Oleg N. Krokhin

Gasdynamic processes in heating of a substance by laser radiation

Abstract Processes of heating and gasdynamic motion of substances under the action of strong laser radiation are studied. The case of heating of a transparent mass of gas is examined. Also the problem of heating by radiation of an initially cold and stationary gas occupying a volume adjacent to a vacuum is solved. The existence of self-similar state of flow is pointed out. Results of self-similar solution and numerical calculations are presented and compared. The possibility is discussed to use the obtained results for describing the heating and vaporization of material from the surface of a solid body in the case of action of strong laser radiation with modulated streangth. In connection with investigation of the possibility of heating metrial to high temperatures by laser radiation with modulated strength, recently the examination of a number of gasdynamic problems is of substantial interest which take place on focusing the radiation on the surface of condensed media. The gasdynamic approach to the solution of indicated problems is dictated by the circumstance that for sufficiently powerful fluxes of laser radiation the temperature increase is associated with an initiation of gasdynamic motion of material (“vaporization”), which in turn exerts substantial influence on the entire process of heating. An essential peculiarity of the process under examination, which complicates the solution of the problem, is the dependence of the absorption power on the state of the substance in its gasdynamic motion. Indeed, if the absorption coefficient per unit mass is a constant value, then the main part of the incident flux is absorbed in a layer of approximately constant mass with optical thickness ~ 1, and in this case it is not dependent on the state of the substance. However, if the coefficient of absorption depends on the density and temperature, which is realized for example in an ionized gas, then the optical density of the substance strongly depends on the distribution of gasdynamic quantities. In the latter case equations of radiation transfer become substantially more complicated because of dependence on gasdynamic quantities. Gasdynamic motion was taken into consideration for heating by laser radiation of small volumes of material with constant mass in [1 and 2]. However, in all these papers gasdynamic equations were not examined. The motion of matter was described by means of quantities averaged over the volume. Heating of matter with varying mass without taking into account gasdynamic motion was examined in [3]. In papers [4 and 5] gasdynamic motion was examined for a given mass of material the optical thickness of which was much smaller than one, while the absorption depended on temperature and density. One-dimensional gasdynamic problem of heating of material is examined below for the case of incident strong monochromatic laser radiation on the surface of gas which is initially cold and immobile and which prior to heating fills a halfspace adjacent to a vacuum. The special feature of the problem examined is, in distinction from those mentioned above, the exact solution of gasdynamic equations in case of changing mass which is included by the motion in the process of action of a laser radiation pulse. The solution of such a problem can be used for approximate description of the process of vaporization and heating of a solid substance placed in a vacuum under the action of laser radiation.

Influence of the radiation of the plasma-focus current sheath on the implosion dynamics of condensed targets

Results are presented from experimental and theoretical studies of the influence of the radiation of the plasma-focus current sheath on the implosion dynamics of condensed targets. Radiative losses from the current sheath of a plasma focus in neon, argon, and hydrogen with a 2% admixture of Xe are calculated with allowance for the line, bremsstrahlung, and recombination radiation. It is shown that the temperature of the neon plasma (10–15 eV) is quite sufficient to evaporate Al2O3 grains of radii 10–20 μm. The use of neon as a working gas makes it possible to alter the cold-start condition in experiments on the implosion of foam liners.

Compact activation detectors for measuring of absolute neutron yield generated by powerful pulsed plasma installations

A system for measuring of absolute neutron yield and its spatial anisotropy is described. The system was developed in the LPI (Lebedev Physical Institute) and destined for study properties of neutron emission of pulse plasma installations such as Plasma Focus (PF),z-pinches and others. The system consists of transported compact activation detectors and small optically isolated neutron activation counter with 4th digit data output to LCD display. The electronic unit is built on the TTL standard. In power supply of the system the small-size accumulators are used.

Application of shock waves for the improvement of current-carrying properties of YBCO(123) and Bi(2223) HTSC tapes in magnetic fields

Shock waves with a leading-edge pressure of ∼1011 Pa, which were produced in a plasma focus setup, were used to increase the critical current density in YBCO(123) and Bi(2223) HTSC tapes. It was shown that the effect of chemically inactive high-temperature high-density plasma on the HTSC tapes leads to an irreversible increase in the critical current in high magnetic fields. The improvement of the current-carrying properties of the YBCO(123) HTSC tape is confirmed also by the results of scanning Hall magnetometry at 77 K. In particular, in a field of 8 T applied perpendicular to the c axis (H ⊥ c), the increase in the critical current after shock-wave treatment is ∼60%. In the case of the Bi(2223) tape, the critical current in a zero field in the sample portion subjected to shock-wave action was found to be twice as high as that in the untreated portion (100 and 50 A, respectively). The increase in the critical current can be related to a number of possible structural transformations of the superconducting core. First of all, an increase in the density of current-carrying core, which leads to an increase in weak bonds at grain boundaries, is possible. In this case, the formation of nanosized defects, which are responsible for an increase in the force of pinning of Abrikosov vortices, is also possible.

Pages from the history of quantum electronics research in the Soviet Union

50 years of quantum electronics is an outstanding event in the history of modern science. Having started in 1954 in two countries, the Soviet Union at Lebedev Physical Institute and in the United S...

Application of plasma focus installations for a study of the influence of deuterium cumulative flows on materials

In this work, as an example of an application of the plasma focus (PF) device, we study the influence on alloys of vanadium of a cumulative flow producing in the PF device. The experiment was done in a 4-kJ PF device with various gas fillings and various anode shapes. It was found that the velocity of the axial cumulative flow depends on the type of gas and is about 5.107 cm/s for deuterium and 2.107 cm/s for argon fillings of plasma focus chamber; the shape of the flow is changed from a broad conical fly for deuterium to a quasi-one-directional stream for argon. The dynamics and structure of such flows are investigated by means of laser diagnostics and an image converter camera. The experiments show that cumulative flows produce various defects in tested samples. The appearance of a large number of cracks on the surface of vanadium under the impulse influence of deuterium plasma shows that pure vanadium cannot be used for the construction of thermonuclear fusion reactors

Compression of Laser-Irradiated Hollow Microspheres

Recent experiments with solid and shell targets proved the existence of high compression of the substance in the center of the target1–7, The ultimate compression and temperature are specified by the mechanisms of absorption and conversion of high-power laser radiation energy to the plasma energy, by the dynamics and stability of compression, and by the processes occurring in a plasma corona8–11.

Changes of internal properties of vanadium and structure of its surface under the effect of pulsed high-temperature deuterium plasma

The work presents experimental research of the influence of a pulse of high-temperature dense plasma on materials, which as are supposed, will be used in future thermonuclear power stations. The experiments were made on 4 kJ Plasma Focus installation. Such type of installations is capable to form pulse of axial plasma flows moving with velocity approximately 108 cm/s and plasma density is about 1018 cm−3. Pure vanadium was chosen in our researches, because it can be used as a basic element at the creation of low-activated alloys. We found that due to a pulse irradiation of vanadium by deuterium plasma, shock waves arises, which result in super-deep penetration of deuterium into a volume of the studied sample. This process is accompanied with the change of vanadium structure, the occurrence of pores in it, and changes of its mechanical properties. The break-away of vanadium particles from the backside surface of the sample and forming of craters were also observed.

Plasma-current structures of plasma focus during the current disruption

In this paper, we present the results of investigation of the plasma structures arising during the current disruption in the Dense Plasma Focus (DPF). The study was done by means of the laser-shadow and interferometry methods, accompanied with the current and X-ray measurements.

To the memory of Aleksandr Alekseevich Manenkov

Aleksandr Alekseevich Manenkov, a well-known Russian physicist, doctor of science in physics and mathematics, professor, one of the pioneers of quantum electronics, died on 26 March 2014.