Laser-plasma Experiments Research Articles

Observation of both wings of the backscattered 3/2 omega 0 emission near threshold laser intensity from long-scale-length laser plasma

Spectral data on 3/2 omega 0 emission from a long-scale-length carbon plasma produced by 1.064 mu m laser light are presented at an average laser intensity of 3.0*1013 W cm-2. Both wings of the spectrum in the backscattering direction, which were observed in the laser-plasma experiments reported earlier only at laser intensities greater than 2*1014 W cm-2, have been observed in the present experiment. The possible mechanisms producing the backscattered 3/2 omega 0 blue photons have been analysed. It is concluded that the specular reflection of the amplified blue plasmons excited by the convecting two-plasmon decay instability is density-scale-length dependent and is the effective mechanism for the observation of the blue wing in the spectrum.

Collisional regimes of radiation-driven Langmuir turbulence

Numerical solutions of the Zakharov equations for a plasma driven above the electron plasma frequency by a long-wavelength radiation pump can be applied to both ionospheric modification experiments and laboratory laser-plasma interactions. A key difference between these two environments is the much larger collisional damping of Langmuir waves near the critical density in laser plasmas. Zakharov equation simulations in one and two dimensions reveal a significant change in the character of the saturated turbulence state of the electromagnetic ion-acoustic decay instability for pump strengths near threshold as the collisional damping is increased to values appropriate to certain (low-intensity) laboratory laser-plasma experiments [Phys. Fluids B 3, 1983 (1991)]. The linear-instability is then characterized by the coupling of the up- and downshifted Langmuir decay modes. A new turbulence regime differing from existing models of both weak and strong turbulence is found, which is characterized by a sequence of narrow peaks in the Langmuir frequency spectrum and a nonlinearly broadened wave-vector spectrum centered near the linearly most unstable modes. Results in this regime may be relevant to second harmonic emission experiments [Phys. Fluids B 3, 1983 (1991)].

The frequency and damping of ion acoustic waves in hydrocarbon (CH) and two-ion-species plasmas

The kinetic theory of ion-acoustic waves in multi-ion-species plasmas is discussed. Particular application is made to hydrocarbon (CH) plasmas, which are widely used in laser–plasma experiments. The mode frequencies and Landau damping of the two, dominant, ion-acoustic modes in CH plasmas are calculated by numerical solution of the kinetic dispersion relation. In addition, some useful results are obtained analytically from expansions of the kinetic dispersion relation and from fluid models. However, these results disagree with the numerical results in domains of particular practical interest. When ion temperatures exceed two-tenths of the electron temperature, the least damped mode is the one with the smaller phase velocity, and this mode is then found to dominate the ponderomotive response of the CH plasma.

Miniature, vacuum compatible 1024×1024 charge-coupled device camera for x-ray, ultraviolet, or optical imaging

We have developed a very compact (60×60×75 mm3), vacuum compatible, large format (25×25 mm2, 1024×1024 pixels) charge-coupled device (CCD) camera for digital imaging of visible and ultraviolet radiation, soft to penetrating x rays (≤20 keV), and charged particles. This camera provides a suitable replacement for film with a linear response, dynamic range, and intrinsic signal-to-noise response superior than current x-ray film, and provides real-time access to the data. The spatial resolution of the camera (<25 μm) is similar to typical digitization slit or step sizes used in processing film data. This new large format CCD camera has immediate applications as the recording device for streak cameras or gated microchannel plate diagnostics, or when used directly as the detector for x-ray, x-ray ultraviolet, or optical signals. This is especially important in studying high-energy plasmas produced in pulse-power, inertial confinement fusion, and high powered laser-plasma experiments, as well as other medical and industrial applications.

High-performance x-ray spectroscopic devices for plasma microsources investigations

X-ray spectroscopy with high spectral (up to Δλ/λ = 10−4) and spatial resolution (up to 1 μm) is discussed. Devices based on crystals, diffraction and Bragg-Fresnel elements and their applications in Z- and X-pinches and laser plasma experiments are described.

Density measurements of a high-density pulsed gas jet for laser-plasma interaction studies

The density profile of a pulsed high-density gas jet developed for laser-plasma interaction experiments has been measured. The measurements were carried out using interferometry and Abet inversion of the resultant fringe shifts to obtain gas densities. The jet produces high densities of up to 6*1020 cm-3 for nitrogen and helium gases at 200 mu m outside the nozzle tip. The dense region is mainly confined to a column of 1 mm diameter above the nozzle accessible to laser-plasma interaction studies. The interferometric technique employed here can be used as an on-line diagnostic for monitoring the gas density in such studies.

Nonlocal electron transport in laser created plasmas

The classical linear Spitzer-Härm formula has been shown to lead to an overestimation of the electron heat flux in laser-plasma interaction experiments. We briefly review the classical theory of heat transport in a plasma, and give a simplified demonstration of the Spitzer-Härm formula. The electron heat conductivity is calculated for a large value of the ion charge Z. Correction due to a finite value of Z is evaluated with a simplified electron-electron collision operator. We then show that in a steep temperature gradient, the collisional mean free path of the electrons that transport the energy may be larger than the scale length of the temperature gradient. In this case the Spitzer-Härm formula overestimates the actual heat flux in the main part of the temperature gradient, and predicts a too small heat flux slightly away from the location of the large temperature gradient.A nonlocal macroscopic formula, which is a sort of convolution of the Spitzer-Härm heat flux by a delocalization function, is shown to accurately describe the electron heat flow in both smooth and steep temperature gradients. This nonlocal formula for the heat flow is analytically justified. A selection of slightly different delocalization functions proposed in the literature is compared to the original one and to the results of Fokker-Planck calculations of the heat flow.

Gated x-ray imager gain correction using a tapered microchannel-plate stripline

Gated imagers used to record x-ray emission in the 1–5 keV range in laser-plasma interaction experiments have suffered from resistive gain degradation between voltage drive pulse input and output ends of a microchannel-plate stripline. The reduction in gain as the pulse travels down the strip results in a lower signal-to-background ratio in the last image on a strip than in the first image. Compensation for this gain nonuniformity using a tapered stripline in place of the normally straight striplines is discussed.

Binary-phase zone plate arrays for the generation of uniform focal profiles

The generation of uniform focal intensity profiles is important for a number of applications, including laser-plasma interaction experiments. We report on a focusing system that uses a novel binary-phase optic capable of producing efficient two-dimensional uniform top-hat intensity optical and x-ray profiles.

Four wave sidescattering absolute instability of stimulated Brillouin backscattering in plasmas

A novel effect of sidescattering absolute instability due to stimulated Brillouin backscattering (SBBS) has been theoretically predicted. The effect is based on four wave mixing of the driver pump, a background propagating Stokes wave and two newly excited counterpropagating sidescattered Stokes waves. It is shown that the instability of the sidescattered Stokes components is absolute. The threshold value for this instability in the model of a homogeneous plasma slab is evaluated. The effect has ultimate importance for the explanation of the driver pump field intensity dependence in SBBS encountered in laser-plasma interaction experiments.

Laser light scattering from fluctuations enhanced by stimulated Raman scattering.

Numerical analysis of the system of Zakharov and electromagnetic wave equations shows that in the nonlinear stage of saturation of stimulated Raman backscattering a wide spectrum of plasma fluctuations is excited. These fluctuations manifest themselves in enhanced levels of Brillouin and forward Raman scattering. The coupling between stimulated Raman scattering and these secondary scattering processes is theoretically described. We derive new scaling laws for enhanced Brillouin and forward Raman scattering, which can be tested in the current laser-plasma interaction experiments.

KrF laser-plasma interaction experiments with ns and ps pulses

We report on KrF laser-plasma interaction studies at focused intensities up to 4 × 1014 W/cm2 for pulse durations of 1–2 ns and up to 1015 W/cm2 for pulse duration of 100 ps. The longer-pulse experiments are concerned with quantifying two important features of the ablating plasma. Stimulated Brillouin scattering at moderately large L/λ has been measured in detail as a function of intensity, target Z, and angle of incidence θ to compare with modeling calculations of backscatter in inhomogeneous plasma. In addition, electrodynamic charge analyzer measurements have been made for varying intensity and target Z to compare with hydrodynamic calculations of ion expansion and recombination. In the short-pulse experiments, we report on X-ray conversion measurements for 100-ps laser-irradiated targets of varying Z at laser intensities of 1·5 × 1014 and 1015 W/cm2. In particular, it is shown that higher laser intensity leads to a substantial increase in X-ray conversion efficiency.

Axial magnetic field generation by ponderomotive force in a laser-produced plasma

Axial magnetic field generation in a plasma due to ponderomotive action of an obliquely incident spatially inhomogeneous laser beam is discussed. The theoretical analysis shows that the field can be excited by the collisional as well as the noncollisional processes via ponderomotive action. The collisional contribution, estimated to be in the range of kilogauss, is dominant for lower intensity (I) and shorter wavelength (λ) lasers and scales as λ−2 for both the s- and p-polarized beams. The noncollisional process can excite the field to 0.5–0.6 MG for a p-polarized intense and longer wavelength laser beam which scales as I4/3λ14/3. Faraday rotation is estimated theoretically to highlight the significance of ponderomotive effect in generating the axial field. Finally, implications of the model are discussed along with its limitations in the context of laser–plasma experiments.

Cooperative instabilities of counterpropagating light waves in homogeneous plasma.

Filamentation and Brillouin-enhanced four-wave-mixing instabilities caused by the cooperative interaction between two counterpropagating electromagnetic waves are shown to possess larger convective growth rates and smaller absolute instability thresholds than the conventional single-pump instabilities. These cooperative instabilities may be important in certain laser-plasma interaction experiments.

Effect of photoelectric fluorescence on the formation of x-ray absorption lines in laser-plasma experiments.

In laser-plasma experiments, conditions in an embedded layer in a target shell can be inferred from the spectral attenuation of x-ray continuum radiation by the layer [B. Yaakobi et al., Opt. Commun. 34, 213 (1980)]. In such experiments, absorption lines are formed by 1s-2p absorption transitions in heliumlike through fluorinelike species of certain ions in the layer to be diagnosed. The areal density of each species can be inferred from the attenuation in the spectrum within its respective band, provided the average 1s-2p absorption cross sections for each species are known and provided competing line-forming mechanisms are taken into account. In photoelectric fluorescence, which is one such potentially important competing mechanism, the formation of a 1s vacancy by photoionization is followed by 2p-1s spontaneous emission within the same spectral range as the 1s-2p absorption band of the same ion. The importance of photoelectric fluorescence depends on the supply of ionizing photons, which is sensitive to the design of the experiment. This process should be considered when analyzing experiments where the continuum is hardened by high core temperatures, by opacity effects, or by core components that radiate efficiently at ionizing energies.

Experimental signatures of localization in Langmuir wave turbulence

Features in certain laser-plasma and ionospheric experiments are identified with the basic properties of Langmuir wave turbulence. Also, a model of caviton nucleation is presented which leads to certain novel scaling predictions.

Intense laser plasma interactions: From Janus to Nova

An overview of intense laser plasma interactions is given. The relevant physical processes range from collisional absorption to the excitation of instabilities, such as stimulated Raman and Brillouin scattering. These processes and important consequences that include suprathermal particle generation and enhanced absorption or scattering have been examined in theory, computer simulation, and experiments. The interaction physics has been studied in experiments using electromagnetic waves with wavelengths ranging from a fraction of a micron to tens of meters. Virtually every major interaction process has now been identified in laser plasma experiments, which have grown remarkably in sophistication over the years. Various control techniques have been demonstrated, and some intensity-wavelength regimes for efficient laser plasma coupling have been successfully identified. Indeed, current implosion experiments show excellent coupling of laser energy into thermal plasma. Future challenges include a better understanding of nonlinear levels, the competition of instabilities in large plasmas, and the beneficial effects of laser beam smoothing.

Investigations of pressure nonuniformity in laser-irradiated targets using incoherent optical smoothing

From analytic models of the ablation process in laser-plasma experiments, as well as simulations with Fokker–Planck and Spitzer codes, we have identified a new effect, thermal inertia smoothing. This effect results in a smoother Fokker–Planck temporal and spatial variation of the ablation pressure than that for Spitzer, for the same applied timevarying laser intensity. It is therefore advantageous for incoherent optical smoothing schemes. This is especially true at early times in the laser pulse (≤100 ps).

X-ray emission caused by Raman scattering in long-scale-length plasmas.

By analysis of data from a specific set of laser-plasma interaction experiments, it is argued that stimulated Raman scattering (SRS) is responsible for the hard (>30-keV) x rays emitted from the targets. The Novette laser [K. R. Manes et al., Laser Part. Beams 3, 173 (1985)] was used to irradiate thick, gold targets with up to 4 kJ of 0.53-\ensuremath{\mu}m light in 1-ns, Gaussian pulses at average intensities of (1--200)\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W/${\mathrm{cm}}^{2}$, producing approximately planar plasmas with temperatures of order 3 keV and density-gradient scale lengths of order 250 \ensuremath{\mu}m. The spectrum, amplitude, and timing of the hard x rays emitted by the plasma were measured along with various properties of the scattered light. All the data are consistent with the hypothesis that SRS is the source of the hot electrons that emit the x rays, and some data conflict with any other plausible hypothesis.

Bistability and hysteresis in a laser beatwave excited relativistic Langmuir wave

It is shown that the large amplitude Langmuir wave driven by two collinear laser beams with a frequency difference approximately equal to the plasma frequency exhibits bistability and hysteresis. The threshold pump intensity for the occurrence of the bistability is rather low compared to most laser-plasma experiments. By using the hysteretic effect, relatively weak pump intensities can result in a very large amplitude of the Langmuir wave.