Search Results - (Author, Cooperation:A. A. Ivanov)

2018-04-11

American Physical Society (APS)

0556-2821

1089-4918

Physics

Astrophysics and astroparticle physics

2018-10-20

Institute of Physics (IOP)

1757-8981

1757-899X

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

2018-10-20

Institute of Physics (IOP)

1757-8981

1757-899X

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

2018-03-29

Nature Publishing Group (NPG)

2045-2322

Natural Sciences in General

2018-02-08

Institute of Physics (IOP)

1755-1307

1755-1315

Geography

Geosciences

Physics

1089-7674

AIP Digital Archive

Physics

A curvature-driven flute instability will be excited in the magnetized plasmas if the magnetic field lines curve toward the entire plasma boundary. Conditions under which it can be effectively stabilized in axisymmetric geometry have been experimentally studied in a gas-dynamic trap (GDT) at Novosibirsk. Flexible design of the experimental device and the availability of neutral beams and ion cyclotron heating enabled the pressure-weighted curvature to be varied over a wide range. The stability limits were thus measured and compared with those predicted by the modified Rosenbluth–Longmire criterion. Characteristics of unstable curvature-driven flute modes were also measured and found to conform to a theory including finite ion Larmor radius (FLR) effects. Stable operation during neutral beam injection was achieved with a cusp end cell, resulting in an increase in Te to 45 eV, limited by end losses rather than anomalous power losses.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

The electron energy distribution function measured at the channel exit of a small Hall-type thruster is interpreted as a beam–plasma electron distribution function with electron beam energy of several tens of electronvolts. Though unstable, the distribution function does not correspond to the completely quasilinear relaxed beam function. Comparison with the measured function in the vicinity of the channel exit shows that the electron beam energy is about 30 eV for the total voltage applied ∼200 V. The contribution of the electron beam is shown to be essential in the ionization of the working gas, even when only the single ionization processes are considered. © 1999 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

In this paper, the observation of magnetohydrodynamic (MHD) stability limit in a fully axisymmetric gas-dynamic trap is reported. Transition through the stability boundary was studied by varying the plasma pressure in the stabilizing cusp end cell and simultaneously measuring the particle and energy lifetimes in the central cell. Energy and particle balance of the neutral beam heated plasma was measured and compared in unstable and stable regimes of operation. It was observed that if the calculations based on the energy principle predicted the plasma to be stable, experimentally measured transverse losses appeared to be smaller than the longitudinal ones. In the opposite case, the transverse losses were dominant thus indicating transition through the MHD stability boundary in the cusp-anchored gas-dynamic trap. © 1997 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

The optimization results of the ion-optics system in a neutral beam injector are considered which is developed for application of beam emission spectroscopy in the TEXTOR tokamak. The injector is capable to deliver a neutral beam of at least 1 A in hydrogen and deuterium at 50 keV for a pulse length of 5 s into the plasma. The simulation results with the AXCEL code for grids of 4 mm thickness are discussed. The ion current of 2 A extracted by a multihole grid system at a current density of 120 mA/cm2 is generated by a rf driven ion source. The beamlet half angular divergence is below 0.6° for the optimized four-electrode ion optics system. The estimated temperature rise of the inertially cooled molybdenum grids with 150 holes and 58% transparency is max. 270 K and the related degradation in beam quality by thermal grid expansion is acceptable. © 1997 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

Low-divergent quasistationary neutral beams are often applied in modern magnetic fusion devices as a diagnostic tool providing unique information about plasma parameters. The most important requirements of these beams are sufficiently large current and energy of the particles, so that the beam can penetrate to the plasma core. Also the duration of the beams must be long enough, i.e., close to that of a plasma discharge, amounting to at least a few seconds for large fusion devices. We developed a neutral beam injector for plasma diagnostics in the tokamak TEXTOR-94 which is capable of meeting these requirements. The maximum beam energy is 50 keV and the source operated in hydrogen delivers an ion current of up to 2 A with a pulse duration of up to 4 s. The low divergent beam (∼0.5°– 0.6°) is geometrically focused 4 m downstream from the source having a 1/e width of ∼ 70 mm at the focal point. The beam can be modulated with a frequency variable up to 500 Hz. The ion source plasma is produced by a radio frequency discharge in hydrogen or helium. The ion beam is extracted by a four-grid system with 163 single holes. The measured beam parameters were compared with those predicted by simulations. © 2000 American Institute of Physics.

Electronic Resource

1435-5914

Springer Online Journal Archives 1860-2000

Computer Science

Mathematics

Abstract The paper contains a survey on some areas of research in algebraic combinatorics done originally in Russian. The main problem under consideration is description of the combinatorial configurations which admit a given automorphism group. A special attention is paid to applications of the Galois correspondence between permutation groups and association schemes (cellular rings in our terminology).

Electronic Resource

1572-9168

51E24 ; 20D08 ; diagram geometries ; sporadic groups

Springer Online Journal Archives 1860-2000

Mathematics

Abstract The largest Fischer 3-transposition group M(24) acts flag-transitively on a 3-local incidence geometry $$\mathcal{G}$$ (M(24)) which is a c-extension of the dual polar space associated with the group O 7(3). The action of the simple commutator subgroup M(24)′ is still flag-transitive. We show that $$\mathcal{G}$$ (M(24)) is characterized by its diagram under the flag-transitivity assumption. The result implies in particular that $$\mathcal{G}$$ (M(24)) is simply connected. The geometry $$\mathcal{G}$$ (M(24)) appears as a subgeometry in the Buekenhout-Fischer 3-local geometry $$\mathcal{G}$$ (F 1) of the Monster group. The simple connectedness of $$\mathcal{G}$$ (M(24)) has played a crucial role in the characterization of $$\mathcal{G}$$ (F 1), which has been achieved recently. When determining the possible structure of the parabolic subgroups we have used an unpublished pushing-up result by U. Meierfrankenfeld.

Electronic Resource

1572-9036

Springer Online Journal Archives 1860-2000

Mathematics

Electronic Resource

1572-9168

Springer Online Journal Archives 1860-2000

Mathematics

Abstract It is shown that an association schemeY=(X, {R i }0⩽i⩽d ), in which the parameters coincide with those of the scheme Her(d, 2) of Hermitian forms ind-dimensional space over GF(22), is isomorphic to Her(d, 2). A principal role in the proof is played by a theorem by P. Terwilliger on the number of 4-vertex configurations of a given type in (P andQ)-polynomial association schemes. Some partial results are obtained in the case of an arbitrary finite field.

Electronic Resource

1572-9168

Springer Online Journal Archives 1860-2000

Mathematics

Abstract We consider tilde-geometries (orT-geometries), which are geometries belonging to diagrams of the following shape: Here the rightmost edge stands for the famous triple cover of the classical generalized quadrangle related to the group Sp4(2). The automorphism group of the cover is the nonsplit extension 3·Sp4(2) − 3 ·S 6. Five examples of flag-transitiveT-geometries were known. These are rank 3 geometries related to the groupsM 24 (the Mathieu group),He (the Held group) and and 37·Sp6(2) (a nonsplit extension); a rank 4 geometry related to the Conway groupCo 1 and a rank 5 geometry related to the Fischer-Griess Monster groupF 1. In the present paper we construct an infinite family of flag-transitiveT-geometries and prove that all the new geometries are simply connected. The automorphism group of the rankn geometry in the family is a nonsplit extension of a 3-group by the symplectic group Sp2n (2). The rank of the 3-group is equal to the number of 2-dimensional subspaces in ann-dimensional vector space over GF(2).

Electronic Resource

1063-7834

Springer Online Journal Archives 1860-2000

Physics

Abstract The properties of the stochastic magnetic structure of ultradispersed ferromagnetic materials are studied. The correlation coefficients of the magnetization of a magnetic material are calculated for various types of chaos in the anisotropy field. The effects of different types of disorder on the parameters of a stochastic magnetic structure are compared. Numerical simulations by various methods confirm the theoretical results.

Electronic Resource

1090-6487

52.80.Pi ; 52.35.Ra ; 52.35.Qz

Springer Online Journal Archives 1860-2000

Physics

Abstract The results of an experimental and theoretical investigation of strong turbulence produced by an external rf field in the plasma of a beam-plasma discharge are presented. The modulation instability of a rf electric field, perpendicular to a constant magnetic field, in the frequency range between the lower-hybrid and ionic Langmuir frequencies, has been investigated. It is shown that the low-frequency fields can be used to control beam relaxation.

Electronic Resource

1090-6487

42.70.Qs ; 42.81.Qb

Springer Online Journal Archives 1860-2000

Physics

Abstract The physical principles of photonic-crystal fibers with a photonic band gap tunable in the visible and near-IR spectral ranges are demonstrated. Direct numerical integration of the Maxwell equations with the use of the finite-difference time-domain technique reveals the possibility of creating holey fibers with a photonic-crystal cladding whose photonic band gap lies within the frequency range characteristic of widespread solid-state femtosecond lasers. The fabrication of holey fibers with a pitch of the two-dimensional periodic structure of the cladding less than 500 nm allowed us to experimentally observe a photonic band gap in transmission spectra of holey fibers tunable within the range of 930–1030 nm. This photonic band gap is satisfactorily described within the framework of the proposed numerical approach based on the finite-difference time-domain method.

Electronic Resource

1090-6509

Springer Online Journal Archives 1860-2000

Physics

Abstract A systematic study of the resistivity and Hall effect in single-crystal films Nd2−x CexCuO4−δ (0.12⩽x⩽0.20) is presented, with special emphasis on the low-temperature dependence of the normal state conductance. Two-dimensional weak localization effects are found both in a normally conducting underdoped sample (x=0.12) and in situ superconducting optimally doped (x=0.15) or overdoped (x=0.18) samples in a high magnetic field B〉B c2. The phase coherence time τ ϒ (5.4 · 10−11 s at 2 K) and the effective thickness of a CuO2 conducting layer d (≃1.5Å) have been estimated by fitting 2D weak localization theory expressions to magnetoresistivity data for magnetic fields perpendicular to the ab plane and in plane. Estimates of the parameter d ensure strong carrier confinement and justify a model consisting of almost decoupled 2D metallic sheets for the Nd2−x CexCuO4−δ single crystal.

Electronic Resource

1090-6487

96.40.Pq

Springer Online Journal Archives 1860-2000

Physics

Abstract The effect of the Earth’s magnetic field on the azimuthal distribution of the number of extensive air showers is measured using data obtained from many years of observations at the Yakutsk array. A uniform azimuthal distribution of the event rates of these showers is rejected at a 10−14 significance level. The amplitude of the harmonic of the distribution depends on the zenith angle as A 1≈0.2 sin2θ and is virtually independent of the primary-particle energy, while the phase coincides with the magnetic meridian. From the magnitude of the measured effect a correction factor for the particle density is determined which depends on the geomagnetic parameter of the shower.

Electronic Resource