Plasma Charge Density Research Articles

Characteristics of Emission Non-nulling Pulsars Through Simulation

We investigate the population and several properties of radio pulsars whose emission does not null (non-nulling) through simulation of a large pulsar sample. Emission from a pulsar is identified as non-nulling if (i) the emission does not cease across the whole pulse profile, and (ii) the emission is detectable. For (i), we adopt a model for switching in the plasma charge density, and emission persists if the charge density is non-zero. For (ii), we assume that detectable emission originates from source points where it is emitted tangentially to the magnetic field-line and parallel to the line-of-sight. We find that pulsars exhibiting non-nulling emission possess obliquity angles with an average of 42.°5, and almost half the samples maintain a duty cycle between 0.05 and 0.2. Furthermore, the pulsar population is not fixed but dependent on the obliquity angle, with the population peaking at 20°. In addition, three evolutionary phases are identified in the pulsar population as the obliquity angle evolves, with the majority of samples having an obliquity angle between 20° and 65°. Our results also suggest that emission from a pulsar may evolve between nulling and non-nulling during its lifetime.

Simulation for Distributions of Several Geometric Parameters in Nulling Pulsars

We investigate the distributions of the obliquity angle and impact parameters of nulling pulsars of different duty cycles based on the simulation of more than 600,000 samples. We adopt a purely geometric approach for pulsar visibility, in which visible emission is emitted tangentially to the magnetic field line and parallel to the line-of-sight direction. The geometry is incorporated with the model for pulsar magnetospheres of multiple emission states, in which the plasma charge density is dependent on the emission state. We assume that an emission state can only exist between two limiting conditions described by the vacuum and corotation models, respectively. In this model, pulse nulling corresponds to emission switching to a state in which the plasma charge density is zero. The event is detectable only if the switching occurs at source points that lie on a trajectory, whose locus defines the locations of visible emission, within an open-field region. Our results show that detectable nulling is dependent on all three parameters, such that nulling pulsars prefer a small obliquity angle and duty cycle, and tend to have positive impact parameters. We find that the total population of nulling pulsars in our samples is around 23%, of which about 47% possess a duty cycle of 0.1 or smaller. The former implies that there are more nulling pulsars than currently known. Our model predicts that the number of nulling pulsars increases as the obliquity angle decreases, which also implies that the occurrence of nulling in a pulsar should evolve over time.

Effect of plasma properties on magnetic bubbles

In this work, a three-dimensional hybrid simulation program is used to investigate the process of the explosion and expansion of a large number of thermal debris ions in a low density background plasma. By quantitatively calculating the variation of the magnetic bubble and the bubble constraint in the motion of the debris cloud, studied are the influences of the background plasma charge density, the background ion atomic weight, and the charge-mass ratio of the fragment ion on the magnetic bubble. The results show that the background charge density has an important effect on the motion of the bubble and debris cloud. In the early stage of the debris cloud expansion, the background ion atomic weight has little effect on the bubble expansion, but it affects the movement of debris cloud at a later time. When the charge-mass ratio of the debris ion is small, the radius of the ion gyration is larger than the bubble radius, and the bubble radius is small so that it cannot restrain the debris cloud. When the charge-mass ratio of the debris ion is large, the radius of the ion gyration is smaller than the bubble radius. If the background charge density is low in this condition, the early expansion of the bubble and the debris cloud are slightly affected by the charge-mass ratio of the fragment ions, and only the subsequent evolution of the system is influenced. If the background charge density is large in this condition, the charge-mass ratio of the fragment ion has a prominent influence on the motion of the bubble and the debris cloud.

Understanding pulsed laser deposition process of copper halides via plasma diagnostics techniques

Due to the raised interest in copper halides for optoelectronic applications, a wide range of plasma diagnostic tools based on Langmuir probe (LP) and optical emission spectroscopy (OES) were implemented to investigate the pulsed laser deposition (PLD) process of selected copper halides. This represents the first report of complete diagnostics of copper halide plasma generated by laser ablation. High plasma potential and electron temperature were observed by a time-resolved LP analysis, which affected the kinetic energy of the ejected ions during expansion. The effect of argon gas addition on the copper halide plasma dynamics was investigated. Angle-resolved measurements revealed a narrow cone plasma expansion with a dual peak charge particle distribution, which was affected only above 2 Pa of Ar atmosphere. Plasma potential, charge density, and electron excitation temperature were increasing with Ar pressure, while electron temperature and expansion velocities were decreasing. Using a semi-empirical model, the estimation of an ion acceleration field, neutral temperature, and densities were computed. Space- and time-resolved OES confirmed the plume splitting indicated by LP measurements and provided insight into the composition of the two-plasma structure. All the plasma parameters were investigated in conditions similar to those used for thin film growth, and the results were discussed with respect to the PLD technology.

Confined hydrogenlike ions in plasma environments

The behavior of H-like ions embedded in astrophysical plasmas in the form of \emph{dense, strongly and weakly coupled} plasmas are investigated. In these, the increase and decrease in temperature is impacted with a change in confinement radius $(r_{c})$. Two independent and generalized scaling ideas have been applied to modulate the effect of plasma screening constant ($\lambda$) and charge of ion ($Z$) on such systems. Several new relations are derived to interconnect the original Hamiltonian and two scaled Hamiltonians. In exponential cosine screened Coulomb potential (ECSCP) (dense) and weakly coupled plasma (WCP) these scaling relations have provided a linear equation connecting the critical screening constant $(\lambda^{(c)})$ and $Z$. Their ratio offers a state-dependent constant, beyond which, a particular state vanishes. Shannon entropy has been employed to understand the plasma effect on the ion. With increase in $\lambda$, the accumulation of opposite charge surrounding the ion increases leading to a reduction in number of bound states. However, with rise in ionic charge $Z$, this effect can be delayed. The competing effect of plasma charge density ($n_e$) and temperature in WCP and ECSCP is investigated. A recently proposed simple virial-like theorem has been established for these systems. Multipole ($k=1-4$) oscillator strength (OS) and polarizabilities for these are studied considering $1s, 2s$ states. As a bonus, analytical closed-form expressions are derived for $f^{(k)}$ and $\alpha^{(k)} (k=1-4)$ involving $1s$ and $2s$ state, for \emph{free H-like ion}.

Modulational instability of dust-ion-acoustic waves and associated first and second-order rogue waves in a super-thermal plasma

A proper theoretical research has been carried out to explore the modulational instability (MI) conditions of dust-ion-acoustic (DIA) waves (DIAWs) in a three-component dusty plasma system containing inertialess κ-distributed electrons, and inertial warm positive ions and negative dust grains. The nonlinear Schrödinger equation (NLSE) has been derived by employing the reductive perturbation method. The numerical analysis under consideration demonstrates two types of modes, namely, fast and slow DIA modes. The MI conditions of DIAWs and the configuration of the energetic rogue waves (RWs) associated with DIAWs in the modulationally unstable parametric regime have been rigorously changed by the variation of various plasma parameters, namely, charge, mass, temperature, and number density of the plasma species. The findings of our present investigation will be useful in understanding the criteria for the formation of electrostatic RWs in both astrophysical environments (viz., Jupiter’s magnetosphere, cometary tails, pulsar magneto-sphere, interstellar medium, Earth’s mesosphere, Saturn’s rings, etc.) and laboratory experiments.

Charge Density and Charge Delivery Characteristics of Sputtered Transition Metal Oxide Films Used As Neural Stimulation and Recording Electrodes

The use of microelectronic devices for electrical stimulation and recording of neural electrical activity is a rapidly emerging technique that intends to restore lost or impaired physiological function in humans, such as spinal cord injury, stroke, sensory deficits and neurological disorders. The electrode material is one the most important components of such devices as it constituents the chronic electrode-electrolyte interface to the physiological environment. Therefore understanding the electrochemical properties of the electrodes is essential to provide electrical stimulation without adversely affecting the surrounding tissue. In this study, we explore the charge delivery mechanism of two transition metal oxides namely, iridium oxide (IrOx) and ruthenium oxide (RuOx) electrodes, systematically sputtered using water vapor as one of the reactive plasma constituents in addition to oxygen and argon. Additionally, we examine the effect of microstructure controlled by the composition of the reactive plasma constituents, on electrochemistry and charge density characteristics. Electrode sites with a geometric surface area (GSA) of 1960 μm2 were fabricated on silicon wafers using photolithography to create conductive gold metallization to define contact pads, electrode sites, and interconnect traces. The electrode sites were coated with sputtered IrOx and RuOx films using DC magnetron sputtering with a range of reactive gas ratios (O2:H2O) in the plasma. Material characterization techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to study the microstructure and composition of the electrode coatings. Electrochemical analysis included open circuit potential (Eoc) measurements, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and voltage transient (VT) measurements in electrolyte solutions that mimic the inorganic constituents of the physiological environment. Charge storage capacity (CSC) as a function of sweep and charge injection capacity (Qinj) as a function of pulse width and interpulse bias where calculated from the CV and VT measurements. Varying the plasma constituents show a systematic change in the morphology of both the IrOx and RuOx films. The magnitudes of CSC and Qinj also have a systematic dependence on the morphology of the films as controlled by the composition of the sputtering gases. Figure 1(a) shows a representative dependence of morphology and Qinj as a function of plasma composition for IrOx deposition with similar results obtained for RuOx films. The Eoc of the films was measured as a function of pH, as shown in figure 1(b), with IrOx and RuOx showing a near-Nernstian 62 mV/pH-unit and 59 mV/pH-unit, respectively. In this study, we demonstrate the dependence of IrOx and RuOx film morphology on the composition of the reactive plasma constituents. The relationship between film microstructure and electrochemistry at near-neutral pH has been investigated and the charge delivery mechanisms of sputtered IrOx and RuOx films characterized. The addition of water vapor as a reactive gas constituent increased the charge injection capacity of both IrOx and RuOx films using typical neural stimulation waveforms. Figure 1

Net electron energy gain induced by superluminal phase velocity and subluminal group velocity of a laser in a plasma channel

We examine electron dynamics induced by laser-plasma interaction in a two-dimensional plasma channel, taking into action the laser phase velocity as well as the group velocity. The coupled effects of phase velocity, group velocity, and plasma channel on electron dynamics are discussed in detail. The superluminal phase velocity and the corresponding subluminal group velocity of the laser result in rich and complex electron dynamics, which are depicted in the plane of the phase velocity and plasma charge density. For weak superluminosity of the phase velocity, the effects of the phase velocity and the group velocity can be neglected. For moderate superluminosity of the phase velocity, a cross-over region can exist, where the highly energetic electron could be found and the net energy gain is several times greater than the energy gain in vacuum. For strong superluminosity of the phase velocity, the dephasing rate increases and thus limits the electron energy gain from the laser. However, the asymmetric laser pulse, attributed by the superluminal phase velocity and the subluminal group velocity, results in the electron getting adjustable net energy gain from the laser. The electron oscillations are no longer limited by the charge density threshold and the electron can always get net energy from the laser. These electron dynamics can also be modified by adjusting the polarization of the laser.

Fully Ionized Plasmas and Nuclear Electron Capture Reactions

Properties of fully ionized plasmas are discussed including plasma charge density oscillations and the screening of Coulombs law especially in the dilute classical Debye regime. In a kinetic model with two charged particle scattering events in the Boltzmann collision rates, the rate of electron capture induced plasma nuclear reactions is exhibited and veri es our previous results based on condensed matter electro-weak quantum eld theory.

Characterization of a Glow Discharge Detector With Terahertz Time Domain Spectroscopy

Recently, there has been much interest in the capability of low cost glow discharge detectors (GDDs) for detection of terahertz radiation. To utilize them in applications such as terahertz imaging these studies have typically focused on the response of the GDD at specific frequencies. To better understand the spectral behavior of glow discharges, we examine the interaction mechanism of GDDs with terahertz radiation using terahertz time domain spectroscopy in a broader range of frequencies between 0.05 and 0.5 THz. Our results show that in addition to terahertz-induced oscillations in the plasma charge density, the structure of the GDD itself plays an important role in the detection mechanism. We observe that by increasing the bias voltage on the gap, not only can we greatly reduce the transmission at a specific frequency, but we can also tune it as well, suggesting that these devices can be used as hybrid plasma filters.

An arbitrary curvilinear-coordinate method for particle-in-cell modeling

A new approach to the kinetic simulation of plasmas in complex geometries, based on the Particle-in- Cell (PIC) simulation method, is explored. In the two dimensional (2d) electrostatic version of our method, called the Arbitrary Curvilinear Coordinate PIC (ACC-PIC) method, all essential PIC operations are carried out in 2d on a uniform grid on the unit square logical domain, and mapped to a nonuniform boundary-fitted grid on the physical domain. As the resulting logical grid equations of motion are not separable, we have developed an extension of the semi-implicit Modified Leapfrog (ML) integration technique to preserve the symplectic nature of the logical grid particle mover. A generalized, curvilinear coordinate formulation of Poisson's equations to solve for the electrostatic fields on the uniform logical grid is also developed. By our formulation, we compute the plasma charge density on the logical grid based on the particles' positions on the logical domain. That is, the plasma particles are weighted to the uniform logical grid and the self-consistent mean electrostatic fields obtained from the solution of the logical grid Poisson equation are interpolated to the particle positions on the logical grid. This process eliminates the complexity associated with the weighting and interpolation processes on the nonuniform physical grid and allows us to run the PIC method on arbitrary boundary-fitted meshes.

In-flight calibration of mesospheric rocket plasma probes

Many effects and factors can influence the efficiency of a rocket plasma probe. These include payload charging, solar illumination, rocket payload orientation and rotation, and dust impact induced secondary charge production. As a consequence, considerable uncertainties can arise in the determination of the effective cross sections of plasma probes and measured electron and ion densities. We present a new method for calibrating mesospheric rocket plasma probes and obtaining reliable measurements of plasma densities. This method can be used if a payload also carries a probe for measuring the dust charge density. It is based on that a dust probe's effective cross section for measuring the charged component of dust normally is nearly equal to its geometric cross section, and it involves the comparison of variations in the dust charge density measured with the dust detector to the corresponding current variations measured with the electron and/or ion probes. In cases in which the dust charge density is significantly smaller than the electron density, the relation between plasma and dust charge density variations can be simplified and used to infer the effective cross sections of the plasma probes. We illustrate the utility of the method by analysing the data from a specific rocket flight of a payload containing both dust and electron probes.

Evolution of discontinuous solutions to transport equations in stellarators

It was shown earlier that, in the range of rare collisions, transport equations for stellarators allow steady discontinuous solutions for the ambipolar electric field and for the plasma density and temperature gradients. Moreover, such solutions are non-single-valued; that is, their explicit form depends on the initial values of the ambipolar electric field. The time-independent transport equations are derived under the conventional quasineutrality condition; i.e., it is assumed that the electron and ion densities, N e and N i , are related by the relationship N e = ZN i (where Z is the ion charge number). In other words, the plasma charge density is assumed to be much less than the product e i N i . Under typical conditions, the corresponding inequality is satisfied by a large margin. However, if the electric field E has discontinuities, then it can be seen from the equation ▿·E = 4πρ that, at the discontinuity points, the charge density becomes infinite and the relationship N e = ZN i fails to hold, so it is necessary to replace it with N e = ZN i + ρ/e e . In the transport equations, this latter replacement produces additional terms, proportional to the second radial derivative of the field E. With these additional terms, the steady solutions are modified substantially. First, the ambipolar field and the derivatives of the density and temperatures all become continuous functions of the coordinates, a result that seems to be quite obvious. The second, not-so-obvious result is that the steady solutions become single-valued, i.e., independent of the initial values of the ambipolar electric field. It turns out that, in this case, two regimes are possible, depending on the values of the plasma parameters. In the first regime, the solution is unique and is independent of the initial conditions. In the second regime, two steady solutions can exist, depending on the initial conditions. One of the solution is similar to that obtained in the first regime, and the other differs from the first one both in the ambipolar field profile and in the dependence of the density and temperatures on the minor plasma radius. It cannot be excluded that different plasma confinement modes revealed in experiments are associated with the existence of such solutions.

Photoionization cross section of non-hydrogenic levels for weakly coupled plasmas

Photoionization process is a subject of special importance in many areas of physics. Numerical methods must be used in order to obtain photoionization cross-sections for non-hydrogenic levels. The atomic data required to calculate them is huge and self-consistent calculations increase computing time considerably, overall when we are studying plasmas where it is found million of ion levels. Analytical potentials are a useful alternative because they avoid the iterative procedures typical in self-consistent models. In this work, we present a relativistic quantum-mechanic calculation of photoionization cross-sections for non-hydrogenic ions immersed in weakly coupled plasmas. Both the energy levels and bound and free wave functions are determined by means of the Dirac equation. It is employed as effective potential an analytical one developed to model non-isolated many-electron ions making use of the Debye–Hückel approach but including the reaction of the plasma-charge density to the optical electron. The effect of the plasma screening in the photoionization cross-section and in the threshold energy is analyzed as a function of the photon energy, the Debye radius, the ionization and the mono-electronic bound level involved in the transition. Some comparisons between our results and others are also made.

Modeling of plasma fibers with radiative processes

A fully three dimensional Particle in Cell model of the plasma flber had been developed. The code is written in FORTRAN 95, implementation CVF (Compaq Visual Fortran) under Microsoft Visual Studio user interface. Five particle solvers and two fleld solvers are included in the model. The solvers have relativistic and non{relativistic variants. The model can deal both with periodical and non{periodical boundary conditions. Plasma flber and its surroundings are generated during the initiation process. Many parameters of the flber, such as temperatures of electrons and ions, electric and magnetic flelds, perturbations in the shape of the flber and of the charged particle positions can be in∞uenced by the input dialog. The PIC program package simulates the behavior of the flber, namely the evolution of the magnetic fleld structures and turbulences. The numerical solution of the particle motion and calculation of the electric and magnetic flelds is the core of the program package. There are many additional routines and collaborating program packages for computer diagnostics, graphical output and other calculations such as radiation, collisions with neutrals, etc. The mechanism of the surface turbulences generation in the plasma flber was successfully simulated with the PIC program package. One of the latest goals of the PIC package was clariflcation of the onset of turbulence structures on the plasma flbers surfaces. It was shown that the development of these structures can be caused by initial a radial perturbation of plasma concentration and charge density. The surface structures simulated by PIC package were observed experimentally in various z{pinch apparatuses.

Chondroitin sulfate in normal human plasma is modified depending on the age. Its evaluation in patients with pseudoxanthoma elasticum

Plasma chondroitin sulfate (CS) amount and charge density were determined in 45 healthy volunteers (control group), 45 pseudoxanthoma elasticum (PXE)-affected patients and 19 healthy carriers by using fluorophore-assisted carbohydrate electrophoresis (FACE) and HPLC equipped with postcolumn derivatization and fluorescence detection. The mean values of CS amount were 4.9 ± 1.21 for volunteers, 4.7 ± 1.40 for PXE subjects and 4.4 ± 1.44 for the carriers. No significant differences were found for the three human subjects groups. On the contrary, by considering the age of normal volunteers, a significant increase of plasma CS amount was measured. In fact, the volunteers aging from 17 to 40 years (mean 32.1) showed a CS concentration of 4.3 ± 1.30 while the group ranging from 50 to 74 years (mean 56.9) had a value of 5.6 ± 1.16 with a significant increase of + 30.2%. The same significant increase in CS plasma content with increasing age was measured for PXE-affected and healthy carriers group. Extracted plasma CS was evaluated for the main two unsaturated disaccharides, non-sulfated and 4-monosulfated, and the charge density determined. The mean values were 0.54 ± 0.13 for volunteers, 0.60 ± 0.15 for PXE subjects and 0.50 ± 0.15 for the carriers. A significant increase of + 11.1% was found between the PXE patients and healthy human group but no differences were calculated between the control group and the carriers. Furthermore, besides a CS amount, the volunteers aging from 17 to 40 years (mean 32.1) showed a charge density of 0.53 ± 0.14 while the group ranging from 50 to 74 years (mean 56.9) had a value of 0.58 ± 0.17 with a significant increase of + 9.4%. The same trend was measured for the healthy carriers group. The CS charge density of PXE-affected subjects was found to increase significantly more than healthy controls depending on the age. In fact, the PXE patients aging from 10 to 40 years (mean 29.3) showed a charge density of 0.56 ± 0.14 while the group ranging from 50 to 74 years (mean 58.6) had a value of 0.67 ± 0.11 with a significant increase of + 19.6%. Furthermore, the group of PXE-affected subjects ranging from 50 to 74 years (mean 58.6) showed a significant increase of 15.5% in comparison with the group matched for age (mean 56.9) of healthy volunteers.

Self‐consistent Studies of Electron Acceleration to Ultrarelativistic Energies by Upper Hybrid Waves

When electrons that are trapped by strong electrostatic waves are carried across a magnetic field, the Lorentz force can in principle accelerate them to ultrahigh energies. This wave accelerator known as the electron surfing acceleration is thus a potential generation mechanism for ultrarelativistic electrons at astrophysical shocks. Here, we present for the first time a self-consistent simulation that follows the growth and saturation of strong electrostatic waves that are triggered by proton beams moving at relativistic speeds relative to a background plasma. We find in our simulation that proton beams moving at a Lorentz factor of 7 can accelerate electrons to 1 GeV by means of electron surfing acceleration. Thereafter the wave collapses, and it scatters some electrons to energies in excess of 10 GeV. The plasma charge density modulations give rise to a strong growth of the fast extraordinary (X) wave in frequency intervals for which its group velocity is comparable to the beam speed.

An effective analytical potential including plasma effects

In this work, we have developed a method to build an effective analytical potential for ions in slightly nonideal plasmas. This proposed potential is obtained from an analytical isolated potential with one or two parameters depending on the total number of electrons of the ion. The plasma effects are included by means of the linearized Debye–Hückel approximation taking into account the reaction of the plasma-charge density to the optical electron. Due to the influence of the plasma over the atomic potential, this permits to obtain level energies and wave functions as a function of the inverse of Debye radius, the quantum numbers, the nuclear charge, the bound electron number and the ionization state of the ion. Also, we compare the analytical effective potential proposed in this paper with other ones very well known in the available literature.

Measurements of striation formation time in an N2 α RF discharge

Measurements have been made of the striation formation time of an α-type RF discharge in nitrogen as a function of pressure, RF frequency, electrode separation and input power. A theoretical model is proposed that yields electron energy and density and striation formation time, given the measured discharge parameters of interelectrode voltage and separation, gas pressure and RF frequency. It is found that increasing the pressure and the electrode separation increase the striation formation time, while increasing the power and the RF frequency decrease the striation formation time. The model predicts that striations in the RF discharge are formed when the plasma charge density reaches some critical value at which the space charge distorts the interelectrode field.

Silicon double-injection electro-optic modulator with junction gate control

Novel structures for carrier-induced electro-optical phase modulation in crystalline silicon are examined. A new dual-injection field-effect transistor structure for guided-wave light modulation at 1.3 μm is proposed and analyzed. It consists of an elongated cathode-anode-gate structure integrated in a rib waveguide. Dual-gate and single-gate control are considered. The overlap between the plasma charge density and the optical guided mode is computed. For a cathode-anode voltage of 0.32 V, the effective refractive index of the waveguide mode changes by ΔN=1×10−3 when the gate voltage is altered by 12 V. Numerical estimates of the bias current, pinchoff voltage, interaction length, and modulator speed are given.