Computer simulation of graphite target ablation under the action of a nanosecond laser pulse

A quasi-gasdynamic approach is used for computer simulation of plasma expansion from a graphite plate subjected to a nanosecond laser pulse. A one-component plasma consisting of carbon molecules alone is considered. This simplifies the experimental conditions used previously to study the dynamics of the gas resulting from evaporation. The results of computer experiment conducted for different initial temperatures and pressures of the plasma are in good qualitative agreement with the real experimental data including in the time instant the density of the expanding gas reaches a maximum. It is shown that high-density clusters are likely to appear in front of the main plasma flux. The results of the computer simulation are compared with the Singh approximation of pressure, velocity, and density of the gas flow. It is concluded that this approximation is valid only within a short (compared with the entire expansion length) plasma expansion interval existing during the initial spread for t = 4 × 10^?9 s.     

A One‐Dimensional Particle‐in‐Cell Model of Plasma Build‐Up in Vacuum Arcs

Understanding the mechanism of plasma build‐up in vacuum arcs is essential in many fields of physics. A one‐dimensional particle‐in‐cell computer simulation model is presented, which models the plasma developing from a field emitter tip under electrical breakdown conditions, taking into account the relevant physical phenomena. As a starting point, only an external electric field and an initial enhancement factor of the tip are assumed. General requirements for plasma formation have been identified and formulated in terms of the initial local field and a critical neutral density. The dependence of plasma build‐up on tip melting current, the evaporation rate of neutrals and external circuit time constant has been investigated for copper and simulations imply that arcing involves melting currents around 0.5–1 A/μm², evaporation of neutrals to electron field emission ratios in the regime 0.01 – 0.05, plasma build‐up timescales in the order of ～ 1 – 10 ns and two different regimes depending on initial conditions, one producing an arc plasma, the other one not. Also the influence of the initial field enhancement factor and the external electric field required for ignition has been explored, and results are consistent with the experimentally measured local field value of ～ 10 GV/m for copper (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multicharged ion source based on the Tornado closed magnetic confinement system

It is proposed to use the Tornado closed magnetic confinement system with microwave plasma heating for creating a pulsed source of multicharged ions. The plasma losses in closed confinement systems are determined by the diffusion across the magnetic field, which substantially increases the plasma lifetime as compared to mirror confinement systems. A plasma heating scenario with the successive switching-on of two oscillators is proposed: an oscillator operating at a frequency of 2.45 GHz produces the initial plasma, which is then heated at a frequency of 15 or 53 GHz. It is shown that it is possible to achieve the distribution of ions over charge states with a maximum at Ar¹⁶⁺ at a plasma density of 2×10¹³ cm⁻³. The extracted ion current in this case can attain 1 A.     

Internal dynamics of a plasma propelled across a magnetic field

When a plasma is pushed across a magnetic field by some nonelectromagnetic force, ions and electrons get turned in opposite directions by the magnetic field. This creates an internal current as well as sheaths at the plasma surfaces and results in an electric field which allows the plasma to maintain some, or even most of its initial momentum in the form of E&oarr;×B&oarr; drift. An exact analysis of that process is presented for the internal region of the plasma. The energy provided by the initial push is used, in part, to create some gyrations inside the plasma. When the rest energy density of the plasma exceeds twice the magnetic energy density (or when the Alfven speed is less than c), there will be enough energy to spare for the plasma to continue across the magnetic field at half its initial momentum. Two cases are considered: an impulsive start and a gentle push such as provided by gravity. The amplitude of the resulting internal gyrations becomes small in the second case. The frequencies of the gyrations are those of extraordinary modes of very long spatial wavelength     

Numerical simulation of the initial plasma formation and current transfer in single-wire electrical explosion in vacuum

In this paper, a computational model is constructed to investigate the phenomenon of the initial plasma formation and current transfer in the single-wire electrical explosion in a vacuum. The process of the single-wire electrical explosion is divided into four stages. Stage Ⅰ: the wire is in solid state. Stage Ⅱ: the melting stage. Stage Ⅲ: the wire melts completely and the initial plasma forms. Stage IV: the core and corona expand separately. The thermodynamic calculation is applied before the wire melts completely in stages Ⅰ and Ⅱ. In stage Ⅲ, a one-dimensional magnetohydrodynamics model comes into play until the instant when the voltage collapse occurs. The temperature, density, and velocity, which are derived from the magnetohydrodynamics calculation, are averaged over the distribution area. The averaged parameters are taken as the initial conditions for stage Ⅳ in which a simplified magnetohydrodynamics model is applied. A wide-range semi-empirical equation of state, which is established based on the Thomas-Fermi-Kirzhnits model, is constructed to describe the phase transition from solid state to plasma state. The initial plasma formation and the phenomenon of current transfer in the electrical explosion of aluminum wire are investigated using the computational model. Experiments of electrical explosion of aluminum wires are carried out to verify this model. Simulation results are also compared with experimental results of the electrical explosion of copper wire.     

Quasi-adiabatic compression of a plasma column by a longitudinal magnetic field

Approximate 1.5-dimensional MHD equations are derived that describe the quasi-adiabatic compression of a thin plasma column by a longitudinal magnetic field. The parameters of the compressed plasma are obtained analytically as functions of the initial conditions and longitudinal field. The stability of plasma compression against the Rayleigh-Taylor instability is investigated. It is shown that, in the Z-Θ-pinch geometry, increasing the longitudinal magnetic field makes it possible to achieve radial compression ratios of 20–30 without violating the cylindrical symmetry of the column. The possibility of thermonuclear ignition in a thin plasma column in a Z-Θ-pinch configuration is studied. The ranges of the initial plasma densities and temperatures and the initial lengths of the plasma column that are needed to achieve ignition in a plasma compressed by a factor of 20–30 are determined. The parameters of the electromagnetic energy source required to achieve such a high plasma compression are estimated.     

Resonant interaction of the Langmuir wave with quasi-stationary plasma electrons

Evolution of the Langmuir wave in quasi-stationary plasma is considered. A consistent solution to the closed system of the Vlasov–Poisson equations is obtained in the adiabatic approximation. Dispersion of the wave evolving in the electron distribution tail with variable electron concentration or plasma temperature is described. It is established that the plasma oscillation energy increases with decreasing electron concentration or increasing temperature. After plasma regains its initial state, the wave parameters also restore their initial values. That is, the wave evolution in the quasi-stationary plasma is a completely reversible process.     

Effect of the ionization process on the focusing of a relativistic electron beam by a gas-plasma lens

A numerical simulation is made of the processes occurring in a plasma lens under conditions when the focusing of a relativistic electron beam is strongly affected by the ionization of the residual gas in the lens region by the beam itself. The paraxial, azimuthally symmetric, 1.5-dimensional, electrostatic kinetic model, taking account of plasma production, expansion of the plasma electrons away from the beam region, and contraction of the ions toward the axis of the beam, was used for the calculation. The dynamics of the formation of a focal spot is studied, and the size and position of the spot are determined as functions of time for different values of the gas pressure, initial plasma density, and energy of the beam electrons. Zh. Tekh. Fiz. 67, 90–94 (October 1997)     

Space- and time-resolved measurements of temperatures and electron densities of plasmas formed during laser ablation of metallic samples

Temperature and electron-density profiles in a laser-produced plasma have been measured at various distances from the sample surface and different delay times from the laser pulse. The plasma is produced with a Nd:YAG laser focused on a low-alloy steel sample in air at atmospheric pressure. The determination of the parameters is made starting from the distinct emission spectra emitted by the plasma along a direction parallel to the sample surface and measured simultaneously by a CCD detector. The experimental relative error is 1.5% for temperatures and 4.5% for electron densities. A small spatial variation of the plasma temperature ((T󾉨 K) is obtained except for the outer regions, were the intensity is weak. A higher spatial variation is obtained for the electron density, especially at initial times ((N_e᜖¹⁷ cm^-3 at t=3 7s).     

Simulation of shock-wave propagation in the argon plasma of a positive column discharge

The computer simulation of shock-wave propagation in the argon plasma of positive column discharge was performed. A one-dimensional model of the gas-discharge plasma is used, which comprises the continuity equations for the electron and ion plasma components and the equation of electrostatics with allowance for initial and boundary conditions. The distribution of plasma parameters in the shock wave was obtained; the effect of its intensity was evaluated. The simulation results were compared with experimental data.     

On the relaxation of a medium after excitation with single fast heavy ions

A plasma model of relaxation of a medium in heavy-ion tracks in condensed matter has been proposed. The model is based on the solution of time-dependent equations of radiative-collisional kinetics. The state of the medium, which is described in the framework of the classical model of multiple ionization of target atoms by a field of fast multiply charged ions, is used as the initial condition. The relaxation in the plasma is investigated using molecular dynamics simulation. An analysis of the results of the calculations performed has made it possible to determine the range of material parameters at which the plasma model actually changes over to the atomic model and to establish the conditions where the atomic model is a very rough approximation. It is demonstrated that the plasma model adequately describes the X-ray spectra recorded upon interaction of ion beams with condensed targets. An X-ray spectral method based on the plasma model is proposed for diagnosing the plasma in fast-ion tracks. The results obtained can be useful in examining the initial stage of defect formation in solids under irradiation with single fast heavy ions.     

Numerical study of the expansion of metallic vapor plasma by a nanosecond laser pulse

The interaction between a laser-produced aluminum plasma and the ambient air, at a pressure of 173.3 Pa, is studied at the plasma thermalization stage. A two-dimensional approach is developed to solve the Navier–Stokes equations, where a finite volume discretization allows for obtaining a numerical solution. The simulation runs over a time representing 10 μs of plasma expansion. It is shown that the shock and drag models are good approximations for the two successive regimes after the initial strong expansion stage, and the calculation makes evident the plume sharpening on the axial direction before its confinement by the ambient gas, which is in good agreement with the experimental observation.     

Equilibrium-based iterative tomography technique for soft X-ray instellarators

A method to deduce local values of plasma emissivity from chord measurements has been developed and tested using data from a 30 detector soft X-ray array in the Wendelstein 7A stellarator. This technique, based on the calculated distribution of toroidal magnetic flux as the initial guess for the plasma emissivity, uses an iterative scheme to algebraically determine the distribution of the detector signals, and solves some drawbacks of the currently available tomography techniques. This method is especially suited for stellarator devices in which plasma cross sections are markedly noncircular and change as a function of the toroidal angle     

Nonlinear theory of ionic sound waves in a hot quantum-degenerate electron-positron-ion plasma

A collisionless nonmagnetized e-p-i plasma consisting of quantum-degenerate gases of ions, electrons, and positrons at nonzero temperatures is considered. The dispersion equation for isothermal ionic sound waves is derived and analyzed, and an exact expression is obtained for the linear velocity of ionic sound. Analysis of the dispersion equation has made it possible to determine the ranges of parameters in which nonlinear solutions in the form of solitons should be sought. A nonlinear theory of isothermal ionic sound waves is developed and used for obtaining and analyzing the exact solution to the system of initial equations. Analysis has been carried out by the method of the Bernoulli pseudopotential. The ranges of phase velocities of periodic ionic sound waves and soliton velocities are determined. It is shown that in the plasma under investigation, these ranges do not overlap and that the soliton velocity cannot be lower than the linear velocity of ionic sound. The profiles of physical quantities in a periodic wave and in a soliton are constructed, as well as the dependences of the velocity of sound and the critical velocity on the ionic concentration in the plasma. It is shown that these velocities increase with the ion concentration.     

Baryon deceleration and partonic plasma creation by strong chromofields in ultrarelativistic heavy-ion collisions

Strong chromofields generated at early stages of ultrarelativistic nuclear collisions may explain not only creation of the quark-gluon plasma but also collective deceleration of net baryons. This is demonstrated by solving classical equations of motion for baryonic slabs under the action of a time-dependent chromofield. We have studied sensitivity of the slab trajectories and their final rapidities to the initial strength and decay time of the chromofield, as well as to the back reaction of the produced plasma. By proper choice of the initial chromofield energy density we can reproduce significant baryon stopping, an average rapidity loss of about two units, observed for Au + Au collisions at RHIC. Using a Bjorken-like hydrodynamical model with the particle production source, we also study the evolution of partonic plasma produced as the result of the chromofield decay. Due to the delayed formation and expansion of plasma its maximum energy density is significantly lower than the initial energy density of the chromofield. It is shown that the fluctuations of the chromofield due to the stochastic distribution of color charges help to populate the midrapidity region in the net-baryon distribution. To fit the midrapidity data we need the chromofields with initial energy densities in the range of 30 to 60 GeV/fm³. Predictions of baryon stopping for Pb + Pb collisions at LHC energies are made.     

Measurements of vacuum field emission from bio-molecular andsemiconductor-metal eutectic composite microstructures

Designs, calculations, and initial data are presented from a program investigating the use of biomolecular and eutectic composite microstructures as vacuum field-emission cathodes. Calculations, supported by the initial data, indicate that these electron sources should be capable of macroscopic beam current densities, J⩾100 A/cm², without the formation of a surface plasma. The absence of a plasma allows these cathodes to operate as a long-pulse to DC electron sources. The designs avoid the formation of surface plasma by forcing the current emitted from the tips to transit a thin layer of silicon. The natural high-field current-limiting nature of the silicon serves to prevent the ablation of tip structures and the subsequent formation of cathode plasma. Initial data obtained with these microstructures has demonstrated Fowler-Nordheim characteristic emission. Coated bio-molecular composite cathodes have demonstrated stable DC current densities of ~100 mA/cm², and the eutectic composite cathodes have demonstrated similarly stable DC current densities of ~1 A/cm²     

脉冲束流引出时的等离子体鞘层变化

采用一维无碰撞的动力学鞘层模型计算了脉冲等离子体在恒压引出时的等离子体鞘层厚度变化,分别对短脉冲和长脉冲放电时的离子源发射面演变进行了分析。结果表明：对于短脉冲放电,发射面位置的变化相对等离子体密度的变化存在一定时间的延迟;对于长脉冲的上升沿和直流放电的开启阶段,鞘层厚度变化的速度与离子初始速度相关,稳定后发射面的位置与离子初始速度和等离子体密度的乘积相关。     

脉冲束流引出时的等离子体鞘层变化

采用一维无碰撞的动力学鞘层模型计算了脉冲等离子体在恒压引出时的等离子体鞘层厚度变化,分别对短脉冲和长脉冲放电时的离子源发射面演变进行了分析。结果表明:对于短脉冲放电,发射面位置的变化相对等离子体密度的变化存在一定时间的延迟;对于长脉冲的上升沿和直流放电的开启阶段,鞘层厚度变化的速度与离子初始速度相关,稳定后发射面的位置与离子初始速度和等离子体密度的乘积相关。     

Electron-scale electrostatic solitary waves and shocks: the role of superthermal electrons

The propagation of electron-acoustic solitary waves and shock structures is investigated in a plasma characterized by a superthermal electron population. A three-component plasma model configuration is employed, consisting of inertial (“cold”) electrons, inertialess κ (kappa) distributed superthermal (“hot”) electrons and stationary ions. A multiscale method is employed, leading to a Korteweg-de Vries (KdV) equation for the electrostatic potential (in the absence of dissipation). Taking into account dissipation, a hybrid Korteweg-de Vries-Burgers (KdVB) equation is derived. Exact negative-potential pulse- and kink-shaped solutions (shocks) are obtained. The relative strength among dispersion, nonlinearity and damping coefficients is discussed. Excitations formed in superthermal plasma (finite κ) are narrower and steeper, compared to the Maxwellian case (infinite κ). A series of numerical simulations confirms that energy initially stored in a solitary pulse which propagates in a stable manner for large κ (Maxwellian plasma) may break down to smaller structures or/and to random oscillations, when it encounters a small-κ (nonthermal) region. On the other hand, shock structures used as initial conditions for numerical simulations were shown to be robust, essentially responding to changed in the environment by a simple profile change (in width).