Steady state ion acceleration by a circularly polarized laser pulse

The steady state ion acceleration at the front of a cold solid target by a circularly polarized flat-top laser pulse is studied with one-dimensional particle-in-cell (PIC) simulation. A model that ions are reflected by a steady laser-driven piston is used by comparing with the electrostatic shock acceleration. A stable profile with a double-flat-top structure in phase space forms after ions enter the undisturbed region of the target with a constant velocity.     

Particle dynamics during adiabatic expansion of a plasma bunch

The renormalization-group approach is used to obtain an exact solution to the self-consistent Vlasov kinetic equations for plasma particles in the quasi-neutral approximation. This solution describes the one-dimensional adiabatic expansion of a plasma bunch into a vacuum for arbitrary initial particle velocity distributions. Ion acceleration is studied for two-temperature Maxwellian and super-Gaussian initial electron distributions, which predetermine distinctly different ion spectra. The solution found is used to describe the acceleration of ions of two types. The relative acceleration efficiency of light and heavy ions as a function of atomic weights and number densities is analyzed. The solutions obtained are of practical importance in describing ion acceleration during the interaction of an ultrashort laser pulse with nanoplasma, for example, cluster plasma or plasma produced when thin foils are irradiated by a laser.     

Ion heating with beating electrostatic waves

The nonlinear interaction of a magnetized ion with two beating electrostatic waves (BEW) whose frequencies differ by a cyclotron harmonic can lead, under some conditions [Phys. Rev. E 69, 046402 (2004)], to vigorous acceleration for an ion with arbitrarily low initial velocity. When applied to an ensemble of ions, this mechanism promises enhanced heating over single electrostatic wave (SEW) heating for comparable wave energy densities. The extension of single ion acceleration to heating (SEWH and BEWH) of an ensemble of initially thermalized ions was carried out to compare the processes. Using a numerical solution of the Vlasov equation as a guideline, an analytical expression for the heating level was derived with Lie transforms and was used to show BEWH's superiority over all parameter space.     

Ion acceleration in a solitary wave by an intense picosecond laser pulse

Acceleration of ions in a solitary wave produced by shock-wave decay in a plasma slab irradiated by an intense picosecond laser pulse is studied via particle-in-cell simulation. Instead of exponential distribution as in known mechanisms of ion acceleration from the target surface, these ions accelerated forwardly form a bunch with relatively low energy spread. The bunch is shown to be a solitary wave moving over expanding plasma; its velocity can exceed the maximal velocity of ions accelerated forward from the rear side of the target.     

A peculiar Maxwell’s Demon observed in a time-dependent stadium-like billiard

The dynamics of a driven stadium-like billiard is considered using the formalism of discrete mappings. The model presents a resonant velocity that depends on the rotation number around fixed points and external boundary perturbation which plays an important separation rule in the model. We show that particles exhibiting Fermi acceleration (initial velocity is above the resonant one) are scaling invariant with respect to the initial velocity and external perturbation. However, initial velocities below the resonant one lead the particles to decelerate therefore unlimited energy growth is not observed. This phenomenon may be interpreted as a specific Maxwell’s Demon which may separate fast and slow billiard particles.     

Generating monoenergetic heavy-ion bunches with laser-induced electrostatic shocks

A method for efficient laser acceleration of heavy ions by electrostatic shock is investigated using particle-in-cell (PIC) simulation and analytical modeling. When a small number of heavy ions are mixed with light ions, the heavy ions can be accelerated to the same velocity as the light ions so that they gain much higher energy because of their large mass. Accordingly, a sandwich target design with a thin compound ion layer between two light-ion layers and a micro-structured target design are proposed for obtaining monoenergetic heavy-ion beams.     

Effects of density profile and multi-species target on laser-heated thermal-pressure-driven shock wave acceleration

The shock wave acceleration of ions driven by laser-heated thermal pressure is studied through one-dimensional particle-in-cell simulation and analysis. The generation of high-energy mono-energetic protons in recent experiments （D. Haberberger et al., 2012 Nat. Phys. 8 95） is attributed to the use of exponentially decaying density profile of the plasma target. It does not only keep the shock velocity stable but also suppresses the normal target normal sheath acceleration. The effects of target composition are also examined, where a similar collective velocity of all ion species is demonstrated. The results also give some reference to future experiments of producing energetic heavy ions.     

Residence time of metal ions generated from microsecond vacuum arcs

Vacuum gaps of 1 mm with lead or copper cathode are fired by a 13 μs duration sinusoidal arc or a 10 μs duration exponentially-decaying arc, and time-of-flight (TOF) ion measurements are made at variable times after the arc ignition. At the lead cathode, Pb⁺ and Pb⁺⁺ ions are generated and the upper limit on the times for Pb⁺ ion detection are 48 μs and 46 μs from the arc ignition for the sinusoidal and exponential arcs, respectively. At the copper cathode, Cu⁺, Cu⁺⁺, and Cu⁺⁺⁺ ions are generated and detected within 15 μs and 13 μs from the arc ignition for the sinusoidal and exponential arcs, respectively. The residence time of the Pb⁺ ions in the ion acceleration region is approximately 35 μs, regardless of the waveform of the arc current. The residence time of the copper ions, described by the time constant of the time-of-flight ion current delay characteristics, is 3 μs     

Analysis of the formation of ion beams in varying electric fields of a stationary plasma thruster

We propose a technique for analyzing the distribution function of the velocity components (radial V _r and azimuthal V _φ) of ions in a beam. This technique is used for studying the ion beam emerging from a stationary plasma thruster (SPT). It is shown that the beam contains ions with a radial velocity component in the range V _r /V _z = ? 1.2 to +0.74, as well as ions with the azimuthal velocity component in the range V _?/V _z = ±0.9. Numerical calculations lead to the conclusion that ions acquire the azimuthal velocity component in the field of the azimuthal wave of the plasma potential evolving in the SPT channel.     

Generation of superthermal plasma flows in current sheets

The profiles of the He II 3203 Å and He II 4686 Å spectral lines of helium ions have been detected and analyzed. Using these data, the processes of acceleration and heating of a plasma in current sheets that are formed in magnetic configurations with X-type singular lines have been analyzed. The generation of plasma flows with energies of 400–1300 eV, which are much higher than the thermal energy of ions, has been revealed. The acceleration of the plasma induced by Ampere forces is likely spatially inhomogeneous, which should lead to shear plasma flows in a current sheet.     

变悬点变摆长单摆运动分析

用两种方法导出了变悬点变摆长单摆的加速度和运动微分方程，分析了小球作周期振动的条件，讨论了角速度最大值的位置，并通过计算机数值计算，画出了小球作周期振动时的相图。     

Towards a predictive scenario of a burning accident in a mining passage

To reveal the inner mechanisms of a combustion accident in a coalmine, the key stages and characteristics of premixed flame front evolution such as the flame shapes, propagation speeds, acceleration rates, run-up distances and flame-generated velocity profiles are scrutinised. The theories of globally spherical, expanding flames and of finger-flame acceleration are combined into a general analytical formulation. Two-dimensional and cylindrical mining passages are studied, with noticeably stronger acceleration found in the cylindrical geometry. The entire acceleration scenario may promote the total burning rate by up to two orders of magnitude, to a near-sonic value. Starting with gaseous combustion, the analysis is subsequently extended to gaseous-dusty environments. Specifically, combustible dust (e.g. coal), inert dust (e.g. sand), and their combination are considered, and the influence of the size and concentration of the dust particles is quantified. In particular, small particles influence flame propagation more than large ones, and flame acceleration increases with the concentration of a combustible dust, until the concentration attains a certain limit.     

Efficient extraction of hydrogen ions from high-velocity plasma bunches by a pulsed electric field

The systematized results are presented of an investigation of the process of the efficient extraction of ions, by a pulsed electric field, from the surface of a plasma moving at a velocity of 10⁴ m/sec under the conditions of small accelerating diodes. A mathematical model is analyzed of the propagation of a plasma bunch formed by a laser pulse (E _las<1 J) in a quasicylindrical diode system. The possibilities are considered of increasing the current of the extracted ions by utilizing the phenomenon of magnetic confinement of electrons, and diode systems with a transparent anode. The designs of small acceleration tubes for generating neutron pulses are presented.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 65–74, April, 1996.     

Theoretical simulation of a rotary motor driven by surface acoustic waves

A simulation model for theoretically studying the operation behavior of a rotary motor driven by surface acoustic waves (SAWs) is proposed. According to the model, some simulation results are obtained as follows: (1) as the rotor is excited by the SAWs, the motor experiences two phases, i.e., the acceleration phase and the steady phase; (2) the normal vibration amplitude A of SAWs has a very weak effect on the acceleration in the first phase but an enhancing effect on the steady velocity of the rotary motor; (3) as the number of the contact points between the rotor and the stator increases, the motor rotates more steadily; (4) as the rotor radius becomes smaller, both the acceleration and the steady angular velocity become greater. These features are in agreement with the experimental results.     

Influence of the gas density on the motion of a free plasma piston in the railgun channel

The influence of the gas density on the acceleration of a plasma armature inside the railgun channel filled with various gases (xenon, air, or helium) under atmospheric pressure is investigated experimentally and theoretically. It is shown that, after the discharge current has reached a steady value, the velocity of the glowing plasma front ceases to grow and remains constant as long as so does the current. The length over which the velocity saturates is equal to a few centimeters, i.e., is much shorter than the railgun channel length. The maximum velocity of the plasma piston meets a predicted limit, which is determined by the drag of the medium and a decrease in the acceleration of the plasma armature when a fraction of the material evaporated from the rails is involved into motion. The plasma composition depends on the electrode material. The velocities measured when the channel is filled with helium (V = 17.5 km/s) or air (V = 9.8 km/s) noticeably exceed the sound speed inside the plasma piston (5–7 km/s).     

Numerical simulation on the operational characteristics of the pulsed inductive acceleration

To analyse the ionization and acceleration properties of an inductive plasma excited by a pulsed current flowing through the planar coil, the extended GLM formulations of the MHD (EGLM‐MHD) model, combined with the high‐temperature thermodynamic and transport model, is employed to simulate the characteristics of the flow. The two‐dimensional axisymmetric calculation captures the generation, growth, and acceleration of the current sheet, and the process is completed in the first half period. The sheet is mainly comprised of lower ionization level ions in the front and higher level ions at the back, and the density is one order higher than that of the residual plasma on the coil surface. As the abscissa value of the sheet is larger than the decoupling distance, a reversed flow emerges, generating a backward impulse, and the negative velocity can be more than 15 km/s at peak intensity B₀ = 0.5 T. In the second 1/4 period, the magnetic field and current density distribute non‐linearly on the surface and regularly in the sheet, caused by the reversing of the changing rate of the magnetic field and the particles' radial diffusion. The results at different intensities show that, for the same coil size, the time at which the maximum velocity V_max appears is advanced as the intensity increases, and V_max can be greater than 20 km/s above 0.5 T.     

Cylindrical cumulation of fast ions in a ring focus of a high-power subpicosecond laser

A new method of cylindrical cumulation of fast ions undergoing ponderomotive acceleration at the focus of a high-power subpicosecond laser is proposed. When a laser beam is focused in a preionized gas at a ring focus, radial acceleration of ions by the ponderomotive force occurs. The ions accelerated from the inner side of the ring form a cylindrical shock wave converging toward the axis. As the shock wave cumulates, the ion density increases rapidly and the ion-ion collision probability increases along with it. A numerical simulation for a ~100 TW subpicosecond laser pulse predicts the generation of up to 200 keV ions and up to 100-fold volume compression of the plasma in a cylinder ~1 μm in diameter. The lifetime of the dense plasma filament over the length of the laser caustic is several picoseconds. It is suggested that laser cumulation of ions be used for the production of a bright and compact subpicosecond source of fast neutrons, media for x-and γ-ray lasers, and multiply-charged ions and for the initiation of nuclear reactions. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 20–25 (10 January 1999)     

Particle Dynamics in an Electrohydrodynamic Flow Field investigated with a two-component laser-doppler velocimeter

A laser-Doppler instrument has been used to measure the migration velocity of NaCl particles in an electrohydrodynamic flow field of an electrical precipitator. The measured average migration velocity of 1.40-μm particles (number distribution median with a geometric standard devitation of 1.46) is approximately five to six times higher than the calculated steady-state velocity for a 1.40-μm particle, provided there is a saturation charge of at least 90f%. Further, the particle velocities in the main flow direction are also influenced by the electrical operation conditions. Both observations demonstrate the important role of the state of the electrohydrodynamic flow field (superposition of moving gas ions and neutral gas molecules) on the particle transport, characterized by the dimensionless electrohydrodynamic number N_EHD. A comparison between six different electrohydrodynamic states revealed that N_EHD ≈? 1 is a critical value for the mutual interactions between the gas ions and the neutral gas phase. Whereas for N_EHD values > 1 the stochastic particle motion is chiefly determined by the nonsteady-state character of the negative corona, for N_EHD values < 1 the particle velocity fluctuations are governed by the turbulence level of the neutral fluid. These finding might be helpful in adjusting the operating conditions in electrical precipitators for and optimized particle separation.     

Reformed Solitary Kinetic Alfven Waves due to Dissipations and Auroral Electron Acceleration

The physical nature of the auroral electron acceleration has been an outstanding problem in space physics for decades. Some recent observations from the auroral orbit satellites, FREJA and FAST, showed that large amplitude solitary kinetic Alfvén waves (SKAWs) are a common electromagnetic active phenomenon in the auroral magnetosphere. In a low-β (i.e., β/2<<m_e/m_i<<1) plasma, the drift velocity of electrons relative to ions within SKAWs is much larger than thermal velocities of both electrons and ions. This leads to instabilities and causes dissipations of SKAWs. In the present work, based on the analogy of classical particle motion in a potential well, it is shown that a shock-like structure can be formed from SKAWs if dissipation effects are included. The reformed SKAWs with a shock-like structure have a local density jump and a net field-aligned electric potential drop of order of m_ev_A²/e over a characteristic width of several λ_e. As a consequence, the reformed SKAWs can efficiently accelerate electrons field-aligned to the order of the local Alfvén velocity. In particular, we argue that this electron acceleration mechanism by reformed SKAWs can play an important role in the auroral electron acceleration problem. The result shows that not only the location of acceleration regions predicted by this model is well consistent with the observed auroral electron acceleration region of 1—2 R_E above the auroral ionosphere, but also the accelerated electrons from this region can obtain an energy of several keV and carry a field-aligned current of several μA/m² which are comparable to the observations of auroral electrons.     

激光驱动飞片加速特征分析

在激光驱动飞片研究中,飞片的加速特征是需要认识的关键问题之一。设计了强激光作用金属膜驱动飞片实验,采用聚偏氟乙烯（PVDF）压电薄膜测量了飞片到达不同距离的时间,计算得到飞片速度和加速度,分析激光能量对飞片加速性能的影响。基于Gurney能理论,建立了激光驱动飞片速度的计算模型,根据实验结果获得了激光能量损失系数和有效吸收系数,分析了激光能量和膜体厚度对飞片速度的影响。实验结果表明:不同激光能量下飞片的加速特征基本相似,激光能量变化对飞片的加速时间影响较小; 激光能量较大的情况下,膜体厚度对飞片最大速度、能量耦合系数的影响更显著; 当膜体超过一定厚度时,能量耦合系数不再增加。