Investigation of preheating of laser thermonuclear targets

The preheating level of the thermonuclear fuel and of the gas-compressing shell is the main criterion of the target-compression efficiency in experiments on laserdriven thermonuclear fusion. This paper contains the result of investigations of electronic, radiative, and impact preheating in experiments on compression and heating of hlgh-aspect shell targets in the Del'fin-l laser-derived thermonuclear facility. A procedure, based on analysis of the epithermal emission of the plasma, is considered for measuring the target preheating by fast electrons. The radiative and impact preheating of compressing shells in DD gas are estimated. The results of the investigation demonstrate the high degree of adiabaticity of the compression of the thermonuclear fuel in experiments with the Del'fin-l facility.Nuclear Plasma Laboratory of the Lebedev Physics Institute. Translated from Preprint No. 3, Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1990.     

Principles of alignment of multibeam lasers for thermonuclear purposes

The concrete object of the investigation is the alignment of the high-power 12-channel Del'fin neodymium-glass laser facility. The objects of the alignment are the optical channel of the laser installation, the system for pointing and focusing the laser radiation on the target, the system for positioning the target in the focal volume, and the system for monitoring the optical quality of the elements of the laser installation and laser beams and also the conditions of the target irradiation in the vacuum chamber. The list of requirements that must be satisfied by the alignment beams, the possible sources of alignment beams, the makeup of the apparatus, and the methods of aligning the laser facility are considered. The principles and the actual systems of automatic alignment of the optical elements are described, as are the operating models of the automatized units. The problem of simulating the working laser beams by the alignment beams and the possibility of automatizing the control of the spatial and angular characteristics of the laser radiation are discussed. The system for controlling the alignment processes by means of a laser is considered, as is also a scheme for incorporating the automatized alignment subsystem in the overall system for the automation of the Del'fln facility.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 103, pp. 52–83, 1978.     

Measurement of Spatial-Frequency and Amplitude Responses of a CCD-Matrix Image-Registration Device

A unique method for studying spatial-frequency and amplitude responses of an image-registration device on the basis of a CCD matrix is developed. A monochromatic light-intensity distribution arising from Fraunhofer diffraction on two equal slits is used as an input signal. A quantitative approach to a spatial resolution of the CCD matrix for registration of a periodic image is developed. The dependence of the rms contrast degree of the output signal on the spatial frequency of the image recorded is studied experimentally. The optical scheme developed and the method proposed for treatment of a digitized signal allow one to determine the amplitude response of the registration device over the whole surface of the CCD matrix and to determine the admissible level of the average intensity of the radiation registered.     

Amplifier module of the “Del'fin” facility for thermonuclear plasma heating

The construction principle and the applicable system are considered for the highpower laser amplifier module of the “Del'fin” facility, intended for spherical heating of thermonuclear targets. Results are presented of investigations of the radiation parameters of the amplifier module under various operating regimes. The system for focusing the laser radiation on the target surfaces is described and the results of experiments on plasma heating by radiation of three composite laser beams, at a laser energy level up to 1 kJ at a flux density up to 10¹⁴ W/cm², are analyzed.     

Dependence of the temperature of stochastically heated electrons on the flux density of pulsed laser radiation on a target

Electron dynamics in the field of focused picosecond multimode laser radiation was considered by numerical methods. It was found that the electron energy during stochastic perturbation of its trajectory can exceed the energy of oscillatory motion in a plane wave by several orders of magnitude. The electron distribution function was studied based on 2D numerical simulation. An analytical expression for the distribution function of relativistic electrons as ideal gas was derived. The distribution function in the radiation flux density range of 10¹⁴–10¹⁷ W/cm² was compared with an analytical expression of the equilibrium distribution; a temperature characteristic for electrons stochastically heated by the laser field was introduced. The dependence of the electron temperature on the radiation flux density was determined.     

Measurement of the dynamics of the compression of high aspect-ratio shell targets in the “Delfin-1” installation

With the help of a specific diagnostic system designed for the “DELFIN-1” installation we have measured implosion velocities ? 250 km/s, which are record values for experiments on compression at sufficiently high hydrodynamic efficiency of ablative shell targets. An empirical scaling is shown to agree well with the measured compression velocities.     

Shell-target laser-plasma images in the light of intrinsic and backlit X radiation

Results are presented of calculation of x-ray plasma images of glass shell targets with diagnostic additives Na, K, and Ca in the light of the intrinsic radiation of the resonance lines of H and He ions with Z_n = 8, ii, 14, and 20, and also in the light of cw radiation in the wavelength range = 1-300 Å. The calculation results are compared with laser-plasma images obtained in experiments with the Del'fin-l thermonuclear facility. The temperature and density of the plasma, the rate of corona expansion, the target mass ablation rate, and the hydrodynamic coefficient of laser-energy transfer into kinetic energy of the ablating shell are determined. The plasma images obtained by backlighting the shell target with an auxiliary x-ray source are analyzed with account taken of the radiation refraction. The required energy, temporal, and spectral characteristics of the auxiliary source are determined.Laser-Plasma Laboratory, Lebedev Physics Institute. Translated from Preprint No. 70 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1989.     

The “Del'fin” high-power laser facility for heating spherical thermonuclear targets

The Del'fin H twelve-channel laser facility for high-temperature heating of thermonuclear targets in a spherical geometry is described. It consists of a neodymium driver laser with maximum energy ~10 kJ, pulse duration 10^-10–10^-9 sec, andbeam divergence ~5·10^–4 rad, a vacuum chamber in which the laser radiation interacts with the plasma, and the devices for the diagnostics of the laser and plasma parameters. The Del'fin focusing system, which ensures a high degree of symmetry of the spherical irradiation of the target at a maximum flux density on its surface ~10¹⁵ W/cm², is described. The problem of realizing the maximum ability of heating spherical thermonuclear targets by radiation from high-power laser systems is considered.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P.N. Lebedeva, Vol. 103, pp. 3–51, 1978.     

Focusing crystal mirrors for X-ray diagnostics of laser-produced plasma

The paper describes an investigation of the characteristics of spherical focusing Bragg-reflection mirrors. The integral reflectivity of bent crystals at Bragg angles 80–90°, the width of the spectrum of the reflected x-rays, and the spatial resolution of the images formed by the focusing crystals are considered. Results are presented of the calculated integral reflectivity of bent silicon and quartz crystals of different orientation and in different orders of reflection. The feasibility is demonstrated of laser-plasma diagnostics using the images obtained with the aid of a multichannel x-ray microscope based on focusing crystals. Possible schemes of active diagnostics of a dense laser plasma using a monochromatic transilluminating x-ray beam are considered.Laser Plasma Laboratory, Lebedev Physics Institute. Translated from FIAN Preprint No. 143, Lebedev Physics Institute of the Academy of Sciences of the USSR, Moscow, 1989.