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A A Ivanov

Publications and source records attributed to A A Ivanov.

At least 19 recordsLinked to original sources
PubMed2006

Frequency-shifted megawatt soliton output of a hollow photonic-crystal fiber for time-resolved coherent anti-Stokes Raman scattering microspectroscopy.

Hollow-core photonic-crystal fibers (PCFs) provide soliton delivery and frequency shifting of 2.8 MW femtosecond pulses with an input central wavelength of 618 nm. The frequency-shifted megawatt soliton output of the hollow PCF is used as a high-peak-power Stokes field for coherent anti-Stokes Raman scattering (CARS) microspectroscopy, providing a dynamic range of nearly four decades for anti-Stokes signal detection, thus enabling time-resolved CARS studies of ultrafast relaxation processes on time scales from tens of femtoseconds up to tens of picoseconds.

Crystallization↗
PubMed2006

Spectral transformation of femtosecond Cr:forsterite laser pulses in a flint-glass photonic-crystal fiber.

Nonlinear-optical performance of photonic-crystal fibers (PCFs) made of highly nonlinear TF10 glass is studied and compared with the general tendencies of nonlinear-optical interactions in fused-silica PCFs. The loss of TF10 glass PCFs prevents the generation of supercontinuum emission with a broad and flat spectrum, which typically requires propagation lengths comparable with or exceeding the attenuation length of the fiber. However, dispersive-wave emission of solitons, induced by high-order dispersion, phase-matched four-wave-mixing processes, and self-phase-modulation-induced spectral broadening are substantially enhanced in TF10 glass PCFs due to the high material nonlinearity, providing a high efficiency of frequency conversion of Cr:forsterite laser pulses.

Journal Article↗
PubMed2006

Multimode anharmonic third-order harmonic generation in a photonic-crystal fiber.

While the third harmonic of a monochromatic pump field with a frequency omega0 is generated exactly at the frequency 3omega0, frequency tripling of broadband ultrashort pump pulses in extended nonlinear media tends to generate isolated spectral peaks substantially shifted from 3omega0. We demonstrate this phenomenon by studying nonlinear spectral transformations of femtosecond Cr:forsterite laser pulses in multimode photonic-crystal fibers. Third-harmonic generation is shown to map adjacent guided modes of the third harmonic onto a manifold of spectral peaks within a 150-THz range around 3omega0. The spectral shifts and the widths of these peaks are controlled by the phase and group-velocity mismatch between the pump field and third-harmonic modes, as well as the length of nonlinear-optical interaction and broadening of the pump spectrum, allowing the spectral content of the third-harmonic signal to be engineered by tailoring the fiber dispersion.

Journal Article↗
PubMed2005

Observation of weak impact of a stochastic magnetic field on fast-ion confinement.

Fast ions are observed to be very well confined in the Madison Symmetric Torus reversed field pinch despite the presence of stochastic magnetic field. The fast-ion energy loss is consistent with the classical slowing down rate, and their confinement time is longer than expected by stochastic estimates. Fast-ion confinement is measured from the decay of d-d neutrons following a short pulse of a 20 keV atomic deuterium beam. Ion confinement agrees with computation of particle trajectories in the stochastic magnetic field, and is understood through consideration of ion guiding center islands.

Journal Article↗
PubMed2005

Cross-phase-modulation-induced instability in photonic-crystal fibers.

Cross-phase-modulation-induced instability is identified as a significant mechanism for efficient parametric four-wave-mixing frequency conversion in photonic-crystal fibers. Fundamental-wavelength femtosecond pulses of a Cr, forsterite laser are used in our experiments to transform the spectrum of copropagating second-harmonic pulses of the same laser in a photonic-crystal fiber. Efficient generation of sidebands shifted by more than 80 THz with respect to the central frequency of the second harmonic is observed in the output spectrum of the probe field.

Journal Article↗
PubMed2005

Tuning the frequency of ultrashort laser pulses by a cross-phase-modulation-induced shift in a photonic crystal fiber.

Femtosecond pulses of fundamental Cr:forsterite laser radiation are used as a pump field to tune the frequency of copropagating second-harmonic pulses of the same laser through cross-phase modulation in a photonic crystal fiber. Sub-100-kW femtosecond pump pulses coupled into a photonic crystal fiber with an appropriate dispersion profile can shift the central frequency of the probe field by more than 100 nm, suggesting a convenient way to control propagation and spectral transformations of ultrashort laser pulses.

Journal Article↗
PubMed2005

Highly diverse, massive organic data as explored by a composite QSPR strategy: an advanced study of boiling point.

An improved strategy of quantitative structure-property relationship (QSPR) studies of diverse and inhomogeneous organic datasets has been proposed. A molecular connectivity term was successively corrected for different structural features encoded in fragmental descriptors. The so-called solvation index 1chis (a weighted Randic index) was used as a "leading" variable and standardized molecular fragments were employed as "corrective" class-specific variables. Performance of the new approach was illustrated by modelling a dataset of experimental normal boiling points of 833 organic compounds belonging to 20 structural classes. Firstly, separate QSPR models were derived for each class and for eight groups of structurally similar classes. Finally, a general model formed by combining all the classes together was derived (r2=0.957, s=12.9degreesC). The strategy outlined can find application in QSPR analyses of massive, highly diverse databases of organic compounds.

Linear Models↗
PubMed2004

Phase-matched four-wave mixing of sub-100-TW/ cm2 femtosecond laser pulses in isolated air-guided modes of a hollow photonic-crystal fiber.

Hollow-core photonic-crystal fibers are shown to allow propagation and nonlinear-optical frequency conversion of high-intensity ultrashort laser pulses in the regime of isolated guided modes confined in the hollow gas-filled fiber core. With a specially designed dispersion of such modes, the 3omega=2omega+2omega-omega four-wave mixing of fundamental (omega) and second-harmonic (2omega) sub-100- TW/ cm(2) femtosecond pulses of a Cr:forsterite laser can be phase matched in a hollow photonic-crystal fiber within a spectral band of more than 10 nm, resulting in the efficient generation of femtosecond pulses in a well-resolved higher-order air-guided mode of 417-nm radiation.

Journal Article↗
PubMed2004

Cross-correlation frequency-resolved optical gating coherent anti-Stokes Raman scattering with frequency-converting photonic-crystal fibers.

Photonic-crystal fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr: forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp, ideally suited for femtosecond coherent nonlinear spectroscopy. These pulses from a photonic-crystal fiber were cross correlated in our experiments with the femtosecond second-harmonic output of the Cr: forsterite laser in toluene solution, used as a test object, in boxcars geometry to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene molecules (XFROG CARS).

Journal Article↗
PubMed2004

Stationary rarefaction wave in magnetized Hall plasmas.

We demonstrate the existence of a stationary rarefaction wave in a current-carrying plasma. The result unexpectedly mismatches with the commonly accepted viewpoint about the impossibility of rarefaction shocks in gases or plasmas. The discovered wave may appear when the magnetic field has penetrated into the plasma and magnetized the electrons. At this stage, the wave front is formed at the cathode and propagates towards the anode through the magnetized quasineutral plasma. The case of low collisionality is investigated analytically. This phenomenon could explain the recent surprising experimental observations of a local plasma density drop in several laboratory plasmas.

Journal Article↗
PubMed2003

Experimental evidence of high-beta plasma confinement in an axially symmetric gas dynamic trap.

In the axially symmetric magnetic mirror device gas dynamic trap (GDT), on-axis transverse beta (ratio of the transverse plasma pressure to magnetic field pressure) exceeding 0.4 in the fast ion turning points has been first achieved. The plasma has been heated by injection of neutral beams, which at the same time produced anisotropic fast ions. Neither enhanced losses of the plasma nor anomalies in the fast ion scattering and slowing down were observed. This observation confirms predicted magnetohydrodynamic stability of plasma in the axially symmetric mirror devices with average min-B, like the GDT is. The measured beta value is rather close to that expected in different versions of the GDT based 14 MeV neutron source for fusion materials testing.

Journal Article↗
PubMed2003

[The long-term results of surgical treatment of critical lower limb ischemia by autotransplantation using different tissues].

Presented herein is an analysis of the 15-year results of surgical treatment of critical lower limb ischemia (stage IV chronic ischemia according to Fontaine - Pokrovsky classification) in patients suffering from chronic occlusive diseases of lower lirtb arteries (CODLLA) with subtotal and total obliteration of the peripheral arterial system. In total lesion of the talotibial segment when the patients could not be rendered assistance using conventional vascular surgery or when roentgenoendovascular deobliterution was not feasible, use was made of indirect revascularizntion of the ischemized limb by autotransplantation of soft tissue complexes. A flap of the broadest muscle of back (FBMB) was employed as an autotransplant in 56 patients, the greater omentum in 12 patients, and a flap of the anterior dentate muscle (FADM) was used in 9 patients. Twenty-five patients with diffuse lesion of the popliteal talotibial segment were provided combined limb revascularization, when the feasibility of standard bypass graft appeared fairly questionable. The majority of patients were placed under annual observation. It is noteworthy that the monitoring of the ankle/brachial index (ABI), oxygen tension on the foot (TcpO2) and of the proximal-distal temperature gradient as well as the assessment of the number of saved limbs and of the patients' quality of life allow to regard the technique as highly effective in the strategy of the salvage of the terminally inchemized limb from amputation. The technique also makes it possible to solve the problem of patch plasty of the trophically and necrotically changed foot and to maintain its support capacity. Combined revascularization makes it possible to unite the effect of direct and indirect reconstruction of the arterial system and to perform bypass graft in non-conventional situations.

Adult↗
PubMed2003

[Clinical implications of DNA-topoisomerases examination in renal biopsies from patients with nephritis].

AIM: To study expression of topoisomerases (TI) I and II alpha (DNA-bound enzymes involved in transcription and replication) in renal tissue as markers of activity and prognosis of glomerulonephritis (GN) decisive for choice of immunodepressive therapy. MATERIAL AND METHODS: TI expression was studied immunohistochemically in renal biopsies from 177 patients with different morphological variants of GN and in the samples of unaffected kidney tissue removed in 12 patients for local tumors. RESULTS: There are definite differences between proliferative and non-proliferative GN variants--elevation of TI levels and monocytic infiltration in proliferative GN. Focal-segmental glomerulosclerosis is characterized by a high TI II alpha level in mesangial cells and monocytic infiltration of the glomeruli which are typical for inflammation. A statistical relationship between TI levels in mesangial cells and glomerular epithelium suggests a pathogenetic relation between these links of the pathological process. Molecular markers of activation and proliferation of cells and direct inductors of the inflammatory process (cells of monocytic infiltrate) closely correlated with the activity index--an integral indicator of inflammatory activity, as well as with the integral indicator of sclerotic processes in renal tissue--sclerosis index. Monocytic infiltration in the interstitium correlated both with morphological manifestations of activity, progression of nephritis and their clinical equivalents. In high TI expression GN resistance to immunodepressive therapy rose. To overcome the resistance, immunodepressive therapy must be more active--large doses and duration of treatment. In patients with lupus nephritis and mesangiocapillary GN renal prognosis was worse in the presence of high TI expression in mesangial cells and epithelium of the renal canaliculi. CONCLUSION: The authors are the first to demonstrate TI expression in renal tissue of GN patients, correlation of its level with activity of renal process as well as its role in prediction of response to treatment and the rate of renal failure progression. It is suggested that high TI expression entails a progressive course of GN.

Biomarkers↗
PubMed2002

[The course and prognosis of mesangioproliferative glomerulonephritis].

AIM: A retrospective analysis of a clinical course of mesangioproliferative glomerulonephritis (MPGN) in patients with glomerular deposition of IgA (IgA nephropathy--IgA-N), with glomerular deposition of other Ig to determine prognostic factors of MpGN progression including IgA-N and to examine the patients' sensitivity to immunodepressive therapy. MATERIAL AND METHODS: 2000 patients with primary MPGN followed up from 1980 to 1999 from the disease onset to development of chronic renal failure (creatinine > 2.5 mg%). Factors affecting kidney survival were studied using the Cox regression model, factors predicting sensitivity to immunodepressive therapy--using multiple logistic regression. RESULTS: IgA-N differed by the course and prognosis from other forms of MPGN. In IgA-N urinary syndrome and macrohematuria were encountered more frequently, in other forms of MPGN more frequent was nephrotic syndrome. Prognosis of patients with IgA-N was worse than in MPGN patients without IgA deposition: 10-year "renal survival" (creatinine < 2.5 mg%) was 64 and 97% (p < 0.05), respectively. Prognosis-deteriorating factors for MPGN patients were the following: male sex, nephritis onset in 40-year-olds and older subjects, acute nephritic syndrome (creatinine > 1.5 mg%), high proteinuria, hematuria (> 50 in sight), the presence of synechia and TIC in renal biopsy, location of immune deposits both in the mesangium and basal glomerular membranes. The responders to the immunodepressive therapy had 10-year renal survival 100%. Positive results of immunodepressive therapy were observed significantly more frequently in patients with normal level of creatinine, moderate hematuria, absence of synechias and TIC in renal biopsy, given large total course dose of corticosteroids and cytostatics. Efficiency of oral cyclophosphamide and its intravenous pulse-therapy did not differ significantly. In pulse therapy an average cumulative dose was lower 6 times, side effects occurred 3 times less frequently. CONCLUSION: The importance of morphological information for prognosis and predicting sensitivity of MPGN patients to immunosuppressive therapy necessitates renal biopsy before therapy. Intravenous pulse therapy with cyclophosphamide is preferable as an active treatment in patients with sclerosis in renal biopsy.

Administration, Oral↗
PubMed2001

Stability of a viscous fluid in an oscillating gravitational field.

The stability of the interface of a viscous incompressible fluid superimposed on a massless fluid is studied for the case of an oscillating gravitational field. For the viscous case, the dispersion relation is shown to represent an infinite determinant of the Hill type, which is investigated analytically. The method presented allows one to find the whole dispersion curve of the instability and its asymptotics in an explicit form. The stabilizing effect of the externally imposed oscillations leads to the appearance of stability windows on the growth rate spectrum. Illustrations are given for the influence of all the parameters of the problem on this effect.

Journal Article↗
PubMed2000

[Primary focal segmental glomerulosclerosis: clinical and morphological prognostic factors].

AIM: To retrospectively analyze clinical course and results of immunodepressive therapy of patients with primary focal-segmental glomerulosclerosis (FSGS), to reveal prognostic factors of the disease progression and patients' sensitivity to immunosuppressive therapy. MATERIAL AND METHODS: Morphological diagnosis was specified, morphological indices of activity and sclerosis were estimated, renal survival was analysed, mono- and multivariate analysis of prognostic factors was made by the evidence obtained in the study of 135 biopsy specimens from CRF patients meeting the criteria of FSGS. RESULTS: At the moment of the disease onset only age of the patients was related to FSGS: 5- and 10-year survival was 100% if the disease started under 16 years of age, if older--the survival was 80 and 65%, respectively. Nephrotic syndrome, hematuria, high creatinine, racemose alterations in the glomeruli worsened the disease prognosis. When cytostatics and corticosteroids were used in combination they produced better results and were associated with better prognosis than each of them in monotherapy. Patients with marked hematuria and low proteinuria were less sensitive to therapy than those with weak hematuria and high proteinemia. Patients with FSGS having high IA and SI required more aggressive therapy for response. CONCLUSION: Renal biopsy with quantitation of IA and IS increases the prognosis accuracy and is important for choice of the treatment policy in patients with primary FSGS.

Adolescent↗