Two-dimensional Radiation Hydrodynamic Code Research Articles

Studies on the motion and radiation of interior plasmas in gas-filled hohlraums with different laser entrance hole sizes

An experiment on 100 kJ laser facility is performed to study the motive features and radiation properties of plasmas from different areas inside gas-filled cylindrical hohlraums. These hohlraums are designed to possess one open end and one laser entrance hole (LEH) with different diameters, which would or not result in the blocking of the LEH. An x-ray streak camera that is set at 16 degrees with respect to the hohlraum axis is applied to acquire the time-resolved x-ray images from the open end. Based on the images, we can study the evolutions of the wall plasma, corona bubble plasma and LEH plasma simultaneously through an equivalent view field of hohlraum interior. Multi-group flat response x-ray detectors are applied to measure the x-ray fluxes. In order to understand these characteristics, our two-dimensional radiation hydrodynamic code is used to simulate the experimental results. For the accuracy of reproduction, dielectronic recombination and two parameter corrections are applied in our code. Based on the comparison between experiments and simulations, we quantitatively understand the blocking process of LEH and the motion effects of other plasmas. The calibrated code is beneficial to design the gas-filled hohlraum in a nearby parameter space, especially the limit size of LEH.

Mitigating laser imprint with a foam overcoating

In direct-drive inertial confinement fusion, laser imprint can cause areal density perturbations on the target shell that seed the Rayleigh–Taylor instability and further degrade the implosion. To mitigate the effect of laser imprint, a foam overcoating layer outside the target shell has been suggested to increase the thermal smoothing of the conduction region (between the ablation front and the critical density surface) and mass ablation of the ablation front. In this paper, we use a two-dimensional radiation hydrodynamic code FLASH to investigate the laser imprint mitigation performance and find other physical mechanisms of foam overcoatings. First, radiation ablation dynamically modulates density distribution not only to increase the frequency of the perturbed ablation front oscillation but also to decrease the amplitude of the oscillation. Second, a larger length of the shocked compression region reduces the amplitude of the perturbed shock front oscillation. The areal density perturbations decrease with the decrease in the perturbations of the ablation front and shock front. Based on the abovementioned physical mechanisms, we propose the optimal ranges of foam parameters to mitigate laser imprint with the aid of dimensional analysis: the foam thickness is about two to three times that of the perturbation wavelength, and the foam density is about 1/2–3/2 times that of the critical density.

Experimental and simulation studies on gold bubble movement in gas-filled hohlraums

A recent experiment on the Shenguang III laser facility has applied open-end gold hohlraums with two gas pressures to study the movement of a plasma bubble. Under a laser intensity and width close to those of an ignition main pulse, the bubbles possess radial scales from several hundred to one thousand micrometers. An x-ray framing camera is used to measure the N-band x-ray images generated from the gold bubble plasma at different moments, from which the edge positions of the expanding bubbles are accurately acquired. The experimental results are simulated by an optimized two-dimensional radiation hydrodynamic code. Based on the classical average atom (AA) model, two phenomenological coefficients Cer and Cop are introduced into the code to correct the bubble evolution. Cer artificially redistributes the energies between radiation and matter, and Cop correlatively adjusts the plasma opacity. The bubble movement simulated by the novel phenomenological model agrees better with the measured result than that by the AA model. This work plays a critical role in our code improvements and advances the reliability of the hohlraum design.

Quantitative analysis of vacuum-ultraviolet radiation from nanosecond laser-zinc interaction

The paper reports measurements of the vacuum-ultraviolet spectral irradiances of a flat zinc target over a wavelength region of 124–164 nm generated by 10 and 60 ns duration low-intensities, 5×109-3×1010 W cm−2, 1.06 μm wavelength laser pulses. Maximum radiation conversion efficiencies of 2.5%/2πsr and 0.8%/2πsr were measured for 60 and 10 ns laser pulses at the intensities of 5×109 and 1.4×1010 W cm−2, respectively. Atomic structure calculations using a relativistic configuration-interaction, flexible atomic code and a developed non-local thermodynamic equilibrium population kinetics model in comparison to the experimental spectra detected by the Seya-Namioka type monochromator reveal the strong broadband experimental emission originates mainly from 3d94p-3d94s, 3d94d-3d94p and 3d84p-3d84s, 3d84d-3d84p unresolved-transition arrays of double and triple ionized zinc, respectively. Two-dimensional radiation-hydrodynamics code is used to investigate time-space plasma evolution and spectral radiation of a 10 ns full-width-at-half-maximum Gaussian laser pulse-zinc interaction.

Radiative cooling induced plasma collapse observed in laser irradiation of a CH-tamped gold micro-disk

Time-resolved x-ray self-emission imaging was used to study the dynamic evolution of a laser-produced gold plasma tamped by plastic (CH), and a significant plasma collapse was observed during the laser irradiation. The plasma collapse, a kind of transverse contraction, has been ascribed to the radial compression caused by the different radiative cooling rates and thus different pressures between the central high-Z gold plasma and the surrounding low-Z CH plasma, and this has been reproduced by numerical simulations using the two-dimensional radiation-hydrodynamics code Multi2D. The experimental results represent an observation of the radiative cooling induced plasma jet within a 1 ns laser pulse duration, much more quickly than those reported previously. In addition, our experiment design may offer a method to study the radiative cooling rates of high-Z plasmas. The measured cooling rate is a factor of 2 higher than the theoretical result [Post et al., At. Data Nucl. Data Tables 20, 397 (1977)], but is within the stated calculational uncertainty.

Measurements and non-local thermodynamic equilibrium modeling of mid-Z plasma emission

The x-ray yields from laser-irradiated thin foils of iron, copper, zinc, and germanium have been measured in the soft and multi-keV x-ray ranges at the OMEGA laser at the Laboratory for Laser Energetics. The incident laser power had a pre-pulse to enhance the x-ray emission of a 1 ns flat-top main pulse. The experimental results have been compared with post-shot simulations performed with the two-dimensional radiation-hydrodynamics code FCI2. A new non-local thermodynamic equilibrium model, NOO-RAD, have been incorporated into FCI2. In this approach, the plasma ionization state is in-line calculated by the atomic physics NOHEL package. In the soft x-ray bands, both simulations using RADIOM [M. Busquet, Phys. Fluids B 5, 4191 (1993)] and NOO-RAD clearly over-predict the powers and energies measured by a broad-band spectrometer. In one case (the iron foil), the discrepancy between the measured and simulated x-ray output is nevertheless significantly reduced when NOO-RAD is used in the simulations. In the multi-keV x-ray bands, the simulations display a strong sensitivity to the coupling between the electron thermal conductivity and the NLTE models, and for some particular combinations of these, provide a close match to the measured emission. The comparison between the measured and simulated H-like to He-like line-intensity ratios deduced from high-resolution spectra indicates higher experimental electron temperatures were achieved, compared to the simulated ones. Measurements of the plasma conditions have been achieved using the Thomson-scattering diagnostic. The electron temperatures are found to range from 3 to 5 keV at the end of the laser pulse and are greater than predicted by the simulations. The measured flow velocities are in reasonable agreement with the calculated ones. This last finding gives us confidence in our numerical predictions for the plasma parameters, which are over that time mainly determined by hydrodynamics, such as the mass densities and the ion temperatures.

Characterizing the hohlraum radiation via one-end driven experiments

A new experiment is designed and performed on the Shenguang III laser facility with the first eight available beams to characterizing the hohlraum radiation, in which the hohlraum with laser entrance holes on both ends is driven through one-end only. The experiment enables us to identify the x-ray radiations originated from the hohlraum reemission wall and high-Z bubble plasmas utilizing their position and spectral characters, which provides a better test on the associated hohlraum models. The total and M-band x-ray radiation fluxes are measured with the flat response x-ray detectors and the filtered M-band x-ray detectors, respectively. Numerical simulations are conducted with the two-dimensional radiation hydrodynamic code LARED-INTEGRATION using the multi-group radiation transfer and/or diffusion models. It is found that the experimentally measured temporal profiles and angular distributions of hohlraum radiation are in good agreement with the predictions of simulation using radiation transfer models, but differ significantly from the results obtained with the multi-group radiation diffusion calculations. We thus note that to accurately represent the hohlraum radiation, a true radiation transfer model is essential.

An improved time-dependent nonlocal electron heat-flux model and its verification by laser-driven Al foil acceleration experiment

In hydrodynamics simulation of laser driven systems, the time-dependent nonlocal electron heat-flux models predict the saturation (flux inhibition) and delocalization of the heat-flux automatically. Therefore it avoids commonly used time and space-independent ad hoc flux limiting. Previously proposed analytical nonlocal heat-flux model of Luciani et al. [Phys. Rev. Lett., 51, p-1664, (1983)] which fits the results of numerical Fokker–Planck calculations is simple and straight forward to implement in a fluid code. The proposed expression, however, is a convolution of Spitzer–Harm heat-flux with a delocalization kernel which depends on classical electron collision mean free path. This is rigorously valid for high temperature non-degenerate plasmas. However, in laser driven systems, the energy transport due to electron thermal conduction is important in regions between the critical density and ablation surface where the plasma is mostly degenerate. We have improved this nonlocal heat-flux model by using a wide-range electron collision frequency model valid from warm-dense matter (degenerate plasmas) to fully ionized plasmas. The effect of this improved nonlocal heat-flux model on the free-surface velocity of laser-accelerated Al foils of thickness 2–10 μm is studied by using a two-dimensional radiation hydrodynamics code. The simulated free surface velocities are compared with our experimental results for laser intensities in the range 4 × 1013–3 × 1014 W/cm2. Preliminary analysis shows that the simulation results obtained with improved nonlocal heat-flux model yields better agreement with our experimental values.

Extreme ultraviolet interferometry of laser plasma material between the critical and ablation surfaces

Interferometric probing using an extreme ultraviolet (EUV) laser has measured both transmission and phase information through laser-irradiated plastic (parylene-N C 8H 8) targets (thickness 350 nm). Unusually, the probe beam is incident longitudinally in approximately the same direction as the incident optical laser. Agreement of the experimental interferometry results has been obtained with two-dimensional radiation hydrodynamic code (h2d) simulations of EUV (21.2 nm) probe transmissions and phase shifts. We show that the transmission of the EUV probe beam provides a measure of the rate of target ablation, as ablated plasma becomes close to transparent when the photon energy is less than the ionization energy of the predominate ion species. Here C 3+ ions with ionization energy 64.5 eV are transparent, while lower carbon ionization stages, present in the unablated target and close to the ablation surface, absorb the 58.5 eV photons. Similarly, we show that refractive indices η below the solid parylene-N ( η solid = 0.946) and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound–free absorption in C +.

Analogy of meteorite impacts in laboratory conditions

In this work, we report on the possibility of using laser-generated craters to investigate planetary events such as meteorite impact craters. Experiments using a 0.44 μm/350 ps wavelength laser beam on aluminum foil targets are performed. We obtain simple and complex craters similar in contour to those formed due to meteorite impacts on the terrestrial surface. Our preliminary results show that the central peak of complex craters seems to be higher because of the turbulence beneath the crater floor. Results obtained using a two-dimensional radiation hydrodynamic code, MULTI, show that the laser ablation surface modifies and rises. This could be one of several factors explaining the central peak existence.

Simulation study of Hohlraum experiments on SGIII-prototype laser facility

The Hohlraum physics experiments performed on the SGIII-prototype laser facility are simulated by using our two-dimensional radiation hydrodynamic code LARED-H, and the influence of laser intensity on the two-dimensional Hohlraum simulations is studied. Both the temporal radiation temperature and the x-ray spectrum from the simulations agree well with the observations, except that the simulated M-band fraction (greater than 2 keV) is obviously smaller than the observation. According to our study, the coupling efficiency from laser to x-ray is around 70% for SGIII-prototype laser facility Hohlraums.

Multi-keV x-ray source development experiments on the National Ignition Facility

We report results from a five shot campaign carried out with Ar–Xe gas-filled targets at the National Ignition Facility (NIF). The targets were shot with ≈350 kJ of 3ω laser energy delivered with a 5 ns trapezoidal laser pulse. We report measured x-ray output from the target in different spectral bands both below and above 1.5 keV photon energies: We find yields of ≈20.5 kJ/sr with peak x-ray power approaching 4 TW/sr over all energies, as measured for the unique viewing angle of our detector, and ≈3.6 kJ/sr with peak x-ray power of 1 TW/sr for x-rays with energies >3 keV. This is a laser-to-x-ray conversion efficiency of 13±1.3% for isotropic x-rays with energies >3 keV. Laser energy reflected by the target plasma for both inner and outer-cone beams is measured and found to be small, between 1% and 4% of the drive energy. The energy emitted in hard x-rays (with energies >25 keV) is measured and found to be ≈1 J/sr. Two-dimensional imaging of the target plasma during the laser pulse confirms a fast, volumetric heating of the entire target, resulting in efficient laser-to-x-ray conversion. Postshot simulations with a two-dimensional radiation-hydrodynamics code reproduce well the observed x-ray flux and fluence, backscattered light, and bulk target motion.

A computational study of x-ray emission from laser-irradiated Ge-doped foams

New advances in fabrication of low-density high-Z-doped foams have opened new windows on understanding how materials that are not in local thermodynamic equilibrium (LTE) are heated and radiate. Simulations are discussed in this paper of the x-ray spectral emissions from laser-irradiated very low-density Ge-doped silica aerogel targets using a two-dimensional radiation-hydrodynamics code incorporating a modern non-LTE superconfiguration atomic model. Details of the computational model are presented, and it is shown that, for the long-scale-length, subcritical-density, ∼2–3 keV electron temperature plasmas created in experiments at the Omega laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], the simulations provide a close match to both the measured Ge L-shell emission (∼1–1.5 keV) and the measured Ge K-shell emission (∼10–11 keV), but only by accounting properly for nonlocal thermal conduction. The older average-atom atomic model is shown to be inadequate for these non-LTE plasmas.

Experimental and computational characterization of hydrodynamic expansion of a preformed plasma from thin-foil target for laser-driven proton acceleration

AbstractWe characterize the electron density distributions of preformed plasma for laser-accelerated proton generation. The preformed plasma of a titanium target 3 μm thick is generated by prepulse and amplified spontaneous emission (ASE) of a high-intensity Ti:sapphire laser and is measured with an interferometer using a second harmonic probe beam. High-energy protons are obtained by reducing the size of the preformed plasma by changing the ASE duration before main pulse at the front side (laser incidence side) of the target. Simulation results with two-dimensional radiation hydrodynamic code are close to the experimental results for low-density region ~4 × 1019 cm−3 at the front side. In the high-density region near to the target surface, the interferometry underestimates the density due to the substantial refraction. The characterization of hydrodynamic expansion with the interferometer and simulation is a useful tool for investigation of high-energy proton generation.

Numerical study of the advanced target design for FIREX-I

In the fast ignition (FI) scheme, at first high density fuel core plasma is assembled by an implosion laser and then heated by a peta watt laser to achieve the fusion burning condition. A typical target for FI is a shell fitted with a re-entrant Au cone to keep a path for the heating laser clean. Recently, for the FIREX-I scale size experiment, the CH tip of the guiding cone has advantages from the viewpoint of the generation of hot electrons and hot electron transport. As a result, from the viewpoint of implosion dynamics, cone-guided implosion with the CH tip is studied using the two-dimensional radiation hydrodynamic code. Two shockwaves caused by the leading shock of implosion and jet from the core determine the breakup time of the tip. With a sophisticated target and laser pulse shape design, CH, CD or other low-Z material tips will be realized without breaking up the cone tip before the maximum compression timing and debasement of the implosion performance.

Studies of supersonic, radiative plasma jet interaction with gases at the Prague Asterix Laser System facility

The interaction of laser driven jets with gas puffs at various pressures is investigated experimentally and is analyzed by means of numerical tools. In the experiment, a combination of two complementary diagnostics allowed to characterize the main structures in the interaction zone. By changing the gas composition and its density, the plasma cooling time can be controlled and one can pass from a quasiadiabatic outflow to a strongly radiation cooling jet. This tuning yields hydrodynamic structures very similar to those seen in astrophysical objects; the bow shock propagating through the gas, the shocked materials, the contact discontinuity, and the Mach disk. From a dimensional analysis, a scaling is made between both systems and shows the study relevance for the jet velocity, the Mach number, the jet-gas density ratio, and the dissipative processes. The use of a two-dimensional radiation hydrodynamic code, confirms the previous analysis and provides detailed structure of the interaction zone and energy repartition between jet and surrounding gases.

Supersonic heat wave propagation in laser-produced underdense plasma for efficient x-ray generation

We have observed supersonic heat wave propagation in a low-density aerogel target (ρ ∼ 3.2 mg/cc) irradiated at the intensity of 4 × 1014 W/cm2. The heat wave propagation was measured with a time-resolved x-ray imaging diagnostics, and the results were compared with simulations made with the two-dimensional radiation-hydrodynamic code, RAICHO. Propagation velocity of the ionization front very slightly decreased as the wave propagates into the target. The reason of decrease is due to increase of laser absorption region as the front propagates and interplay of hydrodynamic motion and reflection of laser propagation in the target. These features are well reproduced with the simulation.

Supersonic propagation of ionization waves in an underdense, laser-produced plasma

A laser-driven supersonic ionization wave propagating through a millimeter-scale plasma of subcritical density up to 2–3keV electron temperatures was observed. Propagation velocities initially ten times the sound speed were measured by means of time-resolved x-ray imaging diagnostics. The measured ionization wave trajectory is modeled analytically and by a two-dimensional radiation-hydrodynamics code. The comparison to the modeling suggests that nonlocal heat transport effects may contribute to the attenuation of the heat-wave propagation.

The ablation-front Rayleigh–Taylor dispersion curve in indirect drive

The Rayleigh–Taylor (RT) instability, which occurs when a lower-density fluid accelerates a higher-density layer, is common in nature. At an ablation front a sharp reduction in the growth rate of the instability at short wavelengths can occur, in marked contrast to the classical case where growth rates are highest at the shortest wavelengths. Theoretical and numerical investigations of the ablative RT instability are numerous and differ considerably on the level of stabilization expected. Presented here are the results of a series of laser experiments designed to measure the RT dispersion curve for a radiatively driven sample. Aluminum foils with imposed sinusoidal perturbations ranging in wavelength from 10 to 70 μm were ablatively accelerated with a radiation drive generated in a gold cylindrical hohlraum. A strong shock wave compresses the package followed by an ∼2 ns period of roughly constant acceleration and the experiment is diagnosed via face-on radiography. Perturbations with wavelengths ⩾20 μm experienced substantial growth during the acceleration phase while shorter wavelengths showed a sharp drop off in overall growth. These experimental results compared favorably to calculations with a two-dimensional radiation-hydrodynamics code, however, the growth is significantly affected by the rippled shock launched by the drive. Due to the influence of the rippled shock transit phase of the experiment and ambiguities associated with directly extracting the physical amplitude of the perturbations at the ablation front from the simulations, direct comparison to the ablation front RT theory of Betti et al. [Phys. Plasmas 5, 1446 (1998)], was difficult. Instead, a numerical “experiment” was constructed that minimized the influence of the shock and this was compared to the Betti model showing quite good agreement.

A nonlocal electron conduction model for multidimensional radiation hydrodynamics codes

Numerical simulation of laser driven Inertial Confinement Fusion (ICF) related experiments require the use of large multidimensional hydro codes. Though these codes include detailed physics for numerous phenomena, they deal poorly with electron conduction, which is the leading energy transport mechanism of these systems. Electron heat flow is known, since the work of Luciani, Mora, and Virmont (LMV) [Phys. Rev. Lett. 51, 1664 (1983)], to be a nonlocal process, which the local Spitzer–Harm theory, even flux limited, is unable to account for. The present work aims at extending the original formula of LMV to two or three dimensions of space. This multidimensional extension leads to an equivalent transport equation suitable for easy implementation in a two-dimensional radiation-hydrodynamic code. Simulations are presented and compared to Fokker–Planck simulations in one and two dimensions of space.