G. A. Vergunova

Soft X-ray spectrum of laser-produced aluminum plasma

Soft X-ray spectra (30–70 Å) of aluminum plasma have been measured in experiments carried out at the Kanal-2 laser facility at laser intensities of (1–7) × 1013 W/cm2. It is shown that the measured spectra satisfactory agree with those calculated using the RADIAN numerical code.

Thermonuclear targets for direct-drive ignition by a megajoule laser pulse

Central ignition of a thin two-layer-shell fusion target that is directly driven by a 2-MJ profiled pulse of Nd laser second-harmonic radiation has been studied. The parameters of the target were selected so as to provide effective acceleration of the shell toward the center, which was sufficient for the onset of ignition under conditions of increased hydrodynamic stability of the ablator acceleration and compression. The aspect ratio of the inner deuterium-tritium layer of the shell does not exceed 15, provided that a major part (above 75%) of the outer layer (plastic ablator) is evaporated by the instant of maximum compression. The investigation is based on two series of numerical calculations that were performed using one-dimensional (1D) hydrodynamic codes. The first 1D code was used to calculate the absorption of the profiled laser-radiation pulse (including calculation of the total absorption coefficient with allowance for the inverse bremsstrahlung and resonance mechanisms) and the spatial distribution of target heating for a real geometry of irradiation using 192 laser beams in a scheme of focusing with a cubo-octahedral symmetry. The second 1D code was used for simulating the total cycle of target evolution under the action of absorbed laser radiation and for determining the thermonuclear gain that was achieved with a given target.

Effect of spatial nonuniformity of heating on compression and burning of a thermonuclear target under direct multibeam irradiation by a megajoule laser pulse

Direct-drive fusion targets are considered at present as an alternative to targets of indirect compression at a laser energy level of about 2 MJ. In this approach, the symmetry of compression and ignition of thermonuclear fuel play the major role. We report on the results of theoretical investigation of compression and burning of spherical direct-drive targets in the conditions of spatial nonuniformity of heating associated with a shift of the target from the beam center of focusing and possible laser radiation energy disbalance in the beams. The investigation involves numerous calculations based on a complex of 1D and 2D codes RAPID, SEND (for determining the target illumination and the dynamics of absorption), DIANA, and NUT (1D and multidimensional hydrodynamics of compression and burning of targets). The target under investigation had the form of a two-layer shell (ablator made of inertial material CH and DT ice) filled with DT gas. We have determined the range of admissible variation of compression and combustion parameters of the target depending on the variation of the spatial nonuniformity of its heating by a multibeam laser system. It has been shown that low-mode (long-wavelength) perturbations deteriorate the characteristics of the central region due to less effective conversion of the kinetic energy of the target shell into the internal energy of the center. Local initiation of burning is also observed in off-center regions of the target in the case of substantial asymmetry of irradiation. In this case, burning is not spread over the entire volume of the DT fuel as a rule, which considerably reduces the thermonuclear yield as compared to that in the case of spherical symmetry and central ignition.

Interaction of laser radiation with a low-density structured absorber

A theoretical model is proposed for computing simulations of laser radiation interaction with inhomogeneous foam materials doped with heavy elements and undoped materials. The model satisfactorily describes many experiments on the interaction of the first and third harmonics of a 200 J pulsed PALS iodine laser with low-density porous cellulose triacetate targets. The model can be used to analyze experimental data and estimate the reality of experimental results.

Mathematical modeling of gas-dynamic and radiative processes in experiments with the use of laser and heavy-ion beams

Results are presented from theoretical and experimental studies of gas-dynamic and radiative processes in the plasma that is planned to be used in future experiments on the stopping of fast heavy-ion beams. These experiments are aimed at measuring the enhanced (as compared to cold substance) plasma stopping power. To reliably interpret the experimental results, it is necessary to create a hydrodynamically stable homogeneous plasma with a uniform temperature and a lifetime exceeding the transit time of the heavy-ion beam (3–5 ns). The method for calculating plasma gas-dynamic characteristics with allowance for radiative heat transfer is described. The specific features of the so-called ion model of plasma, which is used to calculate plasma radiative characteristics, are discussed. The emission spectrum formed as a result of conversion of laser radiation into X-rays and the subsequent passing through a triacetate cellulose (C12H16O8) target is calculated. The simulated spectrum of transmitted radiation satisfactorily agrees with experimental data.

Simple model of the indirect compression of targets under conditions close to the national ignition facility at an energy of 1.5 MJ

The possibility of the analysis and interpretation of the reported experiments with the megajoule National Ignition Facility (NIF) laser on the compression of capsules in indirect-irradiation targets by means of the one-dimensional RADIAN program in the spherical geometry has been studied. The problem of the energy balance in a target and the determination of the laser energy that should be used in the spherical model of the target has been considered. The results of action of pulses differing in energy and time profile (“low-foot” and “high-foot” regimes) have been analyzed. The parameters of the compression of targets with a high-density carbon ablator have been obtained. The results of the simulations are in satisfactory agreement with the measurements and correspond to the range of the observed parameters. The set of compared results can be expanded, in particular, for a more detailed determination of the parameters of a target near the maximum compression of the capsule. The physical foundation of the possibility of using the one-dimensional description is the necessity of the closeness of the last stage of the compression of the capsule to a one-dimensional process. The one-dimensional simulation of the compression of the capsule can be useful in establishing the boundary behind which two-dimensional and three-dimensional simulation should be used.

Monochromatic X-Ray Sources Based on Emitters Controlled by Laser Radiation

The results of study of frequency-tuned monochromatic x-ray source are reported. The source was developed on the basis of a vacuum diode with a laser-plasma cathode. The source proposed is particularly promising, if the range of x-ray energy higher than 5 keV is of interest. The source features a spectral brightness higher than 1019 photons/(cm2·s·sr·keV) and an x-ray pulse duration no larger than 10−8 s. An electromagnetic model of such a cathode is proposed and evaluated in order to assess the feasibility of an x-ray source with a laser-plasma cathode of higher performance. The possibility of using a ferroelectric electron emitter is discussed.

Possible version of the compression degradation of the thermonuclear indirect-irradiation targets at the national ignition facility and a reason for the failure of ignition

The main parameters of compression of a target and tendencies at change in the irradiation conditions are determined by analyzing the published results of experiments at the megajoule National Ignition Facility (NIF) on the compression of capsules in indirect-irradiation targets by means of the one-dimensional RADIAN program in the spherical geometry. A possible version of the “failure of ignition” of an indirect-irradiation target under the NIF conditions is attributed to radiation transfer. The application of onedimensional model to analyze the National Ignition Campaign (NIC) experiments allows identifying conditions corresponding to the future ignition regime and distinguishing them from conditions under which ignition does not occur.

Theoretical and experimental studies of the radiative properties of plasma and their applications to temperature diagnostics of Z-pinch plasma

Important features of the theoretical model known as the ion model of plasma, which is used for quantum mechanical calculations of radiative opacity, are discussed. Reliability of ion-model results was tested with experiment, where measurements of X-pinch radiation energy yield for two exploding wire materials, NiCr and Alloy 188 were made. Theoretical estimations of radiative efficiency were compared with experimental results, and ion-model calculations agree well with the experimental data. Subsequently, the theoretical approach has been applied for theoretical and experimental studies of radiative and gas dynamic properties of plasma at high energy density. As it was found, the theoretical approach can be used for temperature diagnostics of Z-pinch plasma. Calculations of the spectral brightness were made for W plasma radiation at the temperatures 1 and 1.2 keV and the densities 1 and 2 g/cc.

Theoretical and experimental studies of radiative and gas dynamic properties of substances at high energy density in matter

Mathematical modelling of radiative and gas-dynamic processes in substances at high energy density is carried out for experiments, where both laser and heavy ion beams are used. Important features of the theoretical model, known as the ion model (IM), which is used for quantum mechanical calculations of radiative opacity, are discussed. Reliability of (IM) results is tested with experiment, where measurements of x-pinch radiation energy yield for two exploding wire materials, NiCr and Alloy 188 were made. Theoretical estimations of radiative efficiency are compared with experimental results, and (IM) calculations agree well with the experimental data. Subsequently, the theoretical approach was used for temperature diagnostics of CHO plasma target in combined laser-heavy ion beam experiments. Joint radiative and gas-dynamic calculations are performed for comparison with experiment, where hohlraum radiation transmits through the CHO plasma target, and the share of absorbed radiation energy is compared with experiment. Study of radiative properties of CHO plasma with little admixture of gold is carried out as well. Specific dependence of the Rosseland mean on plasma temperature is discussed for gold plasma.