Spatial Distribution Of Plasma Density Research Articles

Spatial distribution of plasma density and magnetic field amplitude in the dayside magnetosheath as a function of the IMF orientation

The properties of the magnetosheath are of pivotal importance in determining the coupling between the magnetosphere and interplanetary medium. In particular, the magnetic flux pileup and plasma depletion layer (PDL) modify the boundary conditions of magnetopause reconnection. However, the spatial distribution of the magnetic field strength and plasma density in the magnetosheath and their functional dependence on the interplanetary magnetic field (IMF) orientation remain poorly understood. This study characterizes these aspects in detail through the statistical processing of decades of data from Cluster, Double Star, THEMIS, and Magnetospheric Multiscale (MMS) missions. The first part of this study focuses on the poorly known variations across the magnetosheath, from the shock to the magnetopause. The magnetic pileup and PDL are significantly correlated, with a strong dependence on the IMF cone angle. Their dependence on the IMF clock angle is found only near the magnetopause, consistent with the expected effect of magnetic reconnection. The second part of this study examines the asymmetry in the magnetic field amplitude and density between the quasi-parallel and quasi-perpendicular sides of the equatorial magnetosheath. These asymmetries are characterized for different relative distances to the magnetopause and bow shock boundaries and for different IMF orientation. The magnetic field amplitude, observed to be higher on the quasi-perpendicular side of the magnetosheath, becomes more symmetric as it approaches the magnetopause. The quasi-parallel magnetosheath exhibits a higher plasma density near the magnetopause. However, this asymmetry reverses at approximately the mid-magnetosheath with a decreasing IMF cone angle.

Effect of focus ring with external circuit on cathode edge sheath dynamics in a capacitively coupled plasma

Capacitively coupled plasmas are widely used in semiconductor processes. The control of plasma to obtain uniform deposition and etching is an open problem, particularly within a few millimeters of the substrate edge. Complex material stacks commonly referred to as focus rings are placed at the wafer edge to provide uniform processes across the entire substrate but have limitations with regard to process window and eventual material erosion. One approach is to combine a focus ring with a tunable external circuit ground path termination to extend the plasma uniformity to the wafer edge over a wider process space. The external circuit coupling focus ring to the ground influences the ion energy profile and the ion angular profile by changing the impedance between the focus ring and the ground and allows wafer edge tuning over a wide range of operating parameters. In this work, it is found that the adjustable external circuit can control the partitioning of bias and RF voltages between the RF powered and passively coupled plasma facing surfaces. The focus ring with an external circuit assembly can also control the spatial distribution of plasma density and, therefore, improve the sheath edge profile. These results point to possible source designs for engineering the distribution of power dissipation and the electric field of the wafer edge in industrial plasma reactors.

Can Radioactive Emanations in a Seismically Active Region Affect Atmospheric Electricity and the Ionosphere?

—Anomalous variations in radon emissions from the soil are sometimes observed before and after strong earthquakes. In this paper, we theoretically analyze the known hypothesis that these variations in radioactive emanations in the surface layer of the atmosphere cause variations in the vertical background current in the atmosphere with subsequent changes in the electron density {{n}_{e}} in the lower ionosphere. For this purpose, we will first assess the relation between radon emission variations and the vertical atmospheric current flowing into the ionosphere. Then we will solve a model problem concerning the spatial distribution of plasma density and electric field perturbations occurring in the E-layer of the ionosphere caused by an inflowing vertical current. It was believed that the current flowing into this layer contains only an electron component and is attenuated only by the recombination of electrons. The estimate of the maximum variation {{n}_{e}} is at least 3–5 orders of magnitude smaller than the observed anomalous variations, which allows us to conclude that this hypothesis is not plausible.

Spatial distribution of plasma parameters by a dual thermal-electrostatic probe in RF and DC magnetron sputtering discharges during deposition of aluminum doped zinc oxide thin films

Aluminum doped zinc oxide thin films deposited by magnetron plasma sputtering are essential for various optoelectronic applications. So far, the oxygen negative ions and the atomic oxygen are regarded as responsible for the poor spatial uniformity of thin film resistivity. While various methods are available for thin film characterization, understanding the growth mechanism requires spatial-resolved measurements of plasma parameters. This work uses a dual thermal-electrostatic probe that is able to reveal the spatial distribution of plasma density, electron temperature and plasma potential. The results exhibit a parabolic profile for plasma density and flat profiles for electron temperature and plasma potential, with no correlation with the strong distribution of thin film resistivity that mirrors the erosion track on the target surface.

Spatial distributions of plasma potential and density in electron cyclotron resonance ion source

The Numerical Advanced Model of Electron Cyclotron Resonance Ion Source (NAM-ECRIS) is applied for studies of the physical processes in the source. Solutions of separately operating electron and ion modules of NAM-ECRIS are matched in iterative ways such as to obtain the spatial distributions of the plasma density and of the plasma potential. Results reveal the complicated profiles with the maximized plasma density close to the ECR surface and on the source axis. The ion-trapping potential dips are calculated to be on the level of ∼(0.01–0.05) V being located at the plasma density maxima. The highly charged ions are also localized close to the ECR surface. The biased electrode effect is due to an ‘electron string’ along the source axis formed by reflection of electrons from the biased electrode and the extraction aperture. The string makes profiles of the highly charged ions more peaked on the source axis, thus increasing the extracted ion currents.

DC radial glow discharge in axial magnetic field at low pressures

The influence of magnetic field on DC radial glow plasma was studied by self-designed coaxial glow discharge device, and the influence of magnetic field on the spatial distribution of plasma density is studied. The experimental results show that the spatial density distribution of plasma from cathode to anode increases gradually in the high-intensity magnetic field, and decreases gradually in the absence of magnetic field. Theoretical analysis of the above results show that the high-intensity magnetic field increases the moving path of the electrons, enhances the collision efficiency between the electrons and the neutral atoms, and makes the discharge plasma density remarkably enhanced.

Double-hole anode glow discharge in axial magnetic field at low pressures

The influence of magnetic field on glow plasma was studied by the self-designed double-hole anode discharge device, and the relationship between ignition voltage and gas pressure under different magnetic fields is obtained. In addition, the influence of magnetic field on the spatial distribution of plasma density is studied by changing different magnetic field configurations. The results show that under the same pressure, the magnetic field can effectively reduce the glow discharge ignition voltage and increase the plasma density; the enhancement of magnetic field is similar to the increase in air pressure in the double-hole anode glow discharge structure; the hole anode and magnetic field are indispensable conditions for glow generation at low pressure; and different magnetic field configurations control the plasma density distribution.

Влияние анода несамостоятельного тлеющего разряда с полым катодом на пространственное распределение концентрации плазмы

Влияние анода несамостоятельного тлеющего разряда с полым катодом на пространственное распределение концентрации плазмы

Quantum electrodynamic cascade structure in a standing linearly polarised wave

We have performed an analysis of plasma structures formed as a result of the quantum electrodynamic (QED) cascade development in the field of a standing linearly polarised plane wave at various field amplitudes. In view of the spatially inhomogeneous cascade dynamics and complicated particle motion, it is proposed to use spatial distributions of plasma density averaged over the half-period of the field, i.e. integral structures. Based on the analysis of of particle trajectories and integral structures with and without allowance for the QED cascade, various plasma structures are revealed and the amplitude thresholds of their occurrence are determined. It is shown that the integral structure maxima may arise not only at the electric field nodes or antinodes, as in a circularly polarised field, but also in the intermediate regions between them.

Numerical simulation and analysis of electromagnetic-wave absorption of a plasma slab created by a direct-current discharge with gridded anode

In this paper, we present investigation of a direct-current discharge with a gridded anode from the point of view of using it as a means of creating plasma coating that could efficiently absorb incident electromagnetic (EM) waves. A single discharge cell consists of two parallel plates, one of which (anode) is gridded. Electrons emitted from the cathode surface are accelerated in the short interelectrode gap and are injected into the post-anode space, where they lose acquired energy on ionization and create plasma. Numerical simulations were used to investigate the discharge structure and obtain spatial distributions of plasma density in the post-anode space. The numerical model of the discharge was based on a simple hybrid approach which takes into account non-local ionization by fast electrons streaming from the cathode sheath. Specially formulated transparency boundary conditions allowed performing simulations in 1D. Simulations were carried out in air at pressures of 10 Torr and higher. Analysis of the discharge structure and discharge formation is presented. It is shown that using cathode materials with lower secondary emission coefficients can allow increasing the thickness of plasma slabs for the same discharge current, which can potentially enhance EM wave absorption. Spatial distributions of electron density obtained during simulations were used to calculate attenuation of an incident EM wave propagating perpendicularly to the plasma slab boundary. It is shown that plasma created by means of a DC discharge with a gridded anode can efficiently absorb EM waves in the low frequency range (6–40 GHz). Increasing gas pressure results in a broader range of wave frequencies (up to 500 GHz) where a considerable attenuation is observed.

The plasma sheath around planar probes: effects of ion collisions

The 2D numerical model of the ion current collection by a planar probe has been developed on the basis of a particle in cell with the Monte Carlo collision method for ion motion. This model is used to reveal the effects of ion collisions on the probe sheath area and spatial distribution of plasma density and to warn against current–voltage probe characteristic misinterpretation at plasma density measurements by commonly used Langmuir probe methods.

A one-dimensional study of the evolution of the microwave breakdown in air

The microwave breakdown in air is simulated numerically within a simple 1D model taking into account a perturbation of electromagnetic field by plasma. The simulations were performed using two qualitatively different codes. One of these codes is based on computation of Maxwell equations, whereas the other one utilizes an approximation of quasi-monochromatic electromagnetic field. There is a good agreement between simulation results obtained by using both codes. Calculations have been carried out in a wide range of air pressures and field frequencies; also varied were initial spatial distributions of plasma density. The results reveal strong dependence of the breakdown evolution on the relation between the field frequency and the gas pressure as well as on the presence of extended rarefied background plasma. At relatively low gas pressures (or high field frequencies), the breakdown process is accompanied by the stationary ionization wave propagating towards the incident electromagnetic wave. In the case of a high gas pressure (or a relatively low field frequency), the peculiarities of the breakdown are associated with a formation of plasma filament array. The extended background plasma can suppress formation of the plasma filament array completely even at high pressures (or low frequencies).

E-H heating mode transition in inductive discharges with different antenna sizes

The spatial distribution of plasma density and the transition power for capacitive (E) to inductive (H) mode transition are studied in planar type inductively coupled plasmas with different antenna sizes. The spatial plasma distribution has a relatively flat profile at a low gas pressure, while the plasma profile is affected by the antenna size at higher gas pressure. The transition power for the E to H mode transition is shown to be critically affected by the antenna size. When the discharge is sustained by a small one-turn antenna coil, the transition power has a minimum value at Ar gas of 20 mTorr. However, the minimum transition power is shown at a relatively high gas pressure (40–60 mTorr) in the case of a large one-turn antenna coil. This change in the transition power can be understood by the thermal transport of the energetic electrons with non-local kinetics to the chamber wall. This non-local kinetic effect indicates that the transition power can also increase even for a small antenna if the antenna is placed near the wall.

2D fluid simulation of capacitively coupled plasma with cylindrical electrode for roll-to-roll processing

RF glow discharges are widely used in deposition, etching and surface treatments for semiconductor materials. Especially, roll-to-roll processing is necessary for flexible display or thin film solar cell modules. For a roll-to-roll thin film deposition processing, it is necessary to use cylindrical grounded electrode. The effects of electrode shapes on the plasma properties of capacitively coupled rf argon discharge at 13.56 MHz are investigated using self-consistent two-dimensional fluid simulation. The simulation model includes blocking capacitor and external circuit effects. Comprehensive analysis has been made in terms of time-varying current–voltage characteristic, spatial distributions of plasma density and ion current density on the substrate. The effects of applied voltage (Vf=200∼800[V]) and gas pressure (p=0.5 ∼ 2 Torr) on the plasma properties are also investigated. We found that flat electrode has a little bit higher impedance than that of cylindrical ground electrode. However the difference between the two configurations is less than 5% therefore we may conclude that the shape of grounded electrode does not significantly affect current–voltage characteristics of the discharge system. For roll-to-roll reactor version, displacement current come from dielectric layer between electrodes reaches more than 75% of total displacement current. Large displacement current results in degradation of power factor and matching circuit efficiency. For the cylindrical electrode, increasing electrode gap length along lateral direction makes that the location of peak electron density moves toward center of the reactor. The effect of the cylindrical electrode on the plasma spatial structure is reflected in the ion current density at substrate (dielectric layer) on grounded electrode. At low voltage and low power condition, about 90% of input power is consumed for electron heating. The efficiency decreases with increasing power and decreasing pressure as a consequence of increasing power to drive ion current. Electron heating efficiency decreased with pressure. The increase of pressure mainly reduces the diffusive component of the loss rate for both charged and neutral species.

Inhomogeneities of plasma density and electric field as sources of electrostatic turbulence in the auroral region

Inhomogeneities of plasma density and non-uniform electric fields are compared as possible sources of a sort of electrostatic ion cyclotron waves that can be identified with broadband extremely low frequency electrostatic turbulence in the topside auroral ionosphere. Such waves are excited by inhomogeneous energy-density-driven instability. To gain a deeper insight in generation of these waves, computational modeling is performed with various plasma parameters. It is demonstrated that inhomogeneities of plasma density can give rise to this instability even in the absence of electric fields. By using both satellite-observed and model spatial distributions of plasma density and electric field in our modeling, we show that specific details of the spatial distributions are of minor importance for the wave generation. The solutions of the nonlocal inhomogeneous energy-density-driven dispersion relation are investigated for various ion-to-electron temperature ratios and directions of wave propagation. The relevance of the solutions to the observed spectra of broadband extremely low frequency emissions is shown.

Influence of heat and particle fluxes nonlocality on spatial distribution of plasma density in two-chamber inductively coupled plasma sources

This study presents 2D simulations of the two-chamber inductively coupled plasma source where power is supplied in the small discharge chamber and extends by electron thermal conductivity mechanism to the big diffusion chamber. Depending on pressure, two main scenarios of plasma density and its spatial distribution behavior were identified. One case is characterized by the localization of plasma in the small driver chamber where power is deposed. Another case describes when the diffusion chamber becomes the main source of plasma with maximum of the electron density. The differences in spatial distribution are caused by local or non-local behavior of electron energy transport in the discharge volume due to different characteristic scale of heat transfer with electronic conductivity.

Experimental study of plasma parameters in a vacuum arc with a hot refractory anode

The spatial distributions of plasma density, electron temperature, plasma potential and ion energy were measured during hot refractory anode vacuum arc (HRAVA) development as functions of arc current and inter-lectrode gap distance. Plasma density increased with arc time and saturated (⩾1014 cm−3) in the developed HRAVA stage. While the ion energy in the anode plasma at the gap exit was relatively small, the ions were accelerated outside the gap in radially expanding plasma to ∼15 eV at 19 cm from the electrode axis. The electron temperature decreased during anode plume development and was ⩾1 eV in the developed HRAVA stage. Measured plasma parameters agreed well with previously developed theory.

Relativistic warm plasma waveguide under the effects of plasma electrons and HF electrical field on Buneman instability

Abstract The stabilization effect of a strong HF (pump) electrical field and plasma electrons on a two-stream (Buneman) instability in a plane relativistic warm plasma waveguide is investigated; using the separation method to solve the two-fluid plasma model we separate the problem into two parts. The “temporal” (dynamical) part enables us to determine the frequencies and growth rates of unstable waves; this part within the redefinition of natural frequencies coincides with the system describing HF suppression of Buneman instability in uniform unbounded plasma. Natural frequencies of oscillations and spatial distribution of the amplitude of the self-consistent electrical field are determined from the solution of a boundary-value problem (“space part”) taking into account specific spatial distribution of plasma density. Plasma electrons are considered to have a relativistic thermal velocity. It is shown that the growth rate of instability in relativistic warm plasma is reduced compared to non-relativistic (cold or warm) plasma and relativistic cold plasma. In addition, it is found that the plasma electrons have no effect on the solution of the space part of the problem.

Compact microwave imaging system to measure spatial distribution of plasma density

We have developed an advanced microwave interferometric system operating in the K band (18–27 GHz) with the use of a fan-shaped microwave based on a heterodyne detection system for measuring the spatial distribution of the plasma density. In order to make a simple, low-cost, and compact microwave interferometer with better spatial resolution, a microwave scattering technique by a microstrip antenna array is employed. Experimental results show that the imaging system with the microstrip antenna array can have finer spatial resolution than one with the diode antenna array and reconstruct a good spatially resolved image of the finite size dielectric phantoms placed between the horn antenna and the micro strip antenna array. The precise two-dimensional electron density distribution of the cylindrical plasma produced by an electron cyclotron resonance has been observed. As a result, the present imaging system is more suitable for a two- or three-dimensional display of the objects or stationary plasmas and it is possible to realize a compact microwave imaging system.

Radio Tomography: A New Experimental Technique

The technique of radio tomography has developed during the past fifteen years from a theoretical concept to an established experimental method, used for geophysical investigations ofsolar-terrestrial processes. It also has potential in the mapping and modelling of the ionised atmosphere for application to practical radio systems. The method involves measurement of the electron content of the ionosphere along a large number of intersecting satellite-to-receiver ray paths, with tomographic inversion of the data to give a two-dimensional image of the spatial distribution of plasma density in the region of ray-path intersection. The emphasis in this review is on experimental tomographic observations, which have highlighted the capabilities and potential of the technique. Examples are presented from the equatorial sector where the equatorial anomaly is a significant feature, the mid-latitude sector where radio propagation is often influenced by the presence of the main ionisation trough, and the auroral and polar regions where footprints of solar-terrestrial coupling processes are frequently to be seen.