Plasma Phenomena Research Articles

Gas-puff induced cold pulse propagation in ADITYA-U tokamak

Short bursts (∼1 ms) of gas, injecting ∼1017–1018 molecules of hydrogen and/or deuterium, lead to the observation of cold pulse propagation phenomenon in hydrogen plasmas of the ADITYA-U tokamak. After every injection, a sharp increase in the chord-averaged density is observed followed by an increase in the core electron temperature. Simultaneously, the electron density and temperature decrease at the edge. All these observations are characteristics of cold pulse propagation due to the pulsed gas application. The increase in the core temperature is observed to depend on the values of both the chord-averaged plasma density at the instant of gas-injection and the amount of gas injected below a threshold value. Increasing the amount of gas-puff leads to higher increments in the core-density and the core-temperature. Interestingly, the rates of rise of density and temperature remain the same. The gas-puff also leads to a fast decrease in the radially outward electric field together with a rapid increase in the loop-voltage suggesting a reduction in the ion-orbit loss and an increase in Ware-pinch. This may explain the sharp density rise, which remains mostly independent of the toroidal magnetic field and plasma current in the experiment. Application of a subsequent gas-puff before the effect of the previous gas-pulse dies down, leads to an increase in the overall electron density and consequently the energy confinement time.

New insight into the formation mechanism of the energetic particle reservoirs in the heliosphere

ABSTRACT The concept of energetic particle reservoirs, essentially based on the assumption of the presence of outer reflecting boundaries/magnetic mirrors or diffusion barriers (deterministic) rather than on the effect of particle diffusive propagation (stochastic) in magnetic turbulence, has been used for decades to describe the space-extended decay phases of energetic particle events within the fields of space physics, solar physics, and plasma physics. Using five-dimensional time-dependent Fokker–Planck transport equation simulations, in this work, we demonstrate that the so-called particle reservoirs are naturally explained and quantitatively reproduced by diffusion processes in turbulent magnetic fields, without invoking the hypothesis of reflecting boundaries. Our results strongly suggest that the so-called reservoir (based on deterministic structure) should be renamed ‘flood’ (based on stochastic diffusion), which symbolizes an authentic shift in thinking and in pragmatic rationale for the studies of energetic particles and relevant plasma phenomena in heliophysics and in astrophysics.

Small amplitude ion-acoustic solitons with regularized κ-distributed electrons

A theoretical investigation of ion-acoustic solitons in an unmagnetized plasma consisting of cold ions and regularized κ-distributed electrons has been carried out. The properties of stationary solitary structures are briefly studied by the reductive perturbation method in the small amplitude limit. It is found that in the small κ region, the ion-acoustic solitary waves propagate more slowly in the case of regularized κ distribution than in the case of standard κ distribution. The regularized κ-distributed electrons affect the width and amplitude of the solitary waves. As the cutoff parameter α increases, the amplitudes of both the compressive and rarefactive solitary waves decrease, and their widths also decrease. In addition, for a given value of κ, positive potential solitons will appear at large α, while negative potential solitons will appear at small α. The results of this paper may be useful for understanding nonlinear electrostatic phenomena in space plasmas.

Research on the Application of Astrophysics in Magnetohydrodynamics under Big Data

Based on the background of big data, the paper establishes the evolution equation of electromagnetic fluid state parameters over time called Magnetohydrodynamics (MHD). Compared with kinetic theory, magnetohydrodynamics is much simpler in mathematical processing, and there are many processes in plasma. For some other phenomena in plasma, such as Landau damping, instability in velocity space, etc., the theory of magnetohydrodynamics is unable to describe. The thesis starts from the dynamic equations and establishes the magnetohydrodynamic equations.

Influence of neutral depletion on blue core in argon helicon plasma

Blue core (BC) is a special spectral phenomenon in argon helicon plasma, with intense blue lights from ion emission around the radial center of discharge tube. In this paper, the characteristics of BC in argon (Ar) helicon plasma were investigated experimentally from aspects of discharge mode transitions, plasma spatial distributions, and spectral features. It is found that the BC generally appears at strong magnetic field (480 G or above in this work) with high plasma density in wave mode, accompanied by exponentially rising of ion line intensity. The electron density and temperature, the neutral density and temperature, and the line emission intensity show a radial profile with a central peak in BC mode. The steep gradient of ion line intensity (corresponding to the ion density) defines a clear boundary of the core. Further, a pressure balance model was developed to investigate the influence of neutral depletion on BC formation. The neutral density is depleted significantly from 7.24×1013 to 0.38 × 1013 cm−3 at magnetic field of 600 G in BC mode, while to 3.13 × 1013 cm−3 at magnetic field of 250 G in normal wave (NW) mode. The ionization rate in BC reaches as high as 70% compared with 9.6% of that in NW mode. The ionization rate and the ion line intensity show similar radial profiles, indicating the BC phenomenon is closely related to the distribution of peaked ion density and hollowed neutral density. Fundamentally, the central electron heating and strong magnetic field contribute to the centrally local high ionization rate and strong neutral heating. The severe neutral depletion with prominent central heating is considered to be the immediate cause of appearance of blue core.

Three-step model for third-harmonic generation in air by nanosecond lasers

Understanding the phenomena involved in harmonic generation in plasmas by high-power pulsed lasers is a paramount task for developing new techniques for generation of coherent radiation in ultrashort bursts. Although first experiments on harmonic generation involved nanosecond lasers and inspired further interest in the subject, numerical simulations on harmonic generation are currently mainly oriented toward ultrashort fs lasers. This paper presents a combined theoretical–experimental approach to the generation mechanisms and the properties of third-harmonic (TH) radiation generated by infrared nanosecond laser pulses in air-breakdown plasma. The paper indicates that, at the microscopic level, the generation of TH can be described by a three-step model, which involves breakdown of nitrogen molecules in the air. First, the nitrogen molecules undergo cascade-impact ionization; then, the ionized molecules are quasi-resonantly excited through three-photon absorption; in the third step, the nitrogen molecules de-excite to the fundamental level with associated emission of TH radiation. At the macroscopic level, the three-step model is implemented considering that the breakdown plasma is a conductive nonlinear medium whose third-order susceptibility and complex conductivity depend upon the cubic root of the driving laser intensity. The 2D numerical simulations performed in the frame of this model are in good agreement with the experimental data in terms of TH generation efficiency, collimation, and polarization of TH radiation, indicating the validity of the theoretical model presented here. The model enables realistic calculations with affordable computing power for prediction and control of the TH generation process driven by nanosecond laser pulses. The results are important from the fundamental and practical points of view, thus providing an efficient tool for prediction of nonlinear optical phenomena in laser-produced plasmas and for noncontact diagnosis of harmonic-generating plasmas.

A nanostructural approach to the interfacial phenomena in spark plasma sintered TiB2 ceramics with vanadium and graphite additives

A novel TiB2-based composite, with metallic vanadium and nano-graphite additives, was fabricated by spark plasma sintering. A comprehensive investigation was carried out to clarify the sintering behavior and interfacial phenomena. The in-situ synthesis of crystalline V3B4, V6C5, and TiC phases was proved. The formation of V6C5 was attributed to the ordering phase transformation promoted by carbon vacancies in the initially-synthesized VC. TiC was formed due to the reduction of TiO2 impurity with graphite additive. Simultaneous addition of vanadium and graphite not only resulted in the reduction of B2O3 impurity, but also the synthesis of V3B4 nano-crystals, which shows the critical role of oxide impurities on the final phase arrangement. The liquid state of B2O3 at high temperatures promoted the possibility of such reactions. The in-situ formation of such phases led to significant improvements in the mechanical properties. Compared to the monolithic TiB2 consolidated through similar conditions, 50% increase in fracture toughness and 27% enhancement in elastic modulus were observed.

Restructuring of rotating spokes in response to changes in the radial electric field and the neutral pressure of a cylindrical magnetron plasma

Nonlinear plasma structures in the partially magnetized E×B plasma of a cylindrical magnetron are investigated using 2D3V particle-in-cell Monte Carlo collision simulations. In the early phase of the discharge, plasma gradients and radial electric fields excite a lower hybrid type instability that forms long wavelength rotating density spokes. As the discharge grows in density by ionization and the cathode gets shielded by the formation of an ion sheath, radial electric fields diminish in the quasineutral region of the discharge. This induces a transition of the spokes into short-scale spoke-on-spoke modes. The short wavelength structures can be reversed back into a long spiral spoke by lowering the neutral pressure, which revives the radial electric fields via a turbulent plasma expansion. Plasma phenomena connected to the rotating spoke include the anomalous radial transport and loss of electrons through the spoke, azimuthal dragging of ions by the spoke’s field, plasma temperature modulations by the spoke structure, and formation of electron vortices around equipotential islands, in some cases with opposing rotations to the underlying E×B drift. Electron scattering from non-ionizing collisions with neutrals also has a minor influence on the instability.

A novel approach in heating phenomena of the drift plasmas in the presence of rotating magnetic field: Appearance of anti-Hermitian part in dielectric tensor

A novel approach in heating phenomena of the drift plasmas in the presence of rotating magnetic field: Appearance of anti-Hermitian part in dielectric tensor

Experimental Study of Plasma Plume Analysis of Long Pulse Laser Irradiates CFRP and GFRP Composite Materials

The application of laser fabrication of fiber-reinforced polymer (FRP) has an irreplaceable advantage. However, the effect of the plasma generated in laser fabrication on the damage process is rarely mentioned. In order to further study the law and mechanism of laser processing, the laser process was measured. CFRP and GFRP materials were damaged by a 1064 nm millisecond pulsed laser. Moreover, the propagation velocity and breakdown time of plasma plume were compared. The results show that GFRP is more vulnerable to breakdown than CFRP under the same conditions. In addition, the variation of plasma plume and material surface temperature with the number of pulses was also studied. The results show that the variation trend is correlated, that is, the singularities occur at the second pulse. Based on the analysis of experimental phenomena, this paper provides guidance for plasma phenomena in laser processing of composite materials.

Multichannel gas electron multiplier based soft x-ray field-programmable gate array measurement system for W-Environment in Steady-state Tokamak (WEST): Hardware, installation, and first plasma acquisition.

The work describes a novel approach to the design of a fast, multichannel measurement system for plasma diagnostics [A. Wojenski et al., Fusion Eng. Des. 123, 727 (2016)]. Its main scope is to provide measurements of soft X-ray (SXR) emission during plasma phenomena at the W-Environment in Steady-state Tokamak (WEST), especially for monitoring and tracing tungsten impurities. This paper describes the vertical Gas Electron Multiplier (GEM) camera installed at the WEST [M. Chernyshova et al., J. Instrument. 10, P10022 (2015)]. The designed GEM detector readout board has more than 100 channels, resulting in high-performance requirements for the data acquisition and processing system. The novel system construction approach is that the unit works on the raw signals providing a high quality of the data, especially in the scope of pileup effect analysis. In the case of doubtful results, the source data can be easily reviewed offline. The data selection and transmission are done in Field-Programmable Gate Arrays (FPGAs) on the custom boards with the custom Peripheral Component Interconnect (PCI)-Express Gen2 switch that allows us to register signals from multiple FPGAs and then process the data by complex algorithms [G. Kasprowicz et al., J. Fusion Energy 38, 480 (2019)]. The firmware is replaceable and different working modes can be applied (some under verification): global trigger mode, high-speed data serialization, and extended signal registration. Low level optimized central processing unit software for data readout was also designed [P. Linczuk et al., J. Instrum. 14, C05001 (2019)]. The installation of the system is described due to complex system components' distribution. The first results of the successful acquisition of the plasma at the WEST are discussed. The corresponding SXR energy and topology spectra were computed. Those are the first technical measurements of the system to ensure verification of data quality.

Some glimpses of the plasma processes involved in power spectra of radio pulsars

ABSTRACT Although more than a half-century since pulsar discovery, the field of pulsar astronomy has attained an unprecedented level of success and has opened several windows in astronomy, very few theories have been put forwarded to investigate the nature of power spectra and emission properties of radio pulsars. There has been a copious amount of observation dedicated to investigating the nature and shape of radio pulsar’s power spectra. In this paper, I have suggested some alternative plasma processes in the frame of general parametric instability, such as Stimulated Raman Scattering (SRS) and Stimulated Compton Scattering (SCS) as a potential probe to generate power spectra theoretically. There are basically three prominent processes in which electromagnetic waves can undergo scattering in an ambient plasma medium. First, one is very well-known Compton scattering; it is when the scattering of radiation occurs by a single electron. The second and third ones are SRS and SCS, which are the relatively less known plasma phenomena. By definition, SRS is a process where the scattering of radiation occurs by longitudinal electron plasma mode, whereas SCS occurs by highly damped electron plasma mode. I have explored the possibility of explaining the radio power spectra of pulsar under different circumstances of plasma. I have computed the growth rates of SRS and SCS instabilities numerically. Thereafter, I have produced full radio power spectra of the pulsars PSRB 2111+46, PSRB 0329+54 theoretically by assuming typical pulsar parameters, dispersion relation associated with the plasma of three-wave interacting parametric instability process and the spatial variation associated with different plasma parameters like plasma density, Lorentz factor of electrons, frequency, and input flux of the pump wave.

λ21-cm Interstellar HI Profiles, Critical Ionization Velocities, and Derived Electron Densities

Analysis of neutral hydrogen λ21-cm profiles at high northern and southern Galactic latitudes reveals unexpected properties, connects unanticipated associations, and confirms the fundamental parameters of the interstellar medium. Gaussian decomposition of profiles from the second data release of the Galactic Arecibo L-Band Feed Array (GALFA) confirms earlier results of a pervasive component with a linewidth of order 34 km s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , consistent with the critical ionization velocity (CIV) of helium. The ratio of the column density of this component to the entire profile is proportional to the interstellar helium abundance. The results are comparable to the accepted value of the cosmic abundance, 0.085, in the southern Galactic hemisphere, but different for directions in the northern Galactic hemisphere. Normalizing to the cosmic abundance allows us to estimate how much HI is “missing” from the λ21-cm observations in the northern sky, confirming an asymmetry that has been known for decades. These CIV results also predict the volume density of interstellar electrons along different lines of site, which match values derived from pulsar dispersion measures and Hα observations. These results provide strong support for the hypothesis that interstellar λ21-cm HI profile linewidths are modulated by the CIV plasma phenomenon.

An analysis of interface trap charges to improve the reliability of a charge-plasma-based nanotube tunnel FET

A new heterodielectric shifted-core-gate nanotube tunneling field-effect transistor (HD-SCG-NT-TFET) is proposed with a higher ON-state current and a better subthreshold swing (SS) compared with the conventional core-gate NT-TFET structure. The charge plasma phenomenon is employed to induce charge carriers inside the channel and source region by applying an appropriate metal workfunction. A brief comparative analysis of the influence of the high-K gate dielectric on the interface trap charges (ITCs) and the resulting effect on the performance of the nanotube structures is also presented for different direct-current (DC) parameters. A reliability analysis for the nanotube TFET is presented for the first time to test how efficiently the proposed device follows the original characteristics. To address reliability concerns for low-power applications, the nanotube TFET structures are investigated in terms of their ION, IOFF, subthreshold swing (SS), and ION/IOFF ratio. All of the analyses are performed in the presence of negative, neutral, and positive ITCs.

Chaos-induced resistivity in different magnetic configurations

It is widely believed that magnetic reconnection plays an important role in various eruptive phenomena of space and astrophysical plasmas. The mechanism of anomalous resistivity, however, has been an open and unsolved problem. The chaos-induced resistivity proposed by Yoshida et al. is one of possible mechanisms for anomalous resistivity. By use of the test particle simulation, the present work studies the chaos-induced resistivity for different configurations of reconnection magnetic fields and its distribution in different chaos regions of reconnection current sheets. The results show that the chaos-induced resistivity can be 6 – 7 orders of magnitude higher than the classical Spitzer resistivity in the X-type chaos regions and 5 orders of magnitude in the O-type chaos regions. Moreover, in the X-type chaos regions the chaos-induced resistivity of the magnetized case is higher by a factor of 2 to 3 times than that of the unmagnetized case, but in the O-type chaos regions the chaos-induced resistivity of the magnetized case is close to or lower than that of the unmagnetized case. The present work is helpful to the understanding of the dynamics of reconnection current sheets, especially of the generation mechanism of the anomalous resistivity of collisionless reconnection regions.

Plasma physics in strong-field regimes: Theories and simulations

In strong electromagnetic fields, unique plasma phenomena and applications emerge whose description requires recently developed theories and simulations [Y. Shi, Ph.D. thesis, Princeton University (2018)]. In the classical regime, to quantify effects of strong magnetic fields on three-wave interactions, a convenient formula is derived by solving the fluid model to the second order in general geometry. As an application, magnetic resonances are exploited to mediate laser pulse compression, whereby higher intensity pulses can be produced in wider frequency ranges, as confirmed by particle-in-cell simulations. In even stronger fields, relativistic-quantum effects become important, and a plasma model based on scalar quantum electrodynamics (QED) is developed which unveils observable corrections to Faraday rotation and cyclotron absorption in strongly magnetized plasmas. Beyond the perturbative regime, lattice QED is extended as a numerical tool for plasma physics, using which the transition from wakefield acceleration to electron-positron pair production is captured when laser intensity exceeds the Schwinger threshold.

Effects of magnetic perturbations and radiation on the runaway avalanche

The electron runaway phenomenon in plasmas depends sensitively on the momentum- space dynamics. However, efficient simulation of the global evolution of systems involving runaway electrons typically requires a reduced fluid description. This is needed, for example, in the design of essential runaway mitigation methods for tokamaks. In this paper, we present a method to include the effect of momentum-dependent spatial transport in the runaway avalanche growth rate. We quantify the reduction of the growth rate in the presence of electron diffusion in stochastic magnetic fields and show that the spatial transport can raise the effective critical electric field. Using a perturbative approach, we derive a set of equations that allows treatment of the effect of spatial transport on runaway dynamics in the presence of radial variation in plasma parameters. This is then used to demonstrate the effect of spatial transport in current quench simulations for ITER-like plasmas with massive material injection. We find that in scenarios with sufficiently slow current quench, owing to moderate impurity and deuterium injection, the presence of magnetic perturbations reduces the final runaway current considerably. Perturbations localised at the edge are not effective in suppressing the runaways, unless the runaway generation is off-axis, in which case they may lead to formation of strong current sheets at the interface of the confined and perturbed regions.

On the rigidity from infinity for nonlinear Alfvén waves

The Alfvén waves are fundamental wave phenomena in magnetized plasmas and the dynamics of Alfvén waves are governed by a system of nonlinear partial differential equations called the MHD system. In this paper, we study the rigidity aspect of the scattering problem for the MHD equations. We prove that the Alfvén waves must vanish if their scattering fields vanish at infinities. The proof is based on a careful study of the null structure and a family of weighted energy estimates.

PicFoam: An OpenFOAM based electrostatic Particle-in-Cell solver

picFoam is a fully kinetic electrostatic Particle-in-Cell (PIC) solver, including Monte Carlo Collisions (MCC), for non-equilibrium plasma research in the open-source framework of OpenFOAM. The solver’s modular design, based on the same principles used in OpenFOAM, makes it highly flexible, by allowing the user to choose different methods at run time, and extendable, by building upon templated modular classes. The implementation of the PIC method employing the finite volume method, allows it to simulate on arbitrary geometries in one to three dimensions. OpenFOAM’s barycentric particle tracking is used effectively to perform charge and field weighting from the Lagrangian particle based description to the Eulerian field description and backwards without computational expensive particle searching algorithm. picFoam also includes open and general circuit boundary models for the description of real plasma devices. Program summaryProgram Title: picFoamCPC Library link to program files:http://dx.doi.org/10.17632/bbsm8tjgjy.1Developer’s repository link:https://github.com/TFDzarm/picFoamLicensing provisions: GPLv3Programming language: C++Nature of problem: The description of equilibrium and non-equilibrium plasma with its complex collective behavior between charged species. As well as their interaction with circuits connected to the boundaries of the plasma. The applications are versatile and lie e.g. in the description of plasma phenomena like discharges in arcjet electric propulsion systems, as well as a general study of plasma phenomena.Solution method: Implementation of the Particle-in-Cell method with Monte Carlo Collisions [1] in the open-source numerical toolbox OpenFOAM [2]. By this the newly implemented solver gains access to OpenFOAM’s powerful tool sets and an easy to set up simulation case structure. Fields are solved by employing OpenFOAM’s cell-centered finite volume solvers, this allows for the simulation on arbitrary structured meshes. The leapfrog scheme [1] is employed for the integration of the particle’s equation of motion combined with relativistic and non-relativistic integration schemes for Newton’s second law of motion [3]. Monte Carlo Collision methods include models for the collision of electrons and neutral species, considered collision events are elastic collision events and inelastic excitation and ionization events. Additional models include Coulomb collisions and isotropic scattering of neutral species [4]. The solver is able to bind circuit models, include open circuit, ideal voltage and current sources, as well as general purpose circuits with an ideal voltage source and a resistance, an impedance and a capacity in series, to the plasma domain [5].Additional comments including restrictions and unusual features: The finite volume method is a relatively rarely used method for implementations of the PIC method, here by employing this method we gain the ability to use arbitrary structured meshes in one to three dimensions. OpenFOAM’s barycentric particle tracking is a novel approach for moving the particles through the mesh. With this method we are able to interpolate (weight) the charges efficiently to the mesh without using computational expensive algorithms to locate the particle’s positions beforehand.

Characterization of descriptors in machine learning for data-based sputtering yield prediction

Sputtering of a single-element material surface by monatomic ion impact is one of the simplest and most fundamental phenomena of plasma–surface interaction. Despite its seemingly simple and well-defined nature, its collision cascade dynamics is so complex that no widely applicable formula of the sputtering yield has ever been derived analytically from the first principles. When the first-principles approach to a complex problem fails to unveil its nature, a data-driven approach, or machine learning, may be used to transform the problem into a tractable model. In this study, regression models of sputtering yields of such systems were constructed based on publicly available data derived from a large number of past experiments. The analysis has also identified the descriptors (i.e., physical variables characterizing the surface and incident ion species) on which the sputtering phenomena depend most strongly and presented quantitative evaluation on how sensitively the regression models depend on each descriptor or group of descriptors. Information obtained in this study can facilitate an understanding of the fundamental workings of the sputtering phenomena in the absence of rigorous analytical theory.