arXiv subjects

Yu. V. Petrov

Publications and source records attributed to Yu. V. Petrov.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear anisotropic equilibrium reconstruction in axisymmetric magnetic mirrors

Magnetic equilibrium reconstruction is a crucial simulation capability for interpreting diagnostic measurements of experimental plasmas. Equilibrium reconstruction has mostly been applied to systems with isotropic pressure and relatively low plasma $β= 2μ_0p/B^2$. This work extends nonlinear equilibrium reconstruction to high-$β$ plasmas with anisotropic pressure and applies it to the Wisconsin High Temperature Superconducting Axisymmetric Magnetic Mirror experiments to infer the presence of sloshing ions. A novel basis set for the plasma profiles and machine learning algorithm using scalable constrained Bayesian optimization allow accurate nonlinear reconstructions with uncertainty quantification to be made more quickly with fewer experimental diagnostics and improves the robustness of reconstructions at high $β$. In addition to WHAM and other mirrors, such reconstruction techniques are potentially attractive in high-performance devices with constrained diagnostic capabilities such as fusion power plants.

physics.plasm-ph↗

arXiv2025

Hot-Ion Modes in Globus-M2 and Saturation of Energy Confinement Time Scaling in Spherical Tokamaks with Toroidal Magnetic Field of 1 T and Above

In a small spherical tokamak with minor radius of 0.22 m and toroidal magnetic field of 1 T, it is possible to heat ions of a sufficiently dense plasma to an extremely high temperature up to 50 million Kelvin. To do this, it is necessary to transfer a sufficiently large torque to stabilize ion-scale turbulence and achieve ion heat transport at neoclassical level reaching extremely low values of plasma collisionality. It is also necessary to ensure good thermal insulation of electrons, which is always determined by turbulent transport. In a spherical tokamak, the toroidal magnetic field has a strong beneficial effect on suppressing turbulent electron heat fluxes. In the Globus-M2 tokamak, when heating plasma with high-energy atomic beams, a significant improvement in the plasma thermal energy confinement is observed with an increase in the toroidal magnetic field from 0.5 to 1 T. The comparison of our results with experiments on ST40 tokamak operating with toroidal magnetic field of 2 T indicates that further strong improvement of thermal insulation of spherical tokamak plasma in the region of higher magnetic fields is not expected.

physics.plasm-ph↗

arXiv2025

Confinement performance predictions for a high field axisymmetric tandem mirror

This paper presents Hammir tandem mirror confinement performance analysis based on Realta Fusion's first-of-a-kind model for axisymmetric magnetic mirror fusion performance. This model uses an integrated end plug simulation model including, heating, equilibrium, and transport combined with a new formulation of the plasma operation contours (POPCONs) technique for the tandem mirror central cell. Using this model in concert with machine learning optimization techniques, it is shown that an end plug utilizing high temperature superconducting magnets and modern neutral beams enables a classical tandem mirror pilot plant producing a fusion gain Q > 5. The approach here represents an important advance in tandem mirror design. The high fidelity end plug model enables calculations of heating and transport in the highly non-Maxwellian end plug to be made more accurately. The detailed end plug modelling performed in this work has highlighted the importance of classical radial transport and neutral beam absorption efficiency on end plug viability. The central cell POPCON technique allows consideration of a wide range of parameters in the relatively simple near-Maxwellian central cell, facilitating the selection of more optimal central cell plasmas. These advances make it possible to find more conservative classical tandem mirror fusion pilot plant operating points with lower temperatures, neutral beam energies, and end plug performance requirements than designs in the literature. Despite being more conservative, it is shown that these operating points have sufficient confinement performance to serve as the basis of a viable fusion pilot plant provided that they can be stabilized against MHD and trapped particle modes.

physics.plasm-ph↗

arXiv2021

Investigation of the effectiveness of non-inductive `multi-harmonic' electron cyclotron current drive through modeling multi-pass absorptions in the EXL-50 spherical tokamak

The effectiveness of multiple electron cyclotron resonance (ECR) harmonics has been thoroughly investigated in context of high current drive efficiency, generally observed in fully non-inductive operation of the low aspect ratio EXL-50 spherical tokamak (ST) powered by electron cyclotron (EC) waves. The Fokker-Plank equation is numerically solved to obtain electron distribution function, under steady state of the relativistic nonlinear Coulomb collision and quasi-linear diffusion operators, for calculating plasma current driven by the injected EC wave. For the extra-ordinary EC wave, simulation results unfold a mechanism by which electrons moving around the cold second harmonic ECR layer strongly resonate with higher harmonics via the relativistic Doppler shifted resonance condition. This feature is in fact evident above a certain value of input EC wave power in simulation, indicating it to be a non-linear phenomenon. Similar to the experimental observation, high efficiency in current drive (over 1 A/W) has indeed been found in simulation for a typical low density ($\sim 1\times10^{18}~m^{-3}$), low temperature ($\lesssim 100$ eV) plasma of EXL-50 by taking into account multi-pass absorptions in our simulation model. However, such characteristic is not found in the ordinary EC-wave study for both single-pass and multi-pass simulations, suggesting it as inefficient in driving current on our ST device.

physics.plasm-ph↗

arXiv2019

Laser ablation in liquid: bridge from a plasma stage to bubble formation

Laser ablation through liquid is an important process that have to be studied for applications which use laser ablation in liquid (LAL) and laser shock peening (LSP). LAL is employed for production of suspensions of nanoparticles, while LSP is applied to increase hardness and fatique/corrosion resistance properties of a surface layer. A bubble appears in liquid around the laser spot focused at a target surface after strong enough laser pulse. In the paper we connect the early quasi-plane heated layer created by a pulse in liquid and the bubble forming at much later stages. In the previous works these early stage from one side and the late stage from another side existed mainly as independent entities. At least, quantitative links between them were unknown. We consider how the quasi-plane heated layer of liquid forms thank to thermal conduction, how gradually conduction becomes weaker, and how the heated layer of liquid nearly adiabatically expands to few orders of magnitude in volume during the drop of pressure. Our molecular dynamics simulations show that the heated layer is filled by the diffusive atomic metal-liquid mixture. Metal atoms began to condense into nanoparticles (NP) when they meet cold liquid outside the edge of a mixing zone. This process limits diffusive expansion of metal atoms, because diffusive ability of NP is less than this ability for individual atoms. Thus the mixture expands together with hot liquid, and the NPs approximately homogeneously fill an interior of a bubble.

physics.comp-ph↗

arXiv2019

Modelling and optimization of the excitonic diffraction grating

Periodical spatial modulation of the excitonic resonance in a quantum well could lead to the formation of a new highly directional and resonant coherent optical response -- resonant diffraction. Such excitonic diffraction gratings were demonstrated in epitaxially grown quantum wells patterned by the low-dose ion beam irradiation before or after the growth. In this paper we present a theoretical model of the resonant diffraction formation based on the step-by-step approximation of the Maxwell equation solution. The resulting theory allows us to reliably describe experimental data, as well as to predict the way to increase the diffraction efficiency.

cond-mat.mes-hall↗

arXiv2018

Condensation of laser produced gold plasma during expansion and cooling in water environment

The ecologically best way to produce nanoparticles (NP) is based on laser ablation in liquid (LAL). In the considered here case the LAL means that a gold target is irradiated through transparent water. During and after irradiation the heated material from surface of a target forms a plume which expands into liquid. In this paper we study a reach set of physical processes mixed with complicated hydrodynamic phenomena which all accompany LAL. These theoretical and simulation investigations are very important for practical applications. Laser pulses with different durations $τ_L$ covering 5-th orders of magnitudes range from 0.1 ps to 0.5 ns and large absorbed fluences $F_{abs}$ near optical breakdown of liquid are compared. It is shown that the trajectory of the contact boundary with liquid at the middle and late stages after passing of the instant of maximum intensity of the longest pulse are rather similar for very different pulse durations (of course at comparable energies $F_{abs});$ we consider the pulses with a Gaussian temporal shape $I\propto \exp(-t^2/τ_L^2).$ We follow how hot (few eV range) dense gold plasma expands, cools down, intersects a saturation curve, and condenses into NPs. These NPs appear first inside the water-gold diffusively mixed intermediate layer where gold vapor has the lowest temperature. Later in time pressure around the gold-water contact drops down below critical pressure for water. Thus NPs find themselves in gaseous water bubble where density of water gradually decreases to $10^{-4}-10^{-5}$ g/cm$\!^3$ at the instant of maximum expansion of a bubble.

physics.comp-ph↗

arXiv2018

Laser-induced ablation of metal in liquid

Laser ablation in liquid (LAL) is important perspective way to compose nanoparticles (NP) necessary for modern technologies. LAL is not fully understood. Deep understanding is necessary to optimize processes and decrease high price of the LAL NPs. Today there are two groups of studies: in one of them scientists go from analyzing of bubble dynamics (thus they proceed from the late stages), while in another one scientists investigate early stages of ablation. In the present paper we consider the process as whole: from ablation and up to formation of a bubble and its inflation. Thus we cover extremely wide range of spatiotemporal scales. We consider role of absorbed energy and duration of pulse (femtosecond, multi-picosecond, nanosecond). Importance of supercritical states is emphasized. Diffusive atomic and hydrodynamic mixing due to Rayleigh-Taylor instability and their mutual interdependence are described. Liquid near contact with metal is heated by dissipation in strong shock and due to small but finite heat conduction in liquid; metal absorbing laser energy is hot and thus it serves as a heater for liquid. Spatial expansion and cooling of atomically mixed liquid and metal causes condensation of metal into NPs when pressure drops below critical pressure for metal. Development of bubble takes place during the next stages of decrease of pressure below critical parameters for liquid and below ambient pressure in liquid. Thin hot layer of liquid near contact expands in volume to many orders of magnitude filling the inflating bubble.

physics.comp-ph↗

arXiv2018

Formation of solitary microstructure and ablation into transparent dielectric by a subnanosecond laser pulse

Laser ablation in liquid (LAL) is important technique used for formation of nanoparticles (NP). The LAL processes cover logarithmically wide range of spatiotemporal scales and is not fully understood. The NP produced by LAL are rather expensive, thus optimization of involved processes is valuable. As the first step to such optimizations more deep understanding is necessary. We employ physical models and computer simulations by thermodynamic, hydrodynamic, and molecular dynamics codes in this direction. Absorbing light metal expanding into transparent solid or liquid dielectrics is considered. We analyze an interplay between diffusion, hydrodynamic instability, and decrease of surface tension down to zero value caused by strong heating and compression transferring matter into state of overcritical fluids. The primary NPs appear during expansion and cooling of diffusion zone when pressure in this zone drops below critical pressure for a metal. Long evolution from the overcritical states to states below a critical point for a metal and down to critical point of liquid and deeply down to surrounding pressure of 1 bar is followed. Conductive heating of liquid from hot metal is significant.

cond-mat.mes-hall↗

arXiv2017

Two-temperature equations of state for d-band metals irradiated by femtosecond laser pulses

The cold curves for energy and pressure of Copper, Iron, and Tantalum were obtained using methods of the density functional theory. We consider hydrostatic and uniaxial deformations in the range from double compression of the initial volume per atom to double stretching. The presence of allotropic transformation from $α$ - phase of Iron to the hexaferrum with the growth of pressure is observed. In the case of hydrostatic deformations we also have obtained analogous cold curves, but with non-zero electronic temperatures in the range up to 5 eV. The similar volume and electronic temperature ranges have been considered recently. The behavior of electronic internal energy, pressure, and density of states was investigated in the volume and temperature ranges called above. The maximum hydrostatic strains and the types of lattice instabilities were theoretically predicted for the considered metals. The influence of high electronic temperature on the electronic heat conductivity and electric resistivity has been provided for d-band metals by the approach based on the solution of Boltzmann kinetic equation in $τ$-approximation. This data is compared with the results of quantum molecular dynamics for Gold.

cond-mat.mtrl-sci↗

arXiv2015

Transport of Massless Dirac Fermions in Non-topological Type Edge States

There are two types of intrinsic surface states in solids. The first type is formed on the surface of topological insulators. Recently, transport of massless Dirac fermions in the band of "topological" states has been demonstrated. States of the second type were predicted by Tamm and Shockley long ago. They do not have a topological background and are therefore strongly dependent on the properties of the surface. We study the problem of the conductivity of Tamm-Shockley edge states through direct transport experiments. Aharonov-Bohm magneto-oscillations of resistance are found on graphene samples that contain a single nanohole. The effect is explained by the conductivity of the massless Dirac fermions in the edge states cycling around the nanohole. The results demonstrate the deep connection between topological and non-topological edge states in 2D systems of massless Dirac fermions.

cond-mat.mes-hall↗

arXiv2015

Orbital Quantization in a System of Edge Dirac Fermions in Nanoperforated Graphene

The dependence of the electric resistance R of nanoperforated graphene samples on the position of the Fermi level, which is varied by the gate voltage Vg, has been studied. Nanoperforation has been performed by irradiating graphene samples on a Si/SiO$_2$ substrate by heavy (xenon) or light (helium) ions. A series of regular peaks have been revealed on the R(Vg) dependence at low temperatures in zero magnetic field. These peaks are attributed to the passage of the Fermi level through an equidistant ladder of levels formed by orbitally quantized states of edge Dirac fermions rotating around each nanohole. The results are in agreement with the theory of edge states for massless Dirac fermions.

cond-mat.mes-hall↗

arXiv2014

Effect of irradiation by He+ and Ga+ ions on 2D-exciton susceptibility of the InGaAs/GaAs quantum-well structures

The effect of irradiation by 30-keV Ga+ and 35-keV He+ ions (in relatively small doses) on the excitonic reflectivity spectra of single InGaAs/GaAs quantum-well structures is studied. It is found that the irradiation results in decreasing intensity and broadening of the excitonic resonances in the reflectivity spectra for all the doses. It is shown that these changes are not related to a decrease of the exciton transition oscillator strength and, therefore, to the irradiation-induced destruction of the excitonic states, but can be rather ascribed to a common cause, namely, to inhomogeneous broadening of the excitonic resonances proportional to the exposure dose.A tentative model of the irradiation-induced broadening is considered, with the mechanism of the process being a consequence of scat-tering of the 2D excitons by structural defects associated with Ga(In) and As vacancies arising upon collisions of the high-energy ions with regular atoms of the crystal structure. The model is used to compare experimental dependence of efficiency of the Ga+-ion-induced broad-ening on distance of the quantum well from the irradiated surface with a similar dependence calculated using the Monte-Carlo technique. A discrepancy between the results of simulation and experimental data is discussed.

cond-mat.mes-hall↗

arXiv2013

Aharonov-Bohm resistance magneto-oscillations on single-nanohole graphite and graphene structures

Graphene is a stable single atomic layer material exhibiting two-dimensional electron gas of massless Dirac fermions of high mobility. One of the intriguing properties of graphene is a possibility of realization of the Tamm-type edge states. These states differ from the usual surface states caused by defects, impurities and other imperfections at the edge of the system, as well as they differ from the magnetic edge states caused by skipping cyclotron orbits. The Tamm states result from breaking of periodic crystal potential at the edge, they can exist even at zero magnetic field and form a conducting band. Until recently those states have been observed in graphene only by local STM technique and there were no direct experiments on their contribution to transport measurements. Here we present the experiments on Aharonov-Bohm (AB) oscillations of resistance in a single-nanohole graphite and graphene structures, it indicates the presence of conducting edge states cycling around nanohole. An estimation show the penetration depth of the edge states to be as short as about 2 nm. The oscillations persist up to temperature T=115 K and the T-range of their existence increases with a decrease of the nanohole diameter. The proposed mechanism of the AB oscillations based on the resonant intervalley backscattering of the Dirac fermions by the nanohole via the Tamm states. The experimental results are consistent with such a scenario. Our findings show a way towards interference devices operating at high temperatures on the edge states in graphene

cond-mat.mes-hall↗

arXiv2012

Threshold fracture energy in solid particle erosion

The effect of geometrical shape of eroding absolutely rigid particles on the threshold rate of failure has been studied. The Shtaerman-Kilchevsky theory of quasi-static blunt impact, which generalizes Hertz's classical impact theory, is used for modeling the frictionless contact interaction of an axially-symmetric particle with an elastic half-space. The incubation time fracture criterion is applied for predicting surface fracture. It is shown that there exist a critical value of the particle shape parameter such that for all its lower values the fracture energy possesses a nonzero minimal value.

physics.class-ph↗

arXiv2009

Spallative ablation of dielectrics by X-ray laser

Short laser pulse in wide range of wavelengths, from infrared to X-ray, disturbs electron-ion equilibrium and rises pressure in a heated layer. The case where pulse duration $τ_L$ is shorter than acoustic relaxation time $t_s$ is considered in the paper. It is shown that this short pulse may cause thermomechanical phenomena such as spallative ablation regardless to wavelength. While the physics of electron-ion relaxation on wavelength and various electron spectra of substances: there are spectra with an energy gap in semiconductors and dielectrics opposed to gapless continuous spectra in metals. The paper describes entire sequence of thermomechanical processes from expansion, nucleation, foaming, and nanostructuring to spallation with particular attention to spallation by X-ray pulse.

physics.optics↗

arXiv2009

Two-dimensional zero-gap electronic states at a magnetic field

This work was firstly published in 1986 \cite{we}. No real two-dimensional object with the zero-gap quasi-relativistic spectrum was known in that time. Such an object is well known now: this is graphene. That is why we decided to present it again as a e-print in a slightly modified form. A density of the two-dimensional zero-gap electronic states at the quantizing magnetic field in the presence the Gaussian random potential has been calculated. The problem is reduced to zero-dimensional spinor field theory using the holomorphic supersymmetric representation. The calculated density of states in the case of the mass perturbation has a delta function peak in the Dirac point.This peak smears due to the potential perturbation.

cond-mat.mes-hall↗

arXiv2008

Two-temperature relaxation and melting after absorption of femtosecond laser pulse

The theory and experiments concerned with the electron-ion thermal relaxation and melting of overheated crystal lattice constitute the subject of this paper. The physical model includes two-temperature equation of state, many-body interatomic potential, the electron-ion energy exchange, electron thermal conductivity, and optical properties of solid, liquid, and two phase solid-liquid mixture. Two-temperature hydrodynamics and molecular dynamics codes are used. An experimental setup with pump-probe technique is used to follow evolution of an irradiated target with a short time step 100 fs between the probe femtosecond laser pulses. Accuracy of measurements of reflection coefficient and phase of reflected probe light are ~1% and $\sim 1\un{nm}$, respectively. It is found that, {\it firstly}, the electron-electron collisions make a minor contribution to a light absorbtion in solid Al at moderate intensities; {\it secondly}, the phase shift of a reflected probe results from heating of ion subsystem and kinetics of melting of Al crystal during $0<t<4\un{ps},$ where $t$ is time delay between the pump and probe pulses measured from the maximum of the pump; {\it thirdly} the optical response of Au to a pump shows a marked contrast to that of Al on account of excitation of \textit{d}-electrons

physics.optics↗