJ of laser energy. Up to ten x-ray pinhole cameras record the x-ray emission from all 60-beam spots. A new set of algorithms has been developed to improve the accuracy of the pointing evaluation. An updated edge-finding procedure allows one to infer the center of the sphere with subpixel accuracy. A new approach was introduced to back-propagate the pixel locations on the 2D image to the 3D surface of the sphere. A fast Fourier transform-based de-noising method significantly improves the signal-to-noise of the data. Based on the beam-pointing analysis, hard-sphere calculations of the laser-drive illumination uniformity on the target surface and the decomposition of the illumination distribution into lower order modes (1-10) are evaluated.

Shock Compression of Liquid Deuterium up to 1 TPa

We present laser-driven shock compression experiments on cryogenic liquid deuterium to 550 GPa along the principal Hugoniot and reflected-shock data up to 1 TPa. High-precision interferometric Doppler velocimetry and impedance-matching analysis were used to determine the compression accurately enough to reveal a significant difference as compared to state-of-the-art ab initio calculations and thus, no single equation of state model fully matches the principal Hugoniot of deuterium over the observed pressure range. In the molecular-to-atomic transition pressure range, models based on density functional theory calculations predict the maximum compression accurately. However, beyond 250 GPa along the principal Hugoniot, first-principles models exhibit a stiffer response than the experimental data. Similarly, above 500 GPa the reflected shock data show 5%-7% higher compression than predicted by all current models.

Direct-drive measurements of laser-imprint-induced shock velocity nonuniformities

Perturbations in the velocity profile of a laser-ablation-driven shock wave seeded by speckle in the spatial beam intensity (i.e., laser imprint) have been measured. Direct measurements of these velocity perturbations were recorded using a two-dimensional high-resolution velocimeter probing plastic material shocked by a 100-ps picket laser pulse from the OMEGA laser system. The measured results for experiments with one, two, and five overlapping beams incident on the target clearly demonstrate a reduction in long-wavelength (>25-μm) perturbations with an increasing number of overlapping laser beams, consistent with theoretical expectations. These experimental measurements are crucial to validate radiation-hydrodynamics simulations of laser imprint for laser direct drive inertial confinement fusion research since they highlight the significant (factor of 3) underestimation of the level of seeded perturbation when the microphysics processes for initial plasma formation, such as multiphoton ionization are neglected.

The Dynamic Compression Sector laser: A 100-J UV laser for dynamic compression research

The Dynamic Compression Sector (DCS) laser is a 100-J ultraviolet Nd:glass system designed and built by the Laboratory for Laser Energetics for experimental research at the DCS located at the Advanced Photon Source (Argonne National Laboratory). Its purpose is to serve as a shock driver to study materials under extreme dynamic pressures. It was designed to deposit energy within a uniformly illuminated 500-μm spot on target, with additional optics provided to implement spot sizes of 250 and 1000 μm. Designed after larger-scale glass lasers such as OMEGA and the National Ignition Facility, the laser consists of a fiber front end with interferometer-based pulse shaping, a Nd:glass regenerative amplifier, a four-pass rod amplifier, and a 15-cm glass disk amplifier, through which six passes are made in a bowtie geometry. The output is frequency tripled from 1053 to 351 nm by using a pair of type-II phase-matched KDP crystals, with a third to increase conversion bandwidth. The super-Gaussian spot in the far field is achieved with a distributed phase plate and a 1-m aspherical focusing lens. Beam smoothing is achieved by smoothing by spectral dispersion and polarization smoothing, resulting in a root-mean-square variation in intensity on target of ±8.7%.

Beam smoothing by a diffraction-weakened lens array combining with induced spatial incoherence

The smoothing scheme combining a diffraction-weakened lens array with the induced spatial incoherence method is proposed and demonstrated to be an efficient smoothing scheme for broadband laser systems. In our simulation, the RMS illumination nonuniformity of the target spot is reduced to 2% after sufficient smoothing time. The temporal characteristics and spatial power spectral density of the scheme are theoretically analyzed. When the incident light has intensity fluctuations, the uniformity of the target spot is stable, which means a robust smoothing scheme, and which predicts practical applications to the smoothing of broadband laser systems.

Precision mapping of laser-driven magnetic fields and their evolution in high-energy-density plasmas

The magnetic fields generated at the surface of a laser-irradiated planar solid target are mapped using ultrafast proton radiography. Thick (50  μm) plastic foils are irradiated with 4-kJ, 2.5-ns laser pulses focused to an intensity of 4×10^{14}  W/cm^{2}. The data show magnetic fields concentrated at the edge of the laser-focal region, well within the expanding coronal plasma. The magnetic-field spatial distribution is tracked and shows good agreement with 2D resistive magnetohydrodynamic simulations using the code draco when the Biermann battery source, fluid and Nernst advection, resistive magnetic diffusion, and Righi-Leduc heat flow are included.

Temperature measurements of shocked silica aerogel foam

We present recent results of equation-of-state (EOS) measurements of shocked silica (SiO_{2}) aerogel foam at the OMEGA laser facility. Silica aerogel is an important low-density pressure standard used in many high energy density experiments, including the novel technique of shock and release. Due to its many applications, it has been a heavily studied material and has a well-known Hugoniot curve. This work then complements the velocity and pressure measurements with additional temperature data providing the full EOS information within the warm dense matter regime for the temperature interval of 1-15 eV and shock velocities between 10 and 40 km/s corresponding to shock pressures of 0.3-2 Mbar. The experimental results were compared with hydrodynamic simulations and EOS models. We found that the measured temperature was systematically lower than suggested by theoretical calculations. Simulations provide a possible explanation that the emission measured by optical pyrometry comes from a radiative precursor rather than from the shock front, which could have important implications for such measurements.

Equation of state measurements of warm dense carbon using laser-driven shock and release technique

We present a new approach to equation of state experiments that utilizes a laser-driven shock and release technique combined with spatially resolved x-ray Thomson scattering, radiography, velocity interferometry, and optical pyrometry to obtain independent measurements of pressure, density, and temperature for carbon at warm dense matter conditions. The uniqueness of this approach relies on using a laser to create very high initial pressures to enable a very deep release when the shock moves into a low-density pressure standard. This results in material at near normal solid density and temperatures around 10 eV. The spatially resolved Thomson scattering measurements facilitate a temperature determination of the released material by isolating the scattering signal from a specific region in the target. Our results are consistent with quantum molecular dynamics calculations for carbon at these conditions and are compared to several equation of state models.

Nonuniformly driven two-plasmon-decay instability in direct-drive implosions

Half-harmonic emission spectra and images taken during directly driven implosions show that the two-plasmon decay (TPD) instability is driven nonuniformly over the target surface and that multibeam effects dominate this instability. The images show a spatially limited extent of the TPD instability. A prominent spectral feature is used to determine the electron temperature in the corona. Near threshold the temperatures agree with one-dimensional hydrodynamic predictions but exceed them by ∼10% above the TPD threshold. Two-dimensional hydrodynamic simulations indicate that a significant part (∼20%) of the laser intensity must be locally absorbed by the TPD instability (i.e., by collisional damping of the electron plasma waves) to maintain these temperature islands.

Spectral and temporal properties of optical signals with multiple sinusoidal phase modulations

Optical signals generated by multiple sinusoidal temporal phase modulations (multi-FMs) applied to a monochromatic field are studied from the viewpoint of their optical spectrum and temporal modulations arising from spectral impairments. Statistical analysis based on the central limit theorem shows that the signals' optical spectrum converges to a normal distribution as the number of modulations increases, allowing one to predict the frequency range containing a given fraction of the total energy with the associated cumulative density function. The conversion of frequency modulation to amplitude modulation is analyzed and simulated for arbitrary multi-FM signals. These developments are of theoretical and practical importance for high-energy laser systems, where optical pulses are phase modulated in the front end to smooth out the on-target beam profile and prevent potentially catastrophic damage to optical components.

Performance of target irradiation in a high-power laser with a continuous phase plate and spectral dispersion

We report on the performance of target irradiation at the SG-II high-power laser facility with a continuous phase plate (CPP) and the technique of smoothing by spectral dispersion (SSD). Simulative and experimental results are presented, where the irradiation uniformity and energy concentration of the target spots are analyzed. The results show that the designed CPP can focus the spot energy into the desired region and shape a profile with steep edge and flat top, but the actual performance of the fabricated CPP needs some improvements. It is also proved that the CPP is insensitive to the long-scale wavefront distortion in the incident beam. The one-dimensional SSD configuration evidently works in smoothing the fine-scale intensity modulation inside the target spot.

First measurements of Rayleigh-Taylor-induced magnetic fields in laser-produced plasmas

The first experimental demonstration of Rayleigh-Taylor-induced magnetic fields due to the Biermann battery effect has been made. Experiments with laser-irradiated plastic foils were performed to investigate these illusive fields using a monoenergetic proton radiography system. Path-integrated B field strength measurements were inferred from radiographs and found to increase from 10 to 100 T μm during the linear growth phase for 120 μm perturbations. Proton fluence modulations were corrected for Coulomb scattering using measured areal density profiles from x-ray radiographs.

Mitigating laser imprint in direct-drive inertial confinement fusion implosions with high-Z dopants

Nonuniformities seeded by both long- and short-wavelength laser perturbations can grow via Rayleigh-Taylor (RT) instability in direct-drive inertial confinement fusion, leading to performance reduction in low-adiabat implosions. To mitigate the effect of laser imprinting on target performance, spherical RT experiments have been performed on OMEGA using Si- or Ge-doped plastic targets in a cone-in-shell configuration. Compared to a pure plastic target, radiation preheating from these high-Z dopants (Si/Ge) increases the ablation velocity and the standoff distance between the ablation front and laser-deposition region, thereby reducing both the imprinting efficiency and the RT growth rate. Experiments showed a factor of 2-3 reduction in the laser-imprinting efficiency and a reduced RT growth rate, leading to significant (3-5 times) reduction in the σ(rms) of shell ρR modulation for Si- or Ge-doped targets. These features are reproduced by radiation-hydrodynamics simulations using the two-dimensional hydrocode DRACO.

Increasing hydrodynamic efficiency by reducing cross-beam energy transfer in direct-drive-implosion experiments

A series of experiments to determine the optimum laser-beam radius by balancing the reduction of cross-beam energy transfer (CBET) with increased illumination nonuniformities shows that the hydrodynamic efficiency is increased by ∼35%, which leads to a factor of 2.6 increase in the neutron yield when the laser-spot size is reduced by 20%. Over this range, the absorption is measured to increase by 15%, resulting in a 17% increase in the implosion velocity and a 10% earlier bang time. When reducing the ratio of laser-spot size to a target radius below 0.8, the rms amplitudes of the nonuniformities imposed by the smaller laser spots are measured at a convergence ratio of 2.5 to exceed 8  μm and the neutron yield saturates despite increasing absorbed energy, implosion velocity, and decreasing bang time. The results agree well with hydrodynamic simulations that include both nonlocal and CBET models.

Using high-intensity laser-generated energetic protons to radiograph directly driven implosions

The recent development of petawatt-class lasers with kilojoule-picosecond pulses, such as OMEGA EP [L. Waxer et al., Opt. Photonics News 16, 30 (2005)], provides a new diagnostic capability to study inertial-confinement-fusion (ICF) and high-energy-density (HED) plasmas. Specifically, petawatt OMEGA EP pulses have been used to backlight OMEGA implosions with energetic proton beams generated through the target normal sheath acceleration (TNSA) mechanism. This allows time-resolved studies of the mass distribution and electromagnetic field structures in ICF and HED plasmas. This principle has been previously demonstrated using Vulcan to backlight six-beam implosions [A. J. Mackinnon et al., Phys. Rev. Lett. 97, 045001 (2006)]. The TNSA proton backlighter offers better spatial and temporal resolution but poorer spatial uniformity and energy resolution than previous D(3)He fusion-based techniques [C. Li et al., Rev. Sci. Instrum. 77, 10E725 (2006)]. A target and the experimental design technique to mitigate potential problems in using TNSA backlighting to study full-energy implosions is discussed. The first proton radiographs of 60-beam spherical OMEGA implosions using the techniques discussed in this paper are presented. Sample radiographs and suggestions for troubleshooting failed radiography shots using TNSA backlighting are given, and future applications of this technique at OMEGA and the NIF are discussed.

Uniform irradiation of adjustable target spots in high-power laser driver

For smoothing and shaping the on-target laser patterns flexibly in high-power laser drivers, a scheme has been developed that includes a zoom lens array and two-dimensional smoothing by spectral dispersion (SSD). The size of the target pattern can be controlled handily by adjusting the focal length of the zoom lens array, while the profile of the pattern can be shaped by fine tuning the distance between the target and the focal plane of the principal focusing lens. High-frequency stripes inside the pattern caused by beamlet interference are wiped off by spectral dispersion. Detailed simulations indicate that SSD works somewhat differently for spots of different sizes. For small spots, SSD mainly smooths the intensity modulation of low-to-middle spatial frequency, while for large spots, SSD sweeps the fine speckle structure to reduce nonuniformity of middle-to-high frequency. Spatial spectra of the target patterns are given and their uniformity is evaluated.

Velocity and timing of multiple spherically converging shock waves in liquid deuterium

The fuel entropy and required drive energy for an inertial confinement fusion implosion are set by a sequence of shocks that must be precisely timed to achieve ignition. This Letter reports measurements of multiple spherical shock waves in liquid deuterium that facilitate timing inertial confinement fusion shocks to the required precision. These experiments produced the highest shock velocity observed in liquid deuterium (U(s) = 135  km/s at ∼2500  GPa) and also the first observation of convergence effects on the shock velocity. Simulations model the shock-timing results well when a nonlocal transport model is used in the coronal plasma.

Experimental demonstration of Generalized Phase Contrast based Gaussian beam-shaper

We report the first experimental demonstration of Gaussian beam-shaping based on the Generalized Phase Contrast (GPC) approach. We show that, when using a dynamic spatial light modulator (SLM), this approach can rapidly generate arbitrarily shaped beams. Moreover, we demonstrate that low-cost binary-phase optics fabricated using photolithography and chemical etching techniques can replace the SLM in static and high power beam shaping applications. The design parameters for the binary-phase elements of the module are chosen according to the results of our previously conducted analysis and numerical demonstrations [Opt. Express 15, 11971 (2007)]. Beams with a variety of cross-sections such as circular, rectangular and square, with near flat-top intensity distributions are demonstrated. GPC-based beam shaping is inherently speckle-free and the shaped beams maintain a flat output phase. The non-absorbing components used in this beam-shaping approach have a high-damage-threshold and are thus ideally suited for high power applications.

Implosion experiments using glass ablators for direct-drive inertial confinement fusion

Direct-drive implosions with 20-microm-thick glass shells were conducted on the Omega Laser Facility to test the performance of high-Z glass ablators for direct-drive, inertial confinement fusion. The x-ray signal caused by hot electrons generated by two-plasmon-decay instability was reduced by more than approximately 40x and hot-electron temperature by approximately 2x in the glass compared to plastic ablators at ignition-relevant drive intensities of approximately 1x10(15) W/cm2, suggesting reduced target preheat. The measured absorption and compression were close to 1D predictions. The measured soft x-ray production in the spectral range of approximately 2 to 4 keV was approximately 2x to 3x lower than 1D predictions, indicating that the shell preheat caused by soft x-rays is less than predicted. A direct-drive-ignition design based on glass ablators is introduced.

Rayleigh-Taylor growth measurements in the acceleration phase of spherical implosions on OMEGA

The Rayleigh-Taylor (RT) growth of 3D broadband nonuniformities was measured using x-ray radiography in spherical plastic shells accelerated by laser light at an intensity of approximately 2 x 10(14) W/cm(2). The 20- and 24-microm-thick spherical shells were imploded with 54 beams on the OMEGA laser system. The shells contained diagnostic openings for backlighter x rays used to image shell modulations. The measured shell trajectories and modulation RT growth were in fair agreement with 2D hydro simulations during the acceleration phase of the implosions with convergence ratios of up to approximately 2.2. Since the ignition designs rely on these simulations, improvements in the numerical codes will be implemented to achieve better agreement with experiments.

Highly efficient nonlinear filter for femtosecond pulse contrast enhancement and pulse shortening

We propose a highly efficient scheme for temporal filters devoted to femtosecond pulse contrast enhancement. The filter is based on cross-polarized wave generation with a spatially suger-Gaussian-shaped beam. In a single nonlinear crystal scheme the energy conversion to the cross-polarized pulse can reach 28%. We demonstrate that the process enables a significant spectral broadening. For an efficiency of 23% the pulse shortening is estimated to 2.2, leading to an intensity transmission of the nonlinear filter of 50%.

First measurements of the absolute neutron spectrum using the magnetic recoil spectrometer at OMEGA (invited)

A neutron spectrometer, called a magnetic recoil spectrometer (MRS), has been built and implemented at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] for absolute measurements of the neutron spectrum in the range of 6-30 MeV, from which fuel areal density (rhoR), ion temperature (T(i)), and yield (Y(n)) can be determined. The results from the first MRS measurements of the absolute neutron spectrum are presented. In addition, measuring rhoR at the National Ignition Facility (NIF) [G. H. Miller et al., Nucl. Fusion 44, S228 (2004)] will be essential for assessing implosion performance during all stages of development from surrogate implosions to cryogenic fizzles to ignited implosions. To accomplish this, we are also developing an MRS for the NIF. As much of the research and development and instrument optimization of the MRS at OMEGA are directly applicable to the MRS at the NIF, a description of the design and characterization of the MRS on the NIF is discussed as well.

Validation of thermal-transport modeling with direct-drive, planar-foil acceleration experiments on OMEGA

We present for the first time the experimental validation of the nonlocal thermal-transport model for a National Ignition Facility relevant laser intensity of approximately 10(15) W/cm(2) on OMEGA. The measured thin target trajectories are in good agreement with predictions based on the nonlocal model over the full range of laser intensities from 2 x 10(14) to 10(15) W/cm(2}) The standard local thermal-transport model with a constant flux limiter of 0.06 disagrees with experimental measurements at a high intensity of approximately 10(15) W/cm(2) but agrees at lower intensities. These results show the significance of nonlocal effects for direct-drive ignition designs.

Rayleigh-Taylor growth stabilization in direct-drive plastic targets at laser intensities of approximately 1 x 10(15) W/cm2

Direct-drive, planar-target Rayleigh-Taylor growth experiments were performed for the first time to test fundamental physics in hydrocodes at peak drive intensities of ignition designs. The unstable modulation growth at a drive intensity of approximately 1 x 10(15) W/cm2 was strongly stabilized compared to the growth at an intensity of approximately 5 x 10(14) W/cm2. The experiments demonstrate that standard simulations based on a local model of electron thermal transport break down at peak intensities of ignition designs (although they work well at lower intensities). The preheating effects by nonlocal electron transport and hot electrons were identified as some of the stabilizing mechanisms.

Studies of plastic-ablator compressibility for direct-drive inertial confinement fusion on OMEGA

The compression of planar plastic targets was studied with x-ray radiography in the range of laser intensities of I approximately 0.5 to 1.5x10(15) W/cm2 using square (low-compression) and shaped (high-compression) pulses. Two-dimensional simulations with the radiative hydrocode DRACO show good agreement with measurements at laser intensities up to I approximately 10(15) W/cm2. These results provide the first experimental evidence for low-entropy, adiabatic compression of plastic shells in the laser intensity regime relevant to direct-drive inertial confinement fusion. A density reduction near the end of the drive at a high intensity of I approximately 1.5x10(15) W/cm2 has been correlated with the hard x-ray signal caused by hot electrons from two-plasmon-decay instability.

Role of hot-electron preheating in the compression of direct-drive imploding targets with cryogenic D2 ablators

The compression of direct-drive, spherical implosions is studied using cryogenic D2 targets on the 60-beam, 351-nm OMEGA laser with intensities ranging from approximately 3x10(14) to approximately 1x10(15) W/cm2. The hard-x-ray signal from hot electrons generated by laser-plasma instabilities increases with laser intensity, while the areal density decreases. Mitigating hot-electron production, by reducing the laser intensity to approximately 3x10(14) W/cm2, results in areal density of the order of approximately 140 mg/cm2, in good agreement with 1D simulations. These results will be considered in future direct-drive-ignition designs.

Fourier transform-based continuous phase-plate design technique: a high-pass phase-plate design as an application for OMEGA and the National Ignition Facility

A technique capable of calculating near-field, continuous phase diffractive optics (or phase plates) without phase dislocations and with optional far-field, speckle-spectrum control is introduced. The design technique improves upon a standard phase-retrieval method by adding convergence enhancements, phase continuity control, and far-field, speckle-spectrum control. The convergence enhancements improve the algorithm's efficiency. Phase continuity control eliminates phase dislocations and mitigates damaging retroreflections and transmissions. Specifying an optional constraint controls the far-field speckle spectrum. Application of these phase plates on the OMEGA and National Ignition Facility laser systems would produce well-controlled far-field spot shapes. High-pass phase-plate designs are compared with designs where the far-field spectrum is not controlled.

Beam-shaping longitudinal range of a binary diffractive optical element

An experimental and theoretical investigation of laser beam shaping using a simple binary diffractive optic is presented. Beam tailoring has been characterized by the experimental determination of two relevant parameters: beam propagation factor M(2) and the beam-shaping longitudinal range, which represents the propagating distance for which the tailored beam remains nearly unchanged.

Measuring E and B fields in laser-produced plasmas with monoenergetic proton radiography

Electromagnetic (E/B) fields generated by the interaction with plasmas of long-pulse, low-intensity laser beams relevant to inertial confinement fusion have been measured for the first time using novel monoenergetic proton radiography methods. High-resolution, time-gated radiography images of a plastic foil driven by a 10(14) W/cm(2) laser implied B fields of approximately 0.5 MG and E fields of approximately 1.5 x 10(8) V/m. Simulations of these experiments with LASNEX+LSP have been performed and are in overall (though not exact) agreement with the data both for field strengths and for spatial distributions; this is the first direct experimental test of the laser-generated B-field package in LASNEX. The experiments also demonstrated that laser phase plates substantially reduce medium-scale chaotic field structure.

Test of thermal transport models through dynamic overpressure stabilization of ablation-front perturbation growth in laser-driven CH foils

Heat-flow-induced dynamic overpressure at the perturbed ablation front of an inertial confinement fusion target can stabilize the ablative Richtmyer-Meshkov-like instability and mitigate the subsequent ablative Rayleigh-Taylor (RT) instability. A series of experiments was performed on the OMEGA laser to quantify the dynamic overpressure stabilization during the shock transit. Analysis of the experimental data using hydrocode simulations shows that the observed oscillatory evolution of the ablation-front perturbations depends on Dc, the size of the thermal conduction zone, and the fluid velocity in the blowoff region Vb1 that are sensitive to the thermal transport model used. We show that the simulations match the experiment well when the time dependence of the heat-flux inhibition is taken into account using a recently developed nonlocal heat-transport model [V. N. Goncharov et al., Phys. Plasmas 13, 012702 (2006)].

Fourier-space nonlinear Rayleigh-Taylor growth measurements of 3D laser-imprinted modulations in planar targets

Nonlinear growth of 3D broadband nonuniformities was measured near saturation levels using x-ray radiography in planar foils accelerated by laser light. The initial target modulations were seeded by laser nonuniformities and later amplified during acceleration by Rayleigh-Taylor instability. The nonlinear saturation velocities are measured for the first time and are found to be in excellent agreement with Haan predictions. The measured growth of long-wavelength modes is consistent with enhanced, nonlinear, long-wavelength generation in ablatively driven targets.

Time-resolved areal-density measurements with proton spectroscopy in spherical implosions

The temporal history of the target areal-density near peak compression of direct-drive spherical target implosions has been inferred with 14.7-MeV deuterium-helium-3 D3He proton spectroscopy of the 60-beam, 30-kJ UV OMEGA laser system. The target areal-density grows by a factor of approximately 8 during the time of neutron-production ( approximately 400 ps) before reaching 123+/-16 mg/cm(2) at peak compression in the implosion of a 950-micrometer-diam, 20-micrometer-thick plastic CH capsule filled with 4 atm of D3He fuel.

Iterative algorithm for the design of diffractive phase elements for laser beam shaping

An improved iterative algorithm for designing diffractive phase elements for laser beam shaping in free space is presented. The algorithm begins with the Gerchberg-Saxton approach to obtain a stable solution. This is followed by several new iterations, in which modified constraining functions are imposed in the Fourier domain while the phase distribution of each iteration remains unchanged. For super-Gaussian beam shaping suitable for inertial confinement fusion applications the mean-square errors of the amplitude and the intensity profile of the entire beam fitted to the corresponding parameters of the 12th-power super-Gaussian beam are approximately 0.035 and 9.75x10(-3), respectively. Approximately 97.4% of the incident energy is converged into the desired region.

Three-directional spectral dispersion for smoothing of a laser irradiance profile

In inertial confinement fusion research, uniform laser irradiation on a fusion target is a key issue. We propose a new method of beam smoothing in which we use three-directional spectral dispersion to reduce the coherent speckle that is unavoidable in the usual two-directional spectral-dispersion scheme. We have used this smoothing technique in a Nd:glass laser system and have demonstrated that the coherent speckle is reduced by a factor of 2.9 from that in two-directional spectral dispersion.

Evolution of shell nonuniformities near peak compression of a spherical implosion

The evolution of shell modulations near peak compression of direct-drive spherical-target implosions has been measured using the 60-beam, 30-kJ UV OMEGA laser system. The spatial size and amplitude of shell-areal-density modulations decrease during the target compression, then increase during its decompression as expected. The shell uniformity at peak compression has been increased by reducing single-beam, laser-drive nonuniformity.

Interface imprinting by a rippled shock using an intense laser

Perturbation imprinting at a flat interface by a rippled shock has been observed in a laser hydrodynamics experiment. A strong shock was driven through a three-layer target, with the first interface rippled, and the second flat. The chosen thickness of the second layer gave instability growth with opposite phases at the two interfaces, consistent with two-dimensional simulations and rippled shock theory.

Characterization of excimer lasers for application to lenslet array homogenizers

We investigate the best method of characterizing high-divergence lasers, such as excimer lasers, to suppress fine-scale intensity nonuniformity that is due to coherence effects of lenslet homogenizers. We show by a detailed analysis of the lenslet homogenizer that, for highest accuracy, a direct measurement of the value of the autocorrelation function should be made at the separation p of the lenslet elements, identified as the critical spatial period. We show that the commonly used characterization of lasers by the 1/e(2) width of the angular divergence is not the most accurate test and may overstate or understate the effectiveness of a given laser.

Pure-phase plates for super-Gaussian focal-plane irradiance profile generations of extremely high order

A set of recursive formulas for diffractive optical plates design is described. The pure-phase plates simulated by this method homogeneously concentrate more than 96% of the incident laser energy in the desired focal-plane region. The intensity focal-plane profile fits a 12th-order super-Gaussian function and has a nearly perfect f lat top. Its fit to the required profile measured in the mean square error is 3.576 x 10(-3).

Designing fully continuous phase screens for tailoring focal-plane irradiance profiles

An iterative algorithm for constructing fully continuous phase screens for tailoring far-field intensity profiles is presented. The algorithm is robust, stable, and, if run properly, maintains the continuous nature of the phase throughout the iterative process. The iterative procedure is applied to generate continuous phase screens to produce a 12th-power super-Gaussian far-field intensity profile.

Design of continuous surface-relief phase plates by surface-based simulated annealing to achieve control of focal-plane irradiance

High-performance phase plates are of vital concern for controlling the far-field irradiance of laser-fusion systems. Several designs for solving this difficult problem have been reported in Optics Letters [e. g., S. N. Dixit et al., Opt. Lett. 19, 417 (1994)]. We report a surface-based form of simulated annealing that significantly improves the irradiance control while eliminating the high-scatter problems that have plagued other methods.