Laser-irradiated Planar Targets Research Articles

Influence of radiative cooling effect on the plasma filamentations in the interaction of high-power laser with planar targets

Interaction of high-power laser with planar target is studied by using radiation-hydrodynamics simulation. When the laser interacts with the tungsten planar target, the laser energy deposition is uneven due to thermal filamentation instability and other reasons, and density fluctuations will appear in the front of the plasma, resulting in obvious plasma filamentation in the later stage. The researches of four materials, i.e. aluminum, copper, tungsten and gold, show that in the high-<i>Z</i> material tungsten and gold, due to the strong radiative cooling effect, the filamentation phenomena of the density distribution, electron temperature distribution and pressure distribution obviously occur. The order of magnitude of filamentous plasma density is different from that of the surrounding plasma. The filamentation phenomenon is closely related to the non-uniform energy deposition of the laser and the radiative cooling effect, although the ray beam will cause inhomogeneity of the laser irradiation to a certain extent, this is not the main reason for the filamentation phenomenon observed in this paper. Owing to refraction, reflection and the thermal filamentation instability when the laser is transmitted in the ablation plasma, the laser energy is deposited unevenly, which generates instability seeds in the early stage of plasma formation. The radiative cooling effect then amplifies this instability seeds, creating a radiative cooling instability that eventually results in a filamentous distribution of physical quantities such as plasma density, temperature, and pressure. This filamentation phenomenon destroys the uniformity of the plasma to a certain extent, and lays the seeds for the growth of fluid instability, which will seriously affect fusion-related research. It is shown that radiative cooling is crucial to the filamentation phenomenon, which causes uneven distribution of the plasma pressure during the evolution of the plasma, thereby affecting its transverse motion and enhancing the density fluctuation. After the laser irradiation ends, the density fluctuation gradually develops into filamentations. We also find that the clear filamentation occurs only for high-<i>Z</i> materials like tungsten and gold, but not for the moderate-<i>Z</i> materials like aluminum and copper. This can be attributed to the fact that radiative cooling is stronger for the high-<i>Z</i> materials. Studying the filamentation effect in laser-irradiated planar targets can contribute to understanding the instability in laser plasma, and then suppressing this instability and improving the gain of fusion. The results here can thus be of reference significance to the research of laser fusion, laboratory astrophysics, and other applications of intense-laserdriving.

Strong magnetic fields generated with a metal wire irradiated by high power laser pulses and its effect on bow shock

Laboratory astrophysics is a rapid developing field studying astrophysical or astronomical processes on a high-power pulsed facility in laboratory. It has been proved that with the similarity criteria, the parameters in astrophysical processes can be transformed into those under laboratory conditions. With appropriate experimental designs the astrophysical processes can be simulated in laboratory in a detailed and controlled way. Magnetic fields play an important role in many astrophysical processes. Recently, the generation of strong magnetic fields and their effects on relevant astrophysics have attracted much interest. According to our previous work, a strong magnetic field can be induced by a huge current formed by the background cold electron flow around the laser spot when high power laser pulses irradiate a metal wire. In this paper we use this scheme to produce a strong magnetic field and observe its effect on a bow shock on the Shenguang II (SG II) laser facility. The strength of the magnetic field is measured by B-dot detectors. With the measured results, the magnetic field distribution is calculated by using a three-dimension code. Another bunch of lasers irradiates a CH planar target to generate a high-speed plasma. A bow shock is formed in the interaction of the high-speed plasma with the metal wire under the strong magnetic condition. The effects of the strong magnetic field on the bow shock are observed by shadowgraphy and interferometry. It is shown that the Mach number of the plasma flow is reduced by the magnetic field, leading to an increase of opening angle of the bow shock and a decrease of the density ratio between downstream and upstream. In addition, according to the similarity criteria, the experimental parameters of plasma are scaled to those in space. The transformed results show that the magnetized plasma around the wire, produced by X-ray emitted from the laser-irradiated planar target in the experiment, is suitable for simulating solar wind in astrophysics. In this paper, we provide another method to produce strong magnetic field, apply it to a bow shock laboratory astrophysical study, and also generate the magnetized plasma which can be used to simulate solar wind in the future experiments.

Suprathermal electron production in laser-irradiated Cu targets characterized by combined methods of x-ray imaging and spectroscopy

In a series of experiments performed with laser-irradiated planar targets at the PALS laser facility, the generation of suprathermal electrons has been studied at conditions relevant for the development of a shock ignition approach to inertial confinement fusion. A simultaneous application of high-collection-efficiency K-shell imaging with high resolution x-ray spectroscopy offers a novel approach to hot electron diagnosis at non-coated or moderately coated, medium-atomic-number targets, where the contribution of suprathermal-electron-generated, frequency-shifted Kα emission from highly ionized atoms cannot be neglected. Based on experimental data provided by these combined techniques and their interpretation via collisional-radiative atomic codes and Monte Carlo modeling of hot electron energy deposition in heated Cu targets, the fraction of the energy converted to hot electrons at laser intensities ≈1016 W cm−2 was measured to be at the level of 0.1–0.8%. The higher values of conversion efficiency found for frequency tripled radiation support a theoretical conjecture of enhanced laser energy absorption by a resonance mechanism and its transport to a flow of fast electrons.

Numerical study of the ablative Richtmyer–Meshkov instability of laser-irradiated deuterium and deuterium-tritium targets

The Richtmyer–Meshkov instability (RMI) at the ablation front of laser-irradiated planar targets is investigated by two-dimensional numerical hydrodynamics simulations. The linear evolution of perturbations seeded either by surface roughness or target inhomogeneity is studied for perturbation wavelengths in the range 10≤λ≤400 μm and laser intensity 4×1012≤I≤4×1014 W/cm2 (with laser wavelength λlaser=0.35 μm). Thin and thick cryogenic deuterium or deuterium-tritium (DT) planar targets are considered. For targets irradiated at constant intensity, it is found that perturbations with wavelength below a given threshold perform damped oscillations, while perturbations above such a threshold are unstable and oscillate with growing amplitude. This is qualitatively in agreement with theoretical predictions by Goncharov et al. [Phys. Plasmas 13, 012702 (2006)], according to which ablation related processes stabilize perturbations with kDc⪢1, where Dc is the distance between the ablation front and critical density for laser propagation. For kDc<1 a weakly growing Landau–Darrieus instability (LDI) is instead excited. The stability threshold increases substantially with laser intensity, given the dependence of Dc on laser intensity I (roughly Dc∝I, according to the present simulations). Direct-drive laser fusion targets are irradiated by time-shaped pulses, with a low intensity initial foot. In this case, perturbations with wavelengths below some threshold (about 10 μm, for typical ignition-class all-DT targets) are damped after an initial growth. In a thin target, initial perturbations, either damped or amplified by RMI and LDI, seed the subsequent Rayleigh–Taylor instability. Finally, it is shown that RMI growth of fusion targets can be reduced by using laser pulses including an initial adiabat-shaping picket (originally proposed to reduce the growth of Rayleigh–Taylor instability).

Penumbral imaging for measurement of the ablation density in laser-driven targets

One-dimensional (1D) penumbral imaging technique with high spatial resolution has been developed, and applied to density profile measurements in laser-driven Rayleigh-Taylor instability experiments at a GEKKO XII-HIPER laser system. A laser-irradiated planar target was observed with side-on x-ray backlighting. A penumbral image of an x-ray radiograph was made by using a knife-edge imager. The x-ray radiograph was, then, reconstructed by differentiating the penumbral image with a proper Wiener filtering. A density profile was deduced from the reconstructed x-ray radiograph. In a proof-of-principle experiment, the density profile of a polystyrene (PS) target before laser irradiation was measured by using this method, and high spatial resolution of 3–4 μm was demonstrated. A laser-irradiated PS target in the acceleration phase was observed. The experimentally observed density profile was found to be consistent with the prediction by a 1D hydrodynamic simulation code. The x-ray penumbral imaging is a very simple and useful technique with high spatial resolution for research in hydrodynamics relevant to inertial fusion energy.

Angular distribution of X-ray line radiation from laser-irradiated planar targets

A study of angular distribution of X-ray line radiation emitted from a laser-produced plasma is presented. Plasma was produced by irradiating a planar magnesium target using single laser pulses of 6 J, 4.5 ns (FWHM) from an Nd:glass laser system. Angular distribution of X-ray emission in the spectral range of 7–10 Å was recorded using a single X-ray crystal spectrograph. X-ray emission intensity for different resonance lines is observed to decrease up to an angle of ∼45° with respect to target normal, followed by a significant increase at higher angles. The observed departure of angular distribution from a monotonically decreasing behavior is broadly in agreement with the calculations of the escape factor of the radiation at different angles from a thick disk-shaped plasma using a spectroscopic code RATION.

Experimental study of X-ray emission from laser-irradiated planar targets on “Mishen” Facility

X-ray emission from planar targets irradiated by 1.054-μm laser pulses was observed with temporal, spatial, and spectral resolution. The main purpose of these measurements was the investigation of energy transfer in multilayer targets and X-ray conversion efficiency. A mass ablation rate was determined from temporal analysis of multicharged ion line emission and a key role of corona X-ray emission in accelerated material preheating was established.

Plasma jets from laser-irradiated planar targets.

We have observed plasma jets from laser-irradiated gold disk targets. The structures originate from low-intensity regions of the incident laser beam but propagate much faster than similar density surfaces from higher-intensity regions. The material is both cold and dense, as evidenced by its emission of soft x rays and absorption of hard x rays.

Determination of Space and Time Resolved Plasma Diagnostics in Laser Produced Plasmas

Spectroscopic data from a variety of Nd glass laser-irradiated planar targets is examined. The experiments were performed for a number of purposes, including development of intense x-ray sources for absorption studies important in ICF, as well as investigations of target material properties. The data are examined with the non-LTE atomic rate equation code package, RATION. RATION solves for and displays detailed theoretical spectra for a number of H-like, He-like, and Li-like atomic transitions and is presently available to users of the MFE Computer Center network. The package has been enhanced to include a larger number of Li-like levels, some in full detail, and the ability to include time-dependent and space-dependent hydrodynamic information. The main results are spectral line ratios and displays of the full spectrum employing the RATION atomic model. Using these tools, time- and space-dependent plasma temperature and density diagnostics are produced and compared with the experimental data.

Dynamics of laser-irradiated planar targets measured by x-ray spectroscopy.

The dynamics of planar plastic targets seeded with small amounts of sulfur as a diagnostic tracer is studied spectroscopically. The time-dependent x-ray emission of the hydrogenic and heliumlike sulfur is used to provide a measure of the plasma parameters. The experiment, which is designed to provide optically thin x-ray lines with small transverse gradients, is analyzed by the coupling of a kinetics model to a hydrodynamics simulation. The results of the analysis show that the time histories of the x-ray line intensities can be used as a plasma diagnostic to constrain the modeling. Intensity ratios between ionization stages cannot be accurately modeled even including time-dependent ionization effects. This disagreement appears to be due to detailed assumptions concerning electron distributions and transport.

Electrical conductivity of a dense plasma.

The ac electrical conductivity of a dense plasma is studied by use of reflectivity measurements on the rear surface of a laser-irradiated planar target. The results show general agreement with theory in which a minimum collision mean free path is assumed, yielding correspondingly a minimum conductivity. However, the results also suggest that the minimum mean free path is greater than the interatomic radius.

Smoothing and instability with magnetic field in a nonuniformly laser-irradiated planar target (plasma)

Calculations are presented of the magneto-hydrodynamic response of a planar target to nonuniformities in energy deposition by a laser. The amplitude of the nonuniformities are assumed small and the equations are linearised in small perturbations about the solution for steady planar ablation driven by uniform laser energy deposition. The grad(n)xgrad(T) magnetic field source is included, along with Nernst convection and the Righi-Leduc heat flow. The magnetic field is shown to give a small increase in smoothing. A source term for magnetic field is induced to simulate the effects of the Weibel instability. The instability is not strong enough to overcome the smoothing processes under the present assumptions.

Temporal and spectral features of the (3/2) ω0 spatial fine structure in laser irradiated planar targets

Both temporally and spectrally resolved fine structures of 90° lateral (3/2)ω0 harmonic emission were experimentally observed. The existence of (3/2)ω0 harmonic filaments is attributed to formation from the moving filament bottom at its transient nc/4 surface. The double hump structure with its intense red hump of laterally emitted (3/2)ω0 spectrum can also be explained by a Doppler correction in addition to the thermal correction. Estimation of both red and blue spectral shifts with our simple model is qualitatively consistent with experimental results. Experiments showed that the (3/2)ω0 emission was evidently suppressed by using a broad band laser. This may be promising for superhot electron suppression.

Radiation heating in laser-irradiated planar targets

Planar targets of different Z were irradiated with 0.53 μm laser light at intensities ≤5×1013 W/cm2. The target rear surface was observed to be first heated by the laser-generated shock wave. This was followed by a strong second heating which was attributed to radiation transport through the decompressed target. This additional heating might also be responsible for enhancing early laser transmission through thick targets.

Filamentation of an annular laser beam in a short wavelength laser produced plasma

We report on the first observations of periodic structures caused by the filamentation instability. These structures were observed on 0.35 μm laser irradiated planar target experiments. Comparison of these observations with self-focusing theory indicates that even though thermal filamentation may initiate the instability, ponderomotive force effects eventually dominate the actual filamentation process.