arXiv subjects

A. Zhukov

Publications and source records attributed to A. Zhukov.

16 recordsLinked to original sources

arXiv2023

The impact of high-dimensional phase space correlations on the beam dynamics in a linear accelerator

Hadron beams develop intensity-dependent transverse-longitudinal correlations within radio-frequency quadrupole (RFQ) accelerating structures. These correlations are only visible in six-dimensional phase space and are destroyed by reconstructions from low-dimensional projections. In this work, we estimate the effect of artificial decorrelation on the beam dynamics in the Spallation Neutron Source (SNS) linac and Beam Test Facility (BTF). We show that the evolution of a realistic initial distribution and its decorrelated twin converge during the early acceleration stages; thus, low-dimensional projections are probably sufficient for detailed predictions in high-power linacs.

physics.acc-ph↗

arXiv2023

Evaluation of Explanation Methods of AI -- CNNs in Image Classification Tasks with Reference-based and No-reference Metrics

The most popular methods in AI-machine learning paradigm are mainly black boxes. This is why explanation of AI decisions is of emergency. Although dedicated explanation tools have been massively developed, the evaluation of their quality remains an open research question. In this paper, we generalize the methodologies of evaluation of post-hoc explainers of CNNs' decisions in visual classification tasks with reference and no-reference based metrics. We apply them on our previously developed explainers (FEM, MLFEM), and popular Grad-CAM. The reference-based metrics are Pearson correlation coefficient and Similarity computed between the explanation map and its ground truth represented by a Gaze Fixation Density Map obtained with a psycho-visual experiment. As a no-reference metric, we use stability metric, proposed by Alvarez-Melis and Jaakkola. We study its behaviour, consensus with reference-based metrics and show that in case of several kinds of degradation on input images, this metric is in agreement with reference-based ones. Therefore, it can be used for evaluation of the quality of explainers when the ground truth is not available.

cs.CV↗

arXiv2020

High dimensional characterization of the longitudinal phase space formed in a radio frequency quadrupole

Modern accelerator front ends almost exclusively include radio-frequency quadrupoles for initial capture and focusing of the low-energy beam. Dynamics in the RFQ define the longitudinal bunch parameters. Simulation of the SNS RFQ with PARMTEQ seeded with a realistic LEBT distribution produces a 2.5 MeV, 40 mA H- beam with root-mean-square emittance of 130 deg-keV. In measurement, a detailed characterization of the longitudinal phase space is made, including a novel study of the dependence of longitudinal emittance on transverse coordinates. This work introduces a new virtual slit technique that provides sub-slit resolution in an energy spectrometer as well as an approach for visualizing 4D phase space data. Through simulation and measurement, the RFQ-formed bunch is confirmed to have significant internal correlated structure. The high-dimensional features are shown to be in qualitative agreement. However, the measured rms emittances are up to 30% lower than predicted, closer to the design value of 95 deg-keV.

physics.acc-ph↗

arXiv2016

Random Forest Based Approach for Concept Drift Handling

Concept drift has potential in smart grid analysis because the socio-economic behaviour of consumers is not governed by the laws of physics. Likewise there are also applications in wind power forecasting. In this paper we present decision tree ensemble classification method based on the Random Forest algorithm for concept drift. The weighted majority voting ensemble aggregation rule is employed based on the ideas of Accuracy Weighted Ensemble (AWE) method. Base learner weight in our case is computed for each sample evaluation using base learners accuracy and intrinsic proximity measure of Random Forest. Our algorithm exploits both temporal weighting of samples and ensemble pruning as a forgetting strategy. We present results of empirical comparison of our method with original random forest with incorporated "replace-the-looser" forgetting andother state-of-the-art concept-drfit classifiers like AWE2.

cs.AI↗

arXiv2015

Hybridized magnetic microwire metacomposites towards microwave cloaking and barcoding applications

The microwave behavior of polymer metacomposites containing parallel Fe-based and continuous/short-cut Co-based microwire arrays has been investigated. A magnetic field-tunable metacomposite feature has been identified in the dense continuous hybrid composite confirmed by the transmission windows in the frequency band of 1 to 3.5 GHz. The complex magnetically tuned redshift-blueshift evolution of the transmission window is reasoned to result from the competition between the dynamic wire-wire interaction and the ferromagnetic resonance of Fe-based wires. Increasing Co-based inter-wire spacing to 10 mm in the continuous hybrid composites, a remarkable dual-band transmission window in the 1.5-3.5 GHz and 9-17 GHz is respectively induced by the ferromagnetic resonance of Fe-based wires and the magnetic resonance arising between Fe-Co wire couples. The hybridization of parallel Fe-based and short-cut Co-based wires in the polymer composite leads to a significant enhancement of the transmission window in the frequency band of 1 to 6 GHz due to the band-stop nature of Co-based wires. The advanced hybridized microwire metacomposites are arguably demonstrated to be particularly attractive for microwave cloaking and radio frequency barcoding applications.

cond-mat.mtrl-sci↗

arXiv2014

Dual-band metacomposites containing hybrid Fe and Co-based ferromagnetic microwires

We investigated the microwave properties of polymer based metacomposites containing hybridized parallel Fe- and Co-based microwire arrays. A dual-band left-handed feature was observed in the frequency bands of 1.5 to 5.5 GHz and 9 to 17 GHz, indicated by two transmission windows associated with ferromagnetic resonance of Fe-based microwires and long range dipolar resonance between the wire arrays. The plasma frequency after hybridization is significantly increased due to the enhanced effective diameter through the wire-wire interactions between the Fe- and Co- microwire couples. These results offer essential perspectives in designing the multi-band metamaterial for microwave applications such as sensors and cloaking devices.

cond-mat.mtrl-sci↗

arXiv2014

Metacomposite characteristics and their influential factors of polymer composites containing orthogonal ferromagnetic microwire arrays

The microwave properties of glass-fibers reinforced polymer composite embedded with an orthogonal array of Fe77Si10B10C3 microwires have been investigated. The composites containing orthogonal wire arrays display a remarkable transmission window in the frequency band of 1 to 6 GHz under zero external magnetic field indicating an intrinsic double-negative-index characteristic. The polymer matrices have proved to exert a synergistic effect on the microwave properties, which is responsible for the disappearance of the transmission windows when Ek is perpendicular to the glass fiber direction. The plasma frequency of the orthogonal microwire array composite is higher than that of the parallel microwire array with identical wire spacing; this could be attributed to the enhanced microwire-wave interaction induced by the axial electrical components on the additional layer of perpendicular wires. All these features make this new kind of orthogonal microwire composites promising for potential cloaking and sensing applications.

cond-mat.mtrl-sci↗

arXiv2011

Interaction-Driven Spectrum Reconstruction in Bilayer Graphene

The nematic phase transition in electronic liquids, driven by Coulomb interactions, represents a new class of strongly correlated electronic ground states. We studied suspended samples of bilayer graphene, annealed so that it achieves very high quasiparticle mobilities. Bilayer graphene is a truly two-dimensional material with complex chiral electronic spectra and the high quality of our samples allowed us to observe strong spectrum reconstructions and electron topological transitions that can be attributed to a nematic phase transition and a decrease in rotational symmetry. These results are especially surprising because no interaction effects have been observed so far in bilayer graphene in the absence of an applied magnetic field.

cond-mat.mes-hall↗

arXiv2011

Low polarized emission from the core of coronal mass ejections

In white-light coronagraph images, cool prominence material is sometimes observed as bright patches in the core of coronal mass ejections (CMEs). If, as generally assumed, this emission is caused by Thomson-scattered light from the solar surface, it should be strongly polarised tangentially to the solar limb. However, the observations of a CME made with the SECCHI/STEREO coronagraphs on 31 August 2007 show that the emission from these bright core patches is exceptionally low polarised. We used the polarisation ratio method of Moran and Davila (2004) to localise the barycentre of the CME cloud. By analysing the data from both STEREO spacecraft we could resolve the plane-of-the-sky ambiguity this method usually suffers from. Stereoscopic triangulation was used to independently localise the low-polarisation patch relative to the cloud. We demonstrated for the first time that the bright core material is located close to the centre of the CME cloud. We show that the major part of the CME core emission, more than 85% in our case, is H$α$ radiation and only a small fraction is Thomson-scattered light. Recent calculations also imply that the plasma density in the patch is 8 10$^8$ cm$^{-3}$ or more compared to 2.6 10$^6$ cm$^{-3}$ for the Thomson-scattering CME environment surrounding the core material.

astro-ph.SR↗

arXiv2007

Four-Probe Measurements of Carbon Nanotubes with Narrow Metal Contacts

We find that electrons in single-wall carbon nanotubes may propagate substantial distances (tens of nanometers) under the metal contacts. We perform four-probe transport measurements of the nanotube conductance and observe significant deviations from the standard Kirchhoff's circuit rules. Most noticeably, injecting current between two neighboring contacts on one end of the nanotube, induces a non-zero voltage difference between two contacts on the other end.

cond-mat.mes-hall↗

arXiv2007

SU(4) and SU(2) Kondo Effects in Carbon Nanotube Quantum Dots

We study the SU(4) Kondo effect in carbon nanotube quantum dots, where doubly degenerate orbitals form 4-electron ``shells''. The SU(4) Kondo behavior is investigated for one, two and three electrons in the topmost shell. While the Kondo state of two electrons is quenched by magnetic field, in case of an odd number of electrons two types of SU(2) Kondo effect may survive. Namely, the spin SU(2) state is realized in the magnetic field parallel to the nanotube (inducing primarily orbital splitting). Application of the perpendicular field (inducing Zeeman splitting) results in the orbital SU(2) Kondo effect.

cond-mat.mes-hall↗

arXiv2005

Terahertz superlattice parametric oscillator

We report a GaAs/AlAs superlattice parametric oscillator. It was pumped by a microwave field (power few mW) and produced 3rd harmonic radiation (frequency near 300 GHz). The nonlinearity of the active superlattice was due to Bragg reflections of conduction electrons at the superlattice planes. A theory of the nonlinearity indicates that parametric oscillation should be possible up to frequencies above 10 THz. The active superlattice may be the object of further studies of predicted extraordinary nonlinearities for THz fields.

cond-mat.mes-hall↗

arXiv2004

Qutrit state engineering with biphotons

The novel experimental realization of three-level optical quantum systems is presented. We use the polarization state of biphotons to generate a specific sequence of states that are used in the extended version of BB84 protocol. We experimentally verify the orthogonality of the basis states and demonstrate the ability to easily switch between them. The tomography procedure is employed to reconstruct the density matrices of generated states.

quant-ph↗

arXiv2003

Giant Magnetoimpedance of Glass-Covered Amorphous Microwires of Co-Mn-Si-B and Co-Si-B

A study of magnetic hysteresis and Giant magnetoimpedance (GMI) in amorphous glass covered Co-Si-B and Co-Mn-Si-B wires is presented. The wires, about 10 microns in diameter, were obtained by glass-coated melt spinning technique. Samples with positive magnetostriction (MS) have a rectangular bistable hysteresis loop. A smooth hysteresis loop is observed for wires with nearly zero MS. When MS is negative, almost no hysteresis is observed. The GMI was measured in the frequency range between 20 Hz and 30 MHz. The shapes of the impedance versus field curves are qualitatively similar to each other for both positive and zero MS samples. Impedance is maximum at zero field, and decreases sharply in the range 10-20 Oe. For the negative MS wires, when the driving current is small, the impedance is maximum at a finite external field. The position of the maximum approaches zero with increasing current. The contributions of the moment rotation and domain wall motion in the three cases are discussed.

cond-mat.mtrl-sci↗

arXiv2003

Novel surface anisotropy term in the FMR spectra of amorphous microwires

Some recent publications on ferromagnetic resonance in amorphous wires mention presumably new kind of anisotropy, called there circumferential anisotropy, as an explanation of various spectral features. In this paper we argue that there is no special reason to speak of the new kind of anisotropy, since the observed spectra can be well described in terms of more traditional uniaxial and surface anisotropies alone.

cond-mat.mtrl-sci↗

arXiv2000

Dynamics of Activated Escape, and Its Observation in a Semiconductor Laser

We report a direct experimental observation and provide a theory of the distribution of trajectories along which a fluctuating system moves over a potential barrier in escape from a metastable state. The experimental results are obtained for a semiconductor laser with optical feedback. The distribution of paths displays a distinct peak, which shows how the escaping system is most likely to move. We argue that the specific features of this distribution may give an insight into the nature of dropout events in lasers.

cond-mat.stat-mech↗