Initial Plasma Parameters Research Articles

THz generation and spatiotemporal variation of two cross‐focused Gaussian laser pulses in plasma

AbstractThe impact of spatial and temporal evolution of nonlinear mixing of two Gaussian laser pulses propagating in plasma on generation of terahertz (THz) radiation have been investigated taking into account the ponderomotive nonlinearity. By calculating the modified electron density of plasma caused by the finite ponderomotive force of the pump lasers and using wave equation and paraxial ray approximation, two coupled governing equations for temporal and spatial pulse‐width parameters have been derived. The electric field of the THz wave as a result of nonlinear current density induced by the beat ponderomotive force of the pulses was extracted. Combined effects of initial laser and plasma parameters on the behavior of self‐compression and self‐focusing as well as THz radiation generation were investigated. The numerical results indicated a considerable spatiotemporal compression takes place within a specific range of laser intensity, exhibiting a saturation intensity point where the compression process reaches its maximum extent. It is observed that the generated THz radiation also strongly depends on the spatiotemporal dynamics of the pump pulses. The maximum THz amplitude corresponds to the strongest pump pulse compression extent.

Assessment of the electron-proton energy relaxation rates extracted from molecular dynamics simulations in weakly-coupled hydrogen plasmas.

Electron-proton energy relaxation rates are assessed using molecular dynamics (MD) simulations in weakly-coupled hydrogen plasmas. To this end, we use various approaches to extract the energy relaxation rate from MD-simulated temperatures, and we find that existing extracting approaches may yield results with a sizable discrepancy larger than the variance between analytical models, which is further verified by well-known case studies. Present results show that two of the extracting approaches can produce identical results, which is attributed to a proper treatment of relaxation evolution. To discriminate the use of various methods, an empirical criterion with respect to initial plasma temperatures is proposed, which can self-consistently explain the cases considered. In addition, for a transient electron-proton plasma, we show that it is possible to extrapolate the Coulomb logarithm from that derived by initial plasma parameters in a single MD calculation, which is reasonably consistent with previous MD data. Our results are helpful to obtain accurate MD-based energy relaxation rates.

Best impedance matching seeking of single-frequency capacitively coupled plasmas by numerical simulations

Impedance matching can maximize the absorbed power transferred to the plasma load and minimize the reflected power, making it critical and indispensable for capacitively coupled plasmas (CCPs). The external circuit usually interacts with the plasma nonlinearly, so the global simulation of the external circuit and plasma and the matching design is very challenging. In this work, an a priori model was proposed to match the plasma impedance and the external circuit impedance for single-frequency CCPs. By calculating the plasma impedance and the matching network, the matching parameters were iteratively updated to find the best matching parameters. By adjusting the capacitance and the inductance of the circuit by numerical simulations, the reflection coefficient can be significantly reduced. At the same time, the plasma power absorption efficiency will be significantly improved. The universality of the method was demonstrated by choosing different initial circuit, discharge, and plasma parameters. The proposed method provides an effective matching design reference for CCPs.

A magnetic reconnection model for hot explosions in the cool atmosphere of the Sun

Context. Ultraviolet (UV) bursts and Ellerman bombs (EBs) are transient brightenings observed in the low solar atmospheres of emerging flux regions. Magnetic reconnection is believed to be the main mechanism leading to formation of the two activities, which are usually formed far apart from each other. However, observations also led to the discovery of co-spatial and co-temporal EBs and UV bursts, and their formation mechanisms are still not clear. The multi-thermal components in these events, which span a large temperature range, challenge our understanding of magnetic reconnection and heating mechanisms in the partially ionized lower solar atmosphere. Aims. We studied magnetic reconnection between the emerging magnetic flux and back ground magnetic fields in the partially ionized and highly stratificated low solar atmosphere. We aim to explain the multi-thermal characteristics of UV bursts, and to find out whether EBs and UV bursts can be generated in the same reconnection process and how they are related with each other. We also aim to unearth the important small-scale physics in these events. Methods. We used the single-fluid magnetohydrodynamic (MHD) code NIRVANA to perform simulations. The background magnetic fields and emerging fields at the solar surface are reasonably strong. The initial plasma parameters are based on the C7 atmosphere model. We simulated cases with different resolutions, and included the effects of ambipolar diffusion, radiative cooling, and heat conduction. We analyzed the current density, plasma density, temperature, and velocity distributions in the main current sheet region, and synthesized the Si IV emission spectrum. Results. After the current sheet with dense photosphere plasma emerges and reaches 0.5 Mm above the solar surface, plasmoid instability appears. The plasmoids collide and coalesce with each other, which causes the plasmas with different densities and temperatures to be mixed up in the turbulent reconnection region. Therefore, the hot plasmas corresponding to the UV emissions and colder plasmas corresponding to the emissions from other wavelengths can move together and occur at about the same height. In the meantime, the hot turbulent structures concentrate above 0.4 Mm, whereas the cool plasmas extend to much lower heights to the bottom of the current sheet. These phenomena are consistent with published observations in which UV bursts have a tendency to be located at greater heights close to corresponding EBs and all the EBs have partial overlap with corresponding UV bursts in space. The synthesized Si IV line profiles are similar to that observed in UV bursts; the enhanced wing of the line profiles can extend to about 100 km s−1. The differences are significant among the numerical results with different resolutions, indicating that the realistic magnetic diffusivity is crucial to revealing the fine structures and realistic plasmas heating in these reconnection events. Our results also show that the reconnection heating contributed by ambipolar diffusion in the low chromosphere around the temperature minimum region is not efficient.

Dense tunable attosecond electron bunch from laser interaction with magnetized plasma

The generation of an isolated relativistic attosecond electron bunch (RAEB) from the interaction of a laser with magnetized subcritical-density plasma is proposed. Particle-in-cell simulations show that a relativistic-intensity right-hand circularly polarized laser pulse can be much shortened and amplified if the magnetic field is along the laser propagation direction and the electron cyclotron frequency is appropriately larger than the laser frequency. In this case the plasma electrons are trapped in and propagate with the laser pulse like a solitary wave. The resulting charge separation field eventually causes the pulse to split into a forward propagating and a backward propagating pulse. Thus, the electrons in the latter are strongly pulled backwards by the ponderomotive force of the tail of the original laser pulse as well as the charge separation force, and eventually acquire the form of a RAEB of several attoseconds duration with a density of more than 5 times the critical density of the pristine plasma. The properties of the RAEB can be controlled by tailoring the initial laser and plasma parameters.

Direct and Inverse Problems of Investigating the Process of Self-Focusing of X-Ray Pulses in Plasma

Methods for solving direct and inverse problems for investigating the process of self-focusing of plane X-ray pulses in plasma are proposed and described. The mathematical model takes into account the dynamics of the electron plasma component in quasi-hydrodynamic approximation; this model is a nonlinear system of four second-order partial differential equations subject to corresponding initial and boundary value conditions. To solve the direct problem, a second-order conservative difference scheme is constructed and an iteration-free algorithm for the computations using this scheme is developed. For solving the inverse problem of determining the initial plasma and pulse parameters given the measured (or desired) characteristics of the X-ray pulse after its self-focusing, it is proposed to use the method of equivalence set designed for solving multiobjective problems in a pseudo-metric space of criteria. An algorithm for applying this method for solving the problem of interest is described.

Design features and commissioning of pulse arc plasma source and power supply system for the VEST neutral beam injection system

A short-pulse, high-power filament arc ion source is developed for the neutral beam injection (NBI) system of the Versatile Experiment Spherical Torus (VEST) device. The 0.6-MW NBI system is designed to deliver positive hydrogen ion beams at 20 keV for a pulse length >10 ms. In addition, the power supply system for the VEST NBI system is designed to change the beam injection energy during the pulse under VEST's initial target plasma parameters to increase beam–plasma coupling efficiency. To satisfy these requirements, a large-area, high-power arc plasma source with a pulse power system based on a Marx generator utilizing high-energy capacitors and a solid-state switching system is developed and successfully commissioned. In this paper, design features of the plasma source and the power supply system for the VEST NBI system is described and the arc plasma discharge characteristics are presented.

Influence of velocity fluctuations on the Kelvin‐Helmholtz instability and its associated mass transport

AbstractKelvin‐Helmholtz instability (KHI) and associated magnetic reconnection and diffusion processes provide plasma transport from solar wind into the magnetosphere. The efficiency of this transport depends on the magnetosheath and magnetospheric plasma and field properties at the vicinity of the magnetopause. Our recent statistical study using data from the Time History of Events and Macroscale Interactions during Substorms spacecraft indicates that the amplitude of the magnetosheath velocity fluctuations perpendicular to the magnetopause can be substantial. We have performed a series of local macroscale 2.5‐dimensional magnetohydrodynamic simulations of the KHI during strongly northward interplanetary magnetic field and with the initial plasma parameters typical to the dayside magnetopause by perturbing the initial equilibrium with time‐dependent perpendicular velocity field fluctuations. The effect of the single‐mode and multimode seed spectrums at different frequencies and amplitudes is studied. The plasma transport in Kelvin‐Helmholtz vortices is quantified. The results show that when large‐amplitude, low‐frequency seed velocity fluctuations exist in the magnetosheath, the resulting KH waves grow faster, get larger in size, and can transport more plasma through magnetic boundary, resulting in diffusion coefficient of the order 109 m2/s. The relevance of these findings to the solar wind‐magnetosphere coupling is discussed.

The Effect of Ar/O2 and H2O Plasma Treatment of SnO2 Nanoparticles and Nanowires on Carbon Monoxide and Benzene Detection.

As the final piece of a broader study on structure-property performance of SnO2 sensors, this study examines the performance of sensors created from tin(IV) oxide (SnO2) nanowires and nanoparticles as a function of temperature for untreated (UT) devices as well as those treated using Ar/O2 and H2O plasmas. Nanoparticle and nanowire sensors were exposed to air, carbon monoxide (CO), or benzene (C6H6) to determine sensor response (Rair/Rgas) and sensitivity (Rair/Rgas > 1 or Rgas/Rair > 1). Although both Ar/O2 and H2O plasma modification minimally increase sensor sensitivity toward CO and C6H6 under most conditions, this study explores initial plasma parameters of a wide array of plasma precursors to better understand the materials properties and gas-phase species that lead to specific sensing capabilities. In particular, certain Ar/O2 and H2O plasma treatment conditions resulted in increased sensitivity over UT nanomaterials at 25 and 50 °C, but of greatest importance is the knowledge gained from the combined materials, gas-phase, and sensor performance analysis that provide greater insight for effectively selecting future materials and modification systems to achieve optimal gas sensor performance.

Development of pulse power supply system based on Marx generator for the arc ion source of the VEST NBI

A short-pulse and high power supply system is developed for the pulse arc ion source of the neutral beam injector (NBI) of the Versatile Experiment Spherical Torus (VEST). The NBI system, with the maximum ion beam power of 20keV, 0.8MW and pulse length of 10ms, was designed for the auxiliary plasma heating and current drive source. However, the beam injection energy needs to be lower than 20keV under the initial target plasma parameters of VEST due to the beam-plasma coupling efficiency. For this reason, modulation of the beam energy in pulse duration is important to increase efficiency of the VEST NBI system. To satisfy these requirements, a pulse power system based on Marx generator is developed by utilizing high-energy capacitors and a solid-state switching system for the ion beam extraction and arc power supply. In addition, a battery-based power supply system is developed for the filament heating power supply which is highly cost-effective. In this paper, circuit structure and features of the power supply system of the ion source for the VEST NBI is described and the results of the discharges test are presented.

Spatial confinement in laser-induced breakdown spectroscopy

The spatial confinement of plasma produced by a nanosecond laser is investigated using time resolved spectroscopy, fast imaging, interferometry, and numerical computation. The dynamics of the plasma, depending on shock waves, laser power, and wall distances, are studied. Experimental results confirm that the plasma is constricted by the reflected shock associated with a temperature and density gradient. The peak laser power determines the initial plasma parameters which affect the spectral intensities and the velocity of the reflective shock waves. The wall distance determines the reflection time of the shocks, which in turn influences the time delay of the collision between the two reflective shocks. The numerical results reveal a fast propagation process surrounding the reflective shocks, which indicates that the velocity of the reflective shock wave is influenced by the density of the plasma. The maximum enhancement factor ~5.2 is realized at a delay time of 11.7 µs under a pulse laser energy of 180 mJ and a wall distance of 9 mm.

Interaction of a self-focused laser beam with a DT fusion target in a plasma-loaded cone-guided ICF scheme

In this paper, the interaction of a self-focused laser beam with a DT fusion target in a plasma-loaded cone-guided ICF scheme has been presented. We propose here to merge a plasma-loaded cone with the precompressed DT target in order to strongly focus the incident laser beam on the core to improve the fusion gain. The WKB approximation is used to derive a differential equation that governs the evolution of beamwidth of the incident laser beam with the distance of propagation in the plasma medium. The effects of initial plasma and laser parameters, such as initial plasma electron temperature, initial radius of the laser beam, initial laser beam intensity and plasma density, on self-focusing and defocusing of the Gaussian laser beam have been studied. Numerical results indicate that with increasing the plasma frequency (or plasma density) in the cone, the laser beam will be self-focused noticeably, while for a thinner laser beam (with small radius), it will diverge as propagate in the cone. By evaluating the energy deposition of the relativistic electron ignitors in the fuel, the importance of electron transportation in the cone-attached shell was demonstrated. Moreover, by lessening the least energy needed for ignition, the electrons coupling with the pellet enhances. Therefore, it increases the fusion efficiency. In this scheme, with employing a plasma-loaded cone, the fusion process improves without needing an ultrahigh-intensity laser beam in a conventional ICF.

The role of recombination in formation of charge composition of ions in a laser plasma upon mass spectrometric analysis of solids

A numerical experiment is used to study expansion of a plasma cloud with complex composition into a vacuum. Under the assumption that the density and temperature of the laser plasma are homogeneous throughout the cloud, the system of differential equations describing the plasma cloud expansion with allowance for inelastic processes is reduced to a system of ordinary differential equations for ions of five elements (Fe, Mn, Cr, Ni, Mo) with the charges z = 1, 2, 3, 4. The velocity of the plasma cloud boundary and its temperature as functions of the expansion time are determined. It is demonstrated that, after recombination stops, thermodynamically nonequilibrium plasma is created as a result of “quenching,” and in this plasma the residual degree of ionization and ion charge distribution depend on the initial plasma parameters T0, N0, and R0. In the course of recombination, the dependence of ion charge distribution on ionization potential weakens.

Generation of attosecond electron bunches in a laser-plasma accelerator using a plasma density upramp

Attosecond electron bunches and attosecond radiation pulses enable the study of ultrafast dynamics of matter in an unprecedented regime. In this paper, the suitability for the experimental realization of a novel scheme producing sub-femtosecond duration electron bunches from laser-wakefield acceleration in plasma with self-injection in a plasma upramp profile has been investigated. While it has previously been predicted that this requires laser power above a few hundred terawatts typically, here we show that the scheme can be extended with reduced driving laser powers down to tens of terawatts, generating accelerated electron pulses with minimum length of around 166attoseconds and picocoulombs charge. Using particle-in-cell simulations and theoretical models, the evolution of the accelerated electron bunch within the plasma as well as simple scalings of the bunch properties with initial laser and plasma parameters are presented.

The ignition design space of magnetized target fusion

The simple magnetized target implosion model of Lindemuth and Kirkpatrick [Nucl. Fusion 23, 263 (1983)] has been extended to survey the potential parameter space in which three types of magnetized targets—cylindrical with axial magnetic field, cylindrical with azimuthal magnetic field, and spherical with azimuthal magnetic field—might achieve ignition and produce large gain at achievable radial convergence ratios. The model has been used to compute the dynamic, time-dependent behavior of many initial parameter sets that have been based upon projected ignition conditions using the quasi-adiabatic and quasi-flux-conserving properties of magnetized target implosions. The time-dependent calculations have shown that energy gains greater than 30 can potentially be achieved for each type of target. By example, it is shown that high gain may be obtained at extremely low convergence ratios, e.g., less than 15, for appropriate initial conditions. It is also shown that reaching the ignition condition, i.e., when fusion deposition rates equal total loss rates, does not necessarily lead to high gain and high fuel burn-up. At the lower densities whereby fusion temperatures can be reached in magnetized targets, the fusion burn rate may be only comparable with the hydrodynamic heating/cooling rates. On the other hand, when the fusion burn rates significantly exceed the hydrodynamic rates, the calculations show a characteristic rapid increase in temperature due to alpha particle deposition with a subsequent increased burn rate and high gain. A major result of this paper is that each type of target operates in a different initial density-energy-velocity range. The results of this paper provide initial target plasma parameters and driver parameters that can be used to guide plasma formation and driver development for magnetized targets. The results indicate that plasmas for spherical, cylindrical with azimuthal field, and cylindrical with axial field targets must have an initial density greater than approximately 1017/cm3, 1018/cm3, and 1020/cm3, respectively, implying constraints on target plasma formation research.

Linear vs. nonlinear acceleration in plasma turbulence. I. Global versus local measures

Magnetized turbulent plasmas are generally characterized as strongly or weakly turbulent based on the average relative strengths of the linear and nonlinear terms. While this description is useful, it does not represent the full picture and can be misleading. We study the variation of linear and nonlinear accelerations in the Fourier space of a magnetohydrodynamic system with a mean magnetic field and broad selection of initial states and plasma parameters. We show that the local picture can show significant departures from what is expected from the general global picture. We find that high cross helicity systems that are traditionally believed to have relatively weaker nonlinearities, compared to low cross helicity systems, can show strong nonlinearities in parts of the Fourier space that are orthogonal to the mean magnetic field direction. In some cases, these nonlinearities can exceed in strength the level of nonlinearities recovered from low cross helicity systems.

Measurements of the electron density and degree of plasma ionization in a plasma target based on a linear electric discharge in hydrogen

Laser interferometry methods were used to measure the density of free electrons and degree of plasma ionization in a hydrogen target intended for experiments on determining energy losses of heavy ion beams in an ionized matter. It is shown that the linear electron density can be varied in the range from 3.3 × 1017 to 1.3 × 1018 cm−2 by varying the initial plasma parameters (the hydrogen pressure in the target and the discharge current). The error in measuring the linear electron density in the entire range of the varied plasma parameters was less than 1%. The maximum degree of plasma ionization achieved at the initial gas pressure of 1 mbar was 0.62 ± 0.05.

Evolution of MHD Instabilities in Plasma Imploding Under Magnetic Field

Two-dimensional 3-ns-gated visible images, recorded at different times during the implosion of plasma under azimuthal magnetic field ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z$</tex></formula> -pinch), revealed ringlike instabilities followed by the development of axially and azimuthally nonuniform structures in the imploding plasma. Remarkably, the evolution in time of all structures was found to be highly repeatable in different shots, which should allow, through 3-D magnetohydrodynamics modeling, for systematically studying the development in time of these complex phenomena and correlating them with the initial plasma parameters. The data are also used to infer the time-dependent outer plasma radius and plasma radial velocities.

Wakefield effects and solitary waves of an intense short laser pulse propagation in a plasma channel

In the presence of relativistic and channel-coupling nonlinearity and wakefield effects, the propagation characteristics and solitary waves of an intense short laser pulse in a preformed plasma channel are investigated. The evolution equation of the laser spot size is derived by using variational technique, the initial laser and plasma parameters for propagation with constant spot size, periodic defocusing and focusing oscillations, and solitary waves are identified. For illustration, some numerical results are also presented. It is found that the laser focusing is enhanced by the wakefield effects that result in a significant reduced focusing power.

Conversion of plasma energy into electrical pulse by magnetic flux compression

A conceptual study of magnetic flux compression inside a cylindrical coil by an expanding inertial fusion plasma sphere across the magnetic field produced by the coil itself have been performed numerically using a two-dimensional magnetohydrodynamic (MHD) simulations. The concept may find application in inertial fusion energy (IFE) system as a direct energy conversion scheme to convert a part of fusion plasma kinetic energy into pulsed electrical energy. Important theoretical and technical issues that has to be addressed are discussed. Preliminary theoretical analysis are given for the analysis of MHD interchange instabilities of expanding plasma across magnetic field. Overall efficiency of the system is determined numerically for a typical set of initial plasma and system parameters. Ultrahigh coil inter-turn voltages are predicted. Therefore, the application of magnetic self-insulation to avoid coil inter-turn break-down is considered. Also, we have analysed the system performance with different load conditions.