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- ItemTerahertz pulse generation by two-color laser fields with circular polarization([London] : IOP, 2020) Tailliez, C.; Stathopulos, A.; Skupin, S.; Buožius, D.; Babushkin, T.; Vaičaitis, I.; Bergé, L.We study the influence of the polarization states of ionizing femtosecond two-color pulses on the emitted terahertz radiation in gases. A local-current model and plane-wave evaluations justify the previously-reported impact on the THz energy yield and a (almost) linearly-polarized THz field when using circularly-polarized laser harmonics. For such pump pulses, the THz yield is independent of the relative phase between the two colors. When the pump pulses have same helicity, the increase in the THz yield is associated with longer ionization sequences and higher electron transverse momenta acquired in the driving field. Reversely, for two color pulses with opposite helicity, the dramatic loss of THz power comes from destructive interferences driven by the highly symmetric response of the photocurrents lined up on the third harmonic of the fundamental pulse. While our experiments confirm an increased THz yield for circularly-polarized pumps of same helicity, surprisingly, the emitted THz radiation is not linearly-polarized. This effect is explained by means of comprehensive 3D numerical simulations highlighting the role of the spatial alignment and non-collinear propagation of the two colors. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
- ItemDynamic vortex dust structures in a nuclear-track plasma([London] : IOP, 2003) Rykov, V. A.; Khudyakov, A. V.; Filinov, V. S.; Vladimirov, V. I.; Deputatova, L. V.; Krutov, D. V.; Fortov, V. E.Results are presented from Monte Carlo calculations of the electric charge on dust grains in a plasma produced during the slowing down of radioactive decay products of californium nuclei in neon. The dust grain charging is explained as being due to the drift of electrons and ions in an external electric field. It is shown that the charges of the grains depend on their coordinates and strongly fluctuate with time. The time-averaged grain charges agree with the experimental data obtained on ordered liquid-like dust structures in a nuclear-track plasma. The time-averaged dust grain charges are used to carry out computer modelling of the formation of dynamic vortex structures observed in experiments. Evidence is obtained for the fact that the electrostatic forces experienced by the dust grains are potential in character. The paper is supplemented by a video clip showing the typical dynamics of the simulated vortex dust structure.
- ItemDust as a surfactant([London] : IOP, 2003) Ignatov, A. M.; Schram, P. P. J. M.; Trigger, S. A.We argue that dust immersed in a plasma sheath acts as a surfactant. By considering the momentum balance in a plasma sheath, we evaluate the dependence of the plasma surface pressure on the dust density. It is shown that the dust may reduce the surface pressure, giving rise to a sufficiently strong tangential force. The latter is capable of confining the dust layer inside the sheath in the direction perpendicular to the ion flow.
- ItemLevitation and agglomeration of magnetic grains in a complex (dusty) plasma with magnetic field([London] : IOP, 2003) Samsonov, D.; Zhdanov, S.; Morfill, G.; Steinberg, V.Interaction of magnetic particles with each other and with a magnetic field was studied experimentally in a complex plasma. Monodisperse plastic microspheres with magnetic filler were suspended in an rf symmetrically driven discharge to form a multilayer dust cloud. The magnetic field induced a magnetic moment in the grains. The particles were pulled upward in the direction of the magnetic field gradient and their levitation height increased. This was used as a new diagnostic method to calculate the particle charge and the thickness of the plasma sheath. It was demonstrated that the particle weight can be compensated for. Some particles formed agglomerates due to magnetic attraction between the grains. Analysis of the particle interaction forces showed that at intermediate magnetic fields (used in the experiment) the particles can agglomerate only if their kinetic energy is high enough to overcome the barrier in the interaction potential. The possibility of magnetically induced formation of a plasma crystal was discussed.
- ItemA fluid model for colloidal plasmas under microgravity conditions([London] : IOP, 2003) Gozadinos, G.; Ivlev, A. V.; Boeuf, J. P.A numerical model is presented to simulate steady states of complex ('dusty') plasmas under microgravity conditions. The model uses a fluid description for the plasma as well as for the dust microparticles. This is achieved by using an appropriate equation of state for the crystalline phase of the dust. The only forces assumed to act on the dust particles are the electric force, ion drag and pressure gradients. The model is used to study the formation of the stable 'void' present in recent microgravity experiments. The structure of the dust clouds when the void is present is examined and explained.
- ItemLaboratory-device configurations for investigating new dusty-plasma equilibria([London] : IOP, 2003) Koepke, M. E.; Sato, N.Two configurations that are designed for laboratory investigations of dusty-plasma equilibria are being prepared for operation. The first configuration has a vertical magnetic field that confines horizontally a vertically oriented, 6.4 cm diameter, low-temperature, alkali-metal-ion plasma column. The plasma is produced via the Q-machine method (Rynn and D'Angelo 1960 Rev. Sci. Instrum. 31 1326) with contact-ionized alkali-metal ions and thermionically emitted electrons. The dust grains will be injected to form a small number of horizontal dusty-plasma layers levitated electrostatically above the plasma sheath. The advantage of using a Q-machine plasma source is the insensitivity of its plasma production to the background pressure of neutral particles. The plan is to study the competition between neutral-particle cooling and streaming-ion energization in dusty-plasma crystallization and decrystallization (i.e., freezing and melting) over a wide range of neutral-particle pressure. The second configuration has a large vacuum chamber (2 m diameter, 4 m length) and a large, solenoidal, magnetic field (0.1 T) that will magnetically confine small-diameter dust grains in a large-volume dusty plasma. The advantage of producing a large-volume, dusty plasma in a strong magnetic field is the ability to meet the criterion that the dust gyroradius is much smaller than the dusty-plasma-column diameter. The plan is to study third-component effects in microinstabilities and the influence of size distribution on magnetized dusty-plasma equilibrium and stability.
- ItemMutual interactions of magnetized particles in complex plasmas([London] : IOP, 2003) Yaroshenko, V. V.; Morfill, G. E.; Samsonov, D.; Vladimirov, S. V.Various mutual dust-dust interactions in complex plasmas, including the forces due to induced magnetic and electric moments of the grains are discussed. It is shown that the dipole short-range forces can be responsible for the formation of field-aligned chains. Such chains may incorporate a few tens of individual particles, as frequently observed in experiments.
- ItemIn situ nanoparticle diagnostics by multi-wavelength Rayleigh-Mie scattering ellipsometry([London] : IOP, 2003) Gebauer, G.; Winter, J.We present and discuss the method of multiple-wavelength Rayleigh-Mie scattering ellipsometry for the in situ analysis of nanoparticles. It is applied to the problem of nanoparticles suspended in low-pressure plasmas. We discuss experimental results demonstrating that the size distribution and the complex refractive index can be determined with high accuracy and present a study on the in situ analysis of etching of melamine-formaldehyde nanoparticles suspended in an oxygen plasma. It is also shown that particles with a shell structure (core plus mantle) can be analysed by Rayleigh-Mie scattering ellipsometry. Rayleigh-Mie scattering ellipsometry is also applicable to in situ analysis of nanoparticles under high gas pressures and in liquids.
- ItemPKE-Nefedov*: Plasma crystal experiments on the International Space Station([London] : IOP, 2003) Nefedov, Anatoli P.; Morfill, Gregor E.; Fortov, Vladimir E.; Thomas, Hubertus M.; Rothermel, Hermann; Hagl, Tanja; Ivlev, Alexei V.; Zuzic, Milenko; Klumov, Boris A.; Lipaev, Andrey M.; Molotkov, Vladimir I.; Petrov, Oleg F.; Gidzenko, Yuri P.; Krikalev, Sergey K.; Shepherd, William; Ivanov, Alexandr I.; Roth, Maria; Binnenbruck, Horst; Goree, John A.; Semenov, Yuri P.The plasma crystal experiment PKE-Nefedov, the first basic science experiment on the International Space Station (ISS), was installed in February 2001 by the first permanent crew. It is designed for long-term investigations of complex plasmas under microgravity conditions. 'Complex plasmas' contain ions, electrons, neutrals and small solid particles - normally in the micrometre range. These microparticles obtain thousands of elementary charges and interact with each other via a 'screened' Coulomb potential. Complex plasmas are of special interest, because they can form liquid and crystalline states (Thomas et al 1994 Phys. Rev. Lett. 73 652-5, Chu and I 1994 Phys. Rev. Lett. 72 4009-12) and are observable at the kinetic level. In experiments on Earth the microparticles are usually suspended against gravity in strong electric fields. This creates asymmetries, stresses and pseudo-equilibrium states with sufficient free energy to readily become unstable. Under microgravity conditions the microparticles move into the bulk of the plasma (Morfill et al 1999 Phys. Rev. Lett. 83 1598), experiencing much weaker volume forces than on Earth. This allows investigations of the thermodynamics of strongly coupled plasma states under substantially stress-free conditions. In this first paper we report our results on plasma crystals, in particular the first experimental observations of bcc lattice structures.
- ItemPlasma interaction with microbes([London] : IOP, 2003) Laroussi, M.; Mendis, D. A.; Rosenberg, M.The germicidal effects of a non-equilibrium atmospheric pressure plasma generated by a novel resistive barrier discharge on representatives of the two classes of bacteria (Gram-negative and Gram-positive) are discussed. The plasma exposure, while being lethal to both bacterial classes, also produced gross structural damage in the Gram-negative E. coli while none was observed in the more structurally robust Gram-positive Bacillus subtilis. An electrophysical process involving the role of the electrostatic tension on a charged body in a plasma is invoked to explain both observations. Since the efficacy of this electrophysical process depends not only on the tensile strength of the bacterial cell wall but also on its shape and texture, the need for more experimental studies, using a wide range of bacteria belonging to various morphological groups, is suggested. Ways to further test the validity of this electrophysical lysis mechanism for Gram-negative bacteria on one hand, and also to extend its operation to the more robust Gram-positive bacteria on the other, are suggested.