A. V. Kirillin

Charge and structures of dust particles in a gas discharge at cryogenic temperatures

A dusty plasma in a dc gas discharge is considered at low (cryogenic) temperatures of the gas. The formation of dusty plasma structures consisting of monodisperse poly(styrene) particles (d = 5.44 μm) in a dc glow discharge is experimentally investigated at cryogenic temperatures in the range from 4.2 to 77 K, and the results obtained are presented. The ion velocity distribution function and the charging of dust particles at cryogenic temperatures are calculated using the molecular dynamics method. The primary attention is focused on the correct inclusion of ion-atom collisions in the analysis. This is essential to the understanding of the main mechanisms of the experimentally observed increase in the density of dust particles with decreasing temperature of the gas in the discharge.

Dust structures in cryogenic gas discharges

Results are given of experimental investigations of dust structures of monodisperse particles in a low-pressure dc glow discharge at temperatures of liquid nitrogen ( T = 77 K ) and liquid helium ( T = 4.2 K ) . It is found that the cooling of discharge at invariant discharge current and neutral gas density causes a decrease in the interparticle spacing in structures and an increase in the kinetic energy of dust particles. The forming of a superdense dust structure with free boundaries is observed for the first time at 4.2 K .

Formation of amorphous carbon on melting of microcrystalline graphite by picosecond laser pulses

Observations of microcrystalline graphite subjected to picosecond laser pulses reveal the formation of a liquid phase with a subsequent transition to a uniform amorphous state of a surface layer upon solidification. This phenomenon is observed on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of the laser pulse. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. A periodic structure with a period of the order of the wavelength of the heating pulse is formed in the heating region. The “rulings” of this periodic structure are oriented in the direction of polarization of the heating pulse. A study of the reflection kinetics of the probe laser pulse showed that the characteristic existence time of the liquid phase and of the solidification process is ∼10−10 s.

Dusty Plasma Structures in Cryogenic DC Discharges

Experimental investigations of dust structures in dc low‐pressure glow discharge at temperatures of liquid nitrogen (T = 77 K) and liquid helium (T = 4.2 K) are presented. Super dense dust structures with dust density np ∼ 108–109 cm−3 formed within striations of cryogenic discharges as well as boundary‐free worm‐like dust structure moved in discharge at 4.2 K were obtained.

Dynamics of first- and second-order phase transitions in amorphous magnetooptic TbFeCo films

The dynamics of phase transformations in thin amorphous TbFeCo films under the action of ∼ 1 ps laser pulses is investigated. The films are heated to the Curie temperature in the amorphous state (TC1), to the crystallization temperature (Tac), and to the Curie temperature in the crystalline phase (TC2). The change in magnetization is detected by Faraday magnetooptic effect during and after the action of the heating pulse. A static external magnetic fieldH∼1−12 kOe, whose flux lines are directed perpendicular to the plane of the film, is used in the experiments. Amorphous TbFeCo films possess a perpendicular magnetic anisotropy, which on crystallization becomes reoriented in the plane of the film. It is observed that crystallization and magnetization reorientation occur during the heating pulse (within ∼ 1 ps). The spin subsystem is heated to the Curie temperature several picoseconds after the end of the laser pulse. The characteristic spin relaxation time is ∼ 10 ps. A model of the dynamics of the electronic, spin, and phonon subsystems that makes it possible to explain the experimental results is proposed on the basis of the data obtained.

Optical anisotropy as a technique for studying ultrafast phase transformations at solid surfaces

A new technique for testing long-range order in high-absorption anisotropic crystals has been developed using conversion of an incident p-(s-)wave to an s-(p-)wave due to optical anisotropy. The technique yields time-resolved measurements of parameters related to phase transformations in thin (10−6–10−5 cm) layers with a high resolution (10−12 s). Using picosecond laser pulses and an “Agat” streak camera, the technique has been applied to an experimental investigation of melting and recrystallization kinetics at zinc and graphite surfaces. It was found that the process of melting takes less than 3 ps and the recrystallization time is about 100 ps.

Dusty plasma in a glow discharge in helium in temperature range of 5–300 K

Dusty plasma structures in glow discharge in helium in the temperature range of 5–300 K are investigated experimentally. We have described the experimental setup that makes it possible to continuously vary the temperature regime. The method for experimental data processing has been described. We have measured interparticle distances in the temperature range of 9–295 K and compared them with the Debye radius. We indicate the ranges of variations in experimental parameters in which plasma–dust structures are formed and various types of their behavior are manifested (rotation, vibrations of structures, formation of vertical linear chains, etc.). The applicability of the Yukawa potential to the description of the structural properties of a dusty plasma in the experimental conditions is discussed.

Evolution of Dust Structures from Room to Cryogenic Temperatures

In this work dusty plasma of dc glow discharge at the temperatures in the range of 4.2–300 K was experimentally and numerically investigated. As it was shown in the experiments, the deep cooling of discharge tube walls leads to dramatic change of dusty plasma properties. In particular, sufficient increase of dust particle kinetic temperature (by about an order) and dust density (by several orders) was observed at low (cryogenic) temperatures. At 4.2 K, this can lead to the forming of a super dense dust structures with novel properties. Numerical simulations of charging process, dust charge fluctuation and screening of dust particle charge in plasma were made in dependence with the neutral gas temperature and dust density. The main attention was given to proper ion‐atom collision analysis that allows us to investigate mechanisms of dust structure transformation observed in the cryogenic experiments.

Production of amorphous graphite in picosecond laser interaction experiments

Formation of a liquid phase with subsequent transition to a uniform amorphous state of surface layer upon solidification is observed under action of picosecond laser pulses on microcrystalline graphite. This phenomenon is registered on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of laser pulse. A study of melting and solidification of graphite is performed using a new method based on partial transmission of incident p-polarized wave into s-polarized reflected wave due to optical anisotropy. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. Periodic surface structures with a period of the order of the wavelength of the heating pulse was detected on amorphous graphite region. The orientation of the structures correlates with polarization of incident laser pulse. The instability to formation of these structures is assumed to be connected with surface electromagnetic wave excitation. The characteristic time of existing of liquid phase and of solidification processes is determined to be approximately 10-10 s.