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The plasma channel formation in the focus of a knife-like nanosecond laser pulse irradiating a gas target is studied theoretically, and in gas-dynamics computer simulations. The distribution of the electromagnetic field in the focus region, obtained analytically, is used to calculate the energy deposition in the plasma, which then is implemented in the magnetohydrodynamic computer code. The modelling of the channel evolution shows that the plasma profile, which can guide the laser pulse, is formed by the tightly focused short knife-like lasers. The results of the simulations show that a proper choice of the convergence angle of a knife-like laser beam (determined by the focal length of the last cylindrical lens), and laser pulse duration may provide a sufficient degree of azimuthal symmetry of the formed plasma channel.

Supercomputer simulations are of fundamental importance for understanding the physics of nonlinear processes in high temperature pulse plasma. The development of predictive codes is an urgent problem in computational plasma physics, as well as many other fields of science. Radiative magneto-hydrodynamics 3D code MARPLE is a full-scale multiphysics research code using the state-of-the-art physics, mathematics, and numerics as well as the up-to-date high performance computing functionality. Scalability study demonstrated that the code can fit existing petaFLOPS supercomputers as well as next-generation exaFLOPS ones. The code is currently used for multiphysics simulations, specifically for high energy density plasma in pulsed-power facilities. Compression of a wire array by a high-current discharge is a valuable tool for fundamental study of matter in extreme states. Different configurations of wire arrays were investigated numerically. A series of high resolution computations helped to create a very compact spherical bright radiation source using dedicated design of the electrodes, the wire array, and the mass distribution along the wires.

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, 11 pages, 9 figures

For several decades the capillary discharges have been under intensive investigations due to various promising applications, e.g. for the laser electron accelerators as well as for the X-ray lasers [1,2]. A major portion of the experiments were done with circular cross-section capillaries. An appropriate theoretical and numerical study of circular capillaries can be greatly simplified to a 1D model [3] assuming rotational and axial symmetries of the plasma flow in a long thin channel. On the other hand, studying capillaries with non-circular cross-section [4], which have been attracting substantially less attention, requires more complicated 2D models. Such capillaries, for example, square one, possess several advantages related to their fabrication as well as for plasma diagnostics The aim of our work is to compare the plasma density and temperature distributions formed at the quasistationary stage of the discharge. We present the results of MHD simulations of hydrogen-filled capillary discharges with circular and rectangular cross-sections under almost the same conditions characterizing the initial configurations and the external electric circuit. The simulation parameters are choosen to correspond to the capillary discharge based waveguide for the laser wakefield accelerator [5]. Bibliography [1] Leemans W. P. et al 2014 Phys. Rev. Lett. 113 245002 [2] Benware B. R. et al 1998 Phys. Rev. Lett. 81 5804 [3] Bobrova N. A. et al 2001 Phys. Rev. E 65 016407 [4] Gonsalves A. J. et al 2007 Phys. Rev. Lett. 98 025002 [5] Esarey E. et al 2009 Rev. Mod. Phys. 81 1229

The project BELLA (LBNL, USA) is aimed to create an experimental facility for further advancing the development of laser-driven electron acceleration1. BELLA's unique attribute is the ability to use laser light to accelerate an electron beam up to 10 GeV level in a comparatively short distance of approximately one meter. The acceleration takes place during the propagation of a high power femtosecond laser pulse in the plasma formed in a capillary discharge. This capillary plasma forms a plasma channel able to guide the laser pulse, which in its turn forms a plasma wake wave that accelerates the injected electrons.

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., 6 pages, 10 figures

A design of fast current switch using wire explosion is a problem of studying by our team experimentally and numerically. The main unit of the experimental setup represents itself two coaxial cylindrical electrodes connected by a thin single wire which is exploded by the current pulse. In the experiments a current with the amplitude up to 80 kA and rise time of 7 μs flowing through the 4-mm tungsten wire. Plasma generated by exploded wire fills the inter-electrode space "circularly" owing to the interaction with magnetic field (up to 30 kG) produced by an external source and directed along the electrodes. The setup construction allows us to change the amplitude of current flowing between the electrodes and to measure the total resistance of inter-electrode volume. Therefore we can study a plasma dynamics by varying the current value. In our experiments the current amplitude is changed in a rather wide range of 10–80 kA, and a current rise time is changed from 109 to 1010 A/s. The experimental data are compared with those obtained via computer simulation carried out by means of the radiative magnetohydrodynamic code MARPLE3D (KIAM RAS). W e studied 3D plasma dynamics numerically (i) at a stage of a wire explosion and filling of the inter-electrode gap by plasma, and (ii) a stage of a plasma switch functioning. Numerical simulations allow to estimate a time which need to fill the volume between the coaxial electrodes with the plasma generated from the wire and to study the switch dynamics dependence on the hydromagnetic instability as well as on the Hall effect. The numerical data correlates with experimental results. The main problems we discuss are: (i) what are possible reasons of the rapid decreasing of the switch resistance and (ii) how we can maintain an appropriately high resistance of the switch during the time needed for its applications.