arXiv subjects

Pavel Sasorov

Publications and source records attributed to Pavel Sasorov.

16 recordsLinked to original sources

arXiv2026

Laser Wakefield Acceleration in a Capillary Gas Cell Producing GeV-Scale High-Quality Electron Beams

Laser Wakefield Acceleration (LWFA) is a promising approach for producing high-brightness electron beams in the GeV energy range, offering significant potential for compact next-generation accelerator facilities. In this work, we present a computational study of LWFA in a specially designed single-stage capillary gas-cell target aimed at producing high-quality, GeV-class electron beams. The capillary cell includes a short ($\sim 2$ mm) injection region at the entrance filled with a helium (He) and nitrogen (N$_2$) gas mixture. This is followed by a longer ($\sim 14$ mm) pure He section, which provides the required acceleration length and limits continuous ionization injection, thereby significantly reducing the energy spread of the accelerated beam. Hydrodynamic simulations are performed to optimize the capillary geometry and generate the required two-section gas-pressure profile. The resulting gas-density distributions for various cases are then directly incorporated in Particle-In-Cell (PIC) simulations to study LWFA. In particular, our hydrodynamic simulations demonstrate how tailored density profiles with longitudinal density tapering in the acceleration section can be realized in a capillary gas cell, while the corresponding PIC simulations reveal how these profiles influence the acceleration process and the resulting beam quality. Using a 100~TW-class laser system with parameters relevant to the L2-DUHA laser at the ELI Beamlines Facility, the PIC results demonstrate electron acceleration to mean energies exceeding $1.0$~GeV with high-quality beam properties. Self-injected He electrons are also observed, and their impact on the main beam quality is evaluated. The findings of this study provide valuable insights for upcoming LWFA experiments planned within the EuPRAXIA Project at the ELI Beamlines Facility.

physics.acc-ph↗

arXiv2026

Generation of High Order Harmonics in Vacuum for Various Configurations of Interacting Electromagnetic Field

High order harmonic (HOH) generation by interacting extremely intense electromagnetic waves in the quantum vacuum is investigated within the framework of the Heisenberg-Euler formalism. We consider here the process in the lowest order of a perturbation theory relative to the electromagnetic (EM) beam intensity, giving contribution to the HOH generation. The main expressions are obtained for a general geometry, whyle polarizations of different sub-beams forming the EM beam focus are almost the same. Nevertheless, explicit expressions for the HOH generation are derived for the $4π$-dipole in-coming waves and for the two crossing Gaussian beams. The former geometry of the EM beam is optimal at a given EM wave power, whereas the latter one is more realistic from the experimental point of view. We consider also a relationship of our present general results with the results, obtained earlier for the HOH generation during of collision of two plane electromagnetic waves.

physics.plasm-ph↗

arXiv2025

Coupling and Acceleration of Externally Injected Electron Beams in Laser-Driven Plasma Wakefields

The multi-stage method of laser wakefield acceleration (LWFA) presents a promising approach for developing stable, full-optical, high-energy electron accelerators. By segmenting the acceleration process into several booster stages, each powered by independent laser drivers, this technique effectively mitigates challenges such as electron dephasing, pump depletion, and laser diffraction. A critical aspect of multi-stage LWFA is the nonlinear interaction between the injected electron beam and the laser-driven wakefields in the booster stage. This study investigates the injection and acceleration of external electron beams within wakefields in the booster stage using multi-dimensional Particle-In-Cell (PIC) simulations. We provide both qualitative and quantitative descriptions of the observed physical processes. Key parameters influencing charge coupling process and the resultant beam quality have been identified. Furthermore, we have examined how off-axis injection relative to the driver laser influences the acceleration process and beam quality. Our findings provide valuable insights for advancing and optimizing multi-stage plasma-based accelerators.

physics.plasm-ph↗

arXiv2023

Short-time large deviations of the spatially averaged height of a KPZ interface on a ring

Using the optimal fluctuation method, we evaluate the short-time probability distribution $P (\bar{H}, L, t=T)$ of the spatially averaged height $\bar{H} = (1/L) \int_0^L h(x, t=T) \, dx$ of a one-dimensional interface $h(x, t)$ governed by the Kardar-Parisi-Zhang equation $$ \partial_th=ν\partial_x^2h+\fracλ{2} \left(\partial_xh\right)^2+\sqrt{D}ξ(x,t) $$ on a ring of length $L$. The process starts from a flat interface, $h(x,t=0)=0$. Both at $λ\bar{H} < 0$, and at sufficiently small positive $λ\bar{H}$ the optimal (that is, the least-action) path $h(x,t)$ of the interface, conditioned on $\bar{H}$, is uniform in space, and the distribution $P (\bar{H}, L, T)$ is Gaussian. However, at sufficiently large $λ\bar{H} > 0$ the spatially uniform solution becomes sub-optimal and gives way to non-uniform optimal paths. We study them, and the resulting non-Gaussian distribution $P (\bar{H}, L, T)$, analytically and numerically. The loss of optimality of the uniform solution occurs via a dynamical phase transition of either first, or second order, depending on the rescaled system size $\ell = L/\sqrt{νT}$, at a critical value $\bar{H}=\bar{H}_{\text{c}}(\ell)$. At large but finite $\ell$ the transition is of first order. Remarkably, it becomes an "accidental" second-order transition in the limit of $\ell \to \infty$, where a large-deviation behavior $-\ln P (\bar{H}, L, T) \simeq (L/T) f(\bar{H})$ (in the units $λ=ν=D=1$) is observed. At small $\ell$ the transition is of second order, while at $\ell =O(1)$ transitions of both types occur.

cond-mat.stat-mech↗

arXiv2022

Probabilities of moderately atypical fluctuations of the size of a swarm of Brownian Bees

The ``Brownian bees'' model describes an ensemble of $N=$~const independent branching Brownian particles. The conservation of $N$ is provided by a modified branching process. When a particle branches into two particles, the particle which is farthest from the origin is eliminated simultaneously. The spatial density of the particles is governed by the solution of a free boundary problem for a reaction-diffusion equation in the limit of $N \gg 1$. At long times, the particle density approaches a spherically symmetric steady state solution with a compact support of radius $\bar{\ell}_0$. However, at finite $N$, the radius of this support, $L$, fluctuates. The variance of these fluctuations appears to exhibit a logarithmic anomaly [Siboni {\em et al}., Phys. Rev. E. {\bf104}, 054131 (2021)]. It is proportional to $N^{-1}\ln N$ at $N\to\infty$. We investigate here the tails of the probability density function (PDF), $P(L)$, of the swarm radius, when the absolute value of the radius fluctuation $ΔL=L-\bar{\ell}_0$ is sufficiently larger than the typical fluctuations' scale determined by the variance. For negative deviations the PDF can be obtained in the framework of the optimal fluctuation method (OFM). This part of the PDF displays the scaling behavior: $\ln P\propto - N ΔL^2\, \ln^{-1}(ΔL^{-2})$, demonstrating a logarithmic anomaly at small negative $ΔL$. For the opposite sign of the fluctuation, $ΔL > 0$, the PDF can be obtained with an approximation of a single particle, running away. We find that $\ln P \propto -N^{1/2}ΔL$. We consider in this paper only the case, when $|ΔL|$ is much less than the typical radius of the swarm at $N\gg 1$.

cond-mat.stat-mech↗

arXiv2021

Fluctuations of a swarm of Brownian bees

The ``Brownian bees" model describes an ensemble of $N$ independent branching Brownian particles. When a particle branches into two particles, the particle farthest from the origin is eliminated so as to keep a constant number of particles. In the limit of $N\to \infty$, the spatial density of the particles is governed by the solution of a free boundary problem for a reaction-diffusion equation. At long times the particle density approaches a spherically symmetric steady state solution with a compact support. Here we study fluctuations of the ``swarm of bees" due to the random character of the branching Brownian motion in the limit of large but finite $N$. We consider a one-dimensional setting and focus on two fluctuating quantities: the swarm center of mass $X(t)$ and the swarm radius $\ell(t)$. Linearizing a pertinent Langevin equation around the deterministic steady state solution, we calculate the two-time covariances of $X(t)$ and $\ell(t)$. The variance of $X(t)$ directly follows from the covariance of $X(t)$, and it scales as $1/N$ as to be expected from the law of large numbers. The variance of $\ell(t)$ behaves differently: it exhibits an anomalous scaling $\ln N/N$. This anomaly appears because all spatial scales, including a narrow region near the edges of the swarm where only a few particles are present, give a significant contribution to the variance. We argue that the variance of $\ell(t)$ can be obtained from the covariance of $\ell(t)$ by introducing a cutoff at the microscopic time $1/N$ where the continuum Langevin description breaks down. Our theoretical predictions are in good agreement with Monte-Carlo simulations of the microscopic model. Generalizations to higher dimensions are briefly discussed.

cond-mat.stat-mech↗

arXiv2021

Observing symmetry-broken optimal paths of stationary Kardar-Parisi-Zhang interface via a large-deviation sampling of directed polymers in random media

Consider the short-time probability distribution $\mathcal{P}(H,t)$ of the one-point interface height difference $h(x=0,τ=t)-h(x=0,τ=0)=H$ of the stationary interface $h(x,τ)$ described by the Kardar-Parisi-Zhang equation. It was previously shown that the optimal path -- the most probable history of the interface $h(x,τ)$ which dominates the upper tail of $\mathcal{P}(H,t)$ -- is described by any of \emph{two} ramp-like structures of $h(x,τ)$ traveling either to the left, or to the right. These two solutions emerge, at a critical value of $H$, via a spontaneous breaking of the mirror symmetry $x\leftrightarrow -x$ of the optimal path, and this symmetry breaking is responsible for a second-order dynamical phase transition in the system. We simulate the interface configurations numerically by employing a large-deviation Monte Carlo sampling algorithm in conjunction with the mapping between the KPZ interface and the directed polymer in a random potential at high temperature. This allows us to observe the optimal paths, which determine each of the two tails of $\mathcal{P}(H,t)$, down to probability densities as small as $10^{-500}$. At short times we observe mirror-symmetry-broken traveling optimal paths for the upper tail, and a single mirror-symmetric path for the lower tail, in good quantitative agreement with analytical predictions. At long times, even at moderate values of $H$, where the optimal fluctuation method is \emph{not} supposed to apply, we still observe two well-defined dominating paths. Each of them violates the mirror symmetry $x\leftrightarrow -x$ and is a mirror image of the other.

cond-mat.stat-mech↗

arXiv2021

Generation of High Order Harmonics in Heisenberg-Euler Electrodynamics

High order harmonic generation by extremely intense, interacting, electromagnetic waves in the quantum vacuum is investigated within the framework of the Heisenberg-Euler formalism. Two intersecting plane waves of finite duration are considered in the case of general polarizations. Detailed finite expressions are obtained for the case where only the first Poincaré invariant does not vanish. Yields of high harmonics in this case are most effective.

hep-th↗

arXiv2021

Persistent fluctuations of the swarm size of Brownian bees

The "Brownian bees" model describes a system of $N$ independent branching Brownian particles. At each branching event the particle farthest from the origin is removed, so that the number of particles remains constant at all times. Berestycki et al. (2020) proved that, at $N\to \infty$, the coarse-grained spatial density of this particle system lives in a spherically symmetric domain and is described by the solution of a free boundary problem for a deterministic reaction-diffusion equation. Further, they showed that, at long times, this solution approaches a unique spherically symmetric steady state with compact support: a sphere which radius $\ell_0$ depends on the spatial dimension $d$. Here we study fluctuations in this system in the limit of large $N$ due to the stochastic character of the branching Brownian motion, and we focus on persistent fluctuations of the swarm size. We evaluate the probability density $\mathcal{P}(\ell,N,T)$ that the maximum distance of a particle from the origin remains smaller than a specified value $\ell<\ell_0$, or larger than a specified value $\ell>\ell_0$, on a time interval $0<t<T$, where $T$ is very large. We argue that $\mathcal{P}(\ell,N,T)$ exhibits the large-deviation form $-\ln \mathcal{P} \simeq N T R_d(\ell)$. For all $d$ we obtain asymptotics of the rate function $R_d(\ell)$ in the regimes $\ell \ll \ell_0$, $\ell\gg \ell_0$ and $|\ell-\ell_0|\ll \ell_0$. For $d=1$ the whole rate function can be calculated analytically. We obtain these results by determining the optimal (most probable) density profile of the swarm, conditioned on the specified $\ell$, and by arguing that this density profile is spherically symmetric with its center at the origin.

cond-mat.stat-mech↗

arXiv2020

Velocity fluctuations of stochastic reaction fronts propagating into an unstable state: strongly pushed fronts

The empirical velocity of a reaction-diffusion front, propagating into an unstable state, fluctuates because of the shot noises of the reactions and diffusion. Under certain conditions these fluctuations can be described as a diffusion process in the reference frame moving with the average velocity of the front. Here we address pushed fronts, where the front velocity in the deterministic limit is affected by higher-order reactions and is therefore larger than the linear spread velocity. For a subclass of these fronts -- strongly pushed fronts -- the effective diffusion constant $D_f\sim 1/N$ of the front can be calculated, in the leading order, via a perturbation theory in $1/N \ll 1$, where $N\gg 1$ is the typical number of particles in the transition region. This perturbation theory, however, overestimates the contribution of a few fast particles in the leading edge of the front. We suggest a more consistent calculation by introducing a spatial integration cutoff at a distance beyond which the average number of particles is of order 1. This leads to a non-perturbative correction to $D_f$ which even becomes dominant close to the transition point between the strongly and weakly pushed fronts. At the transition point we obtain a logarithmic correction to the $1/N$ scaling of $D_f$. We also uncover another, and quite surprising, effect of the fast particles in the leading edge of the front. Because of these particles, the position fluctuations of the front can be described as a diffusion process only on very long time intervals with a duration $Δt \gg τ_N$, where $τ_N$ scales as $N$. At intermediate times the position fluctuations of the front are anomalously large and non-diffusive. Our extensive Monte-Carlo simulations of a particular reacting lattice gas model support these conclusions.

cond-mat.stat-mech↗

arXiv2019

Optimal paths of non-equilibrium stochastic fields: the Kardar-Parisi-Zhang interface as a test case

Atypically large fluctuations in macroscopic non-equilibrium systems continue to attract interest. Their probability can often be determined by the optimal fluctuation method (OFM). The OFM brings about a conditional variational problem, the solution of which describes the "optimal path" of the system which dominates the contribution of different stochastic paths to the desired statistics. The OFM proved efficient in evaluating the probabilities of rare events in a host of systems. However, theoretically predicted optimal paths were observed in stochastic simulations only in diffusive lattice gases, where the predicted optimal density patterns are either stationary, or travel with constant speed. Here we focus on the one-point height distribution of the paradigmatic Kardar-Parisi-Zhang interface. Here the optimal paths, corresponding to the distribution tails at short times, are intrinsically non-stationary and can be predicted analytically. Using the mapping to the directed polymer in a random potential at high temperature, we obtain "snapshots" of the optimal paths in Monte-Carlo simulations which probe the tails with an importance sampling algorithm. For each tail we observe a very narrow "tube" of height profiles around a single optimal path which agrees with the analytical prediction. The agreement holds even at long times, supporting earlier assertions of the validity of the OFM in the tails well beyond the weak-noise limit.

cond-mat.stat-mech↗

arXiv2018

Finite-size effects in the short-time height distribution of the Kardar-Parisi-Zhang equation

We use the optimal fluctuation method to evaluate the short-time probability distribution $\mathcal{P}\left(H,L,t\right)$ of height at a single point, $H=h\left(x=0,t\right)$, of the evolving Kardar-Parisi-Zhang (KPZ) interface $h\left(x,t\right)$ on a ring of length $2L$. The process starts from a flat interface. At short times typical (small) height fluctuations are unaffected by the KPZ nonlinearity and belong to the Edwards-Wilkinson universality class. The nonlinearity, however, strongly affects the (asymmetric) tails of $\mathcal{P}(H)$. At large $L/\sqrt{t}$ the faster-decaying tail has a double structure: it is $L$-independent, $-\ln\mathcal{P}\sim\left|H\right|^{5/2}/t^{1/2}$, at intermediately large $|H|$, and $L$-dependent, $-\ln\mathcal{P}\sim \left|H\right|^{2}L/t$, at very large $|H|$. The transition between these two regimes is sharp and, in the large $L/\sqrt{t}$ limit, behaves as a fractional-order phase transition. The transition point $H=H_{c}^{+}$ depends on $L/\sqrt{t}$. At small $L/\sqrt{t}$, the double structure of the faster tail disappears, and only the very large-$H$ tail, $-\ln\mathcal{P}\sim \left|H\right|^{2}L/t$, is observed. The slower-decaying tail does not show any $L$-dependence at large $L/\sqrt{t}$, where it coincides with the slower tail of the GOE Tracy-Widom distribution. At small $L/\sqrt{t}$ this tail also has a double structure. The transition between the two regimes occurs at a value of height $H=H_{c}^{-}$ which depends on $L/\sqrt{t}$. At $L/\sqrt{t} \to 0$ the transition behaves as a mean-field-like second-order phase transition. At $|H|<|H_c^{-}|$ the slower tail behaves as $-\ln\mathcal{P}\sim \left|H\right|^{2}L/t$, whereas at $|H|>|H_c^{-}|$ it coincides with the slower tail of the GOE Tracy-Widom distribution.

cond-mat.stat-mech↗

arXiv2017

Plasma Equilibrium inside Various Cross-Section Capillary Discharges

Plasma properties inside a hydrogen-filled capillary discharge waveguide were modeled with dissipative magnetohydrodynamic simulations to enable analysis of capillaries of circular and square cross-sections implying that square capillaries can be used to guide circularly-symmetric laser beams. When the quasistationary stage of the discharge is reached, the plasma and temperature in the vicinity of the capillary axis has almost the same profile for both the circular and square capillaries. The effect of cross-section on the electron beam focusing properties were studied using the simulation-derived magnetic field map. Particle tracking simulations showed only slight effects on the electron beam symmetry in the horizontal and diagonal directions for square capillary.

physics.plasm-ph↗

arXiv2017

Laser beam coupling with capillary discharge plasma for laser wakefield acceleration applications

One of the most robust methods, demonstrated up to date, of accelerating electron beams by laser-plasma sources is the utilization of plasma channels generated by the capillary discharges. These channels, i.e., plasma columns with a minimum density along the laser pulse propagation axis, may optically guide short laser pulses, thereby increasing the acceleration length, leading to a more efficient electron acceleration. Although the spatial structure of the installation is simple in principle, there may be some important effects caused by the open ends of the capillary, by the supplying channels etc., which require a detailed 3D modeling of the processes taking place in order to get a detailed understanding and improve the operation. However, the discharge plasma, being one of the most crucial components of the laser-plasma accelerator, is not simulated with the accuracy and resolution required to advance this promising technology. In the present work, such simulations are performed using the code MARPLE. First, the process of the capillary filling with a cold hydrogen before the discharge is fired, through the side supply channels is simulated. The main goal of this simulation is to get a spatial distribution of the filling gas in the region near the open ends of the capillary. A realistic geometry is used for this and the next stage simulations, including the insulators, the supplying channels as well as the electrodes. Second, the simulation of the capillary discharge is performed with the goal to obtain a time-dependent spatial distribution of the electron density near the open ends of the capillary as well as inside the capillary. Finally, to evaluate effectiveness of the beam coupling with the channeling plasma wave guide and electron acceleration, modeling of laser-plasma interaction was performed with the code INF&RNO

physics.plasm-ph↗

arXiv2017

Large deviations of surface height in the $1+1$-dimensional Kardar-Parisi-Zhang equation: exact long-time results for $λH<0$

We study atypically large fluctuations of height $H$ in the 1+1-dimensional Kardar-Parisi-Zhang (KPZ) equation at long times $t$, when starting from a "droplet" initial condition. We derive exact large deviation function of height for $λH<0$, where $λ$ is the nonlinearity coefficient of the KPZ equation. This large deviation function describes a crossover from the Tracy-Widom distribution tail at small $|H|/t$, which scales as $|H|^3/t$, to a different tail at large $|H|/t$, which scales as $|H|^{5/2}/t^{1/2}$. The latter tail exists at all times $t>0$. It was previously obtained in the framework of the optimal fluctuation method. It was also obtained at short times from exact representation of the complete height statistics. The crossover between the two tails, at long times, occurs at $|H|\sim t$ as previously conjectured. Our analytical findings are supported by numerical evaluations using exact representation of the complete height statistics.

cond-mat.stat-mech↗

arXiv2011

Negative velocity fluctuations of pulled reaction fronts

The position of a reaction front, propagating into an unstable state, fluctuates because of the shot noise. What is the probability that the fluctuating front moves considerably slower than its deterministic counterpart? Can the noise arrest the front motion for some time, or even make it move in the wrong direction? We present a WKB theory that assumes many particles in the front region and answers these questions for the microscopic model A->2A, 2A->A and random walk.

cond-mat.stat-mech↗