Extreme plasma states in laser-governed vacuum breakdown

Triggering vacuum breakdown at the upcoming laser facilities can provide rapid electron-positron pair production for studies in laboratory astrophysics and fundamental physics. However, the density of the emerging plasma should seemingly stop rising at the relativistic critical density, when the plasma becomes opaque. Here we identify the opportunity of breaking this limit using optimal beam configuration of petawatt-class lasers. Tightly focused laser fields allow plasma generation in a small focal volume much less than ${\lambda}^3$, and creating extreme plasma states in terms of density and produced currents. These states can be regarded as a new object of nonlinear plasma physics. Using 3D QED-PIC simulations we demonstrate the possibility of reaching densities of more than $10^{25}$ cm$^{-3}$, which is an order of magnitude higher than previously expected. Controlling the process via the initial target parameters gives the opportunity to reach the discovered plasma states at the upcoming laser facilities

Improved vectorization of OpenCV algorithms for RISC-V CPUs

The development of an open and free RISC-V architecture is of great interest for a wide range of areas, including high-performance computing and numerical simulation in mathematics, physics, chemistry and other problem domains. In this paper, we discuss the possibilities of accelerating computations on available RISC-V processors by improving the vectorization of several computer vision and machine learning algorithms in the widely used OpenCV library. It is shown that improved vectorization speeds up computations on existing prototypes of RISC-V devices by tens of percent

Extended particle-in-cell schemes for physics in ultrastrong laser fields: Review and developments.

We review common extensions of particle-in-cell (PIC) schemes which account for strong field phenomena in laser-plasma interactions. After describing the physical processes of interest and their numerical implementation, we provide solutions for several associated methodological and algorithmic problems. We propose a modified event generator that precisely models the entire spectrum of incoherent particle emission without any low-energy cutoff, and which imposes close to the weakest possible demands on the numerical time step. Based on this, we also develop an adaptive event generator that subdivides the time step for locally resolving QED events, allowing for efficient simulation of cascades. Further, we present a unified technical interface for including the processes of interest in different PIC implementations. Two PIC codes which support this interface, PICADOR and ELMIS, are also briefly reviewed

Quantum subdiffusion with two- and three-body interactions

We study the dynamics of a few-quantum-particle cloud in the presence of two- and three-body interactions in weakly disordered one-dimensional lattices. The interaction is dramatically enhancing the Anderson localization length ξ1 of noninteracting particles. We launch compact wave packets and show that few-body interactions lead to transient subdiffusion of wave packets, m2 ∼ tα, α < 1, on length scales beyond ξ1. The subdiffusion exponent is independent of the number of particles. Two-body interactions yield α ≈ 0.5 for two and three particles, while three-body interactions decrease it to α ≈ 0.2. The tails of expanding wave packets exhibit exponential localization with a slowly decreasing exponent. We relate our results to subdiffusion in nonlinear random lattices, and to results on restricted diffusion in high-dimensional spaces like e.g. on comb lattices. c EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 20171221sciescopu

Quantum subdiffusion with two- and three-body interactions

We study the dynamics of a few-quantum-particle cloud in the presence of two- and three-body interactions in weakly disordered one-dimensional lattices. The interaction is dramatically enhancing the Anderson localization length ξ1 of noninteracting particles. We launch compact wave packets and show that few-body interactions lead to transient subdiffusion of wave packets, m2 ∼ tα, α < 1, on length scales beyond ξ1. The subdiffusion exponent is independent of the number of particles. Two-body interactions yield α ≈ 0.5 for two and three particles, while three-body interactions decrease it to α ≈ 0.2. The tails of expanding wave packets exhibit exponential localization with a slowly decreasing exponent. We relate our results to subdiffusion in nonlinear random lattices, and to results on restricted diffusion in high-dimensional spaces like e.g. on comb lattices. c EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 20171221sciescopu

Generation of current sheets and giant quasistatic magnetic fields at the ionization of vacuum in extremely strong light fields

The self-consistent dynamics of an electron–positron plasma, which is formed during the generation of quantum-electrodynamic cascades, in a superstrong field of counterpropagating linearly polarized waves is examined. It is shown that the formation of thin (on a wavelength scale) current sheets which generate quasistatic magnetic fields comparable to the corresponding fields of incident waves plays an important role in the dynamics of a cascade for fields above a certain threshold. The fraction of the laser energy transformed into the energy of quasistatic magnetic fields can exceed 20%

Approaches to the optimization and parallelization of computations in the problem of detecting objects of different classes in the image

E.A. Kozinov, Nizhny Novgorod State University (Nizhny Novgorod, Russian Federation), V.D. Kustikova, Nizhny Novgorod State University (Nizhny Novgorod, Russian Federation), I.B. Meyerov, Nizhny Novgorod State University (Nizhny Novgorod, Russian Federation), A.N. Polovinkin, Nizhny Novgorod State University (Nizhny Novgorod, Russian Federation), A.A. Sidnev, Nizhny Novgorod State University (Nizhny Novgorod, Russian Federation)Рассматривается задача детектирования объектов разных классов на статических изображениях: фотографиях или отдельных кадрах видеопотока. Описывается схема решения данной задачи с использованием алгоритма Latent SVM. Используется известный подход к ускорению вычислений - построение каскада классификаторов. Описывается вычислительная схема решения задачи детектирования с помощью каскадного Latent SVM. Обсуждаются проблемы распараллеливания и оптимизации времени поиска объектов одного класса на изображении. Проводится анализ вариантов решения указанных проблем. Выделяются наиболее трудоемкие участки реализаций, рассматриваются различные схемы распараллеливания, оцениваются их преимущества и недостатки. Приводятся результаты вычислительных экспериментов на базе изображений PASCAL Visual Object Challenge 2007, дается их анализ, а также формулируются выводы и планы по дальнейшему развитию. This paper considers the problem of object detection in static images. We describe a state-of-the-art method based on Latent SVM algorithm. A well-known approach to speed up calculations, the construction of cascade classifiers, is used. We describe a computational scheme that uses cascade modification of the original Latent SVM algorithm The issues of parallelization and performance optimization are discussed. We analyze the most timeconsuming parts of implementation, consider several parallelization schemes and aspects of their performance. The results of numerical experiments on PASCAL Visual Object Challenge 2007 image dataset are given