Plasma Sheath Properties Research Articles

Modeling of magnetized collisional plasma sheath with nonextensive electron distribution and ionization source

The properties of an atmospheric-pressure collisional plasma sheath with nonextensively distributed electrons and hypothetical ionization source terms are studied in this work. The Bohm criterion for the magnetized plasma is extended in the presence of an ion–neutral collisional force and ionization source. The effects of electron nonextensive distribution, ionization frequency, ion–neutral collision, magnetic field angle and ion temperature on the Bohm criterion of the plasma sheath are numerically analyzed. The fluid equations are solved numerically in the plasma–wall transition region using a modified Bohm criterion as the boundary condition. The plasma sheath properties such as charged particle density, floating sheath potential and thickness are thoroughly investigated under different kinds of ion source terms, contributions of collisions, and magnetic fields. The results show that the effect of the ion source term on the properties of atmospheric-pressure collisional plasma sheath is significant. As the ionization frequency increases, the Mach number of the Bohm criterion decreases and the range of possible values narrows. When the ion source is considered, the space charge density increases, the sheath potential drops more rapidly, and the sheath thickness becomes narrower. In addition, ion–neutral collision, magnetic field angle and ion temperature also significantly affect the sheath potential profile and sheath thickness.

Analysis of the influence of sheath positions, flight parameters and incident wave parameters on the wave propagation in plasma sheath

The plasma sheath covering hypersonic vehicles has a significant effect on the propagation of electromagnetic waves. Based on the calculation of the flow field of a conical cylindrical, this work studies the propagation of electromagnetic waves in plasma sheath at L-band and Ku-band, and discusses the propagation characteristics in the head, side and tail of the sheath. The dielectric properties of plasma sheath are related to flight speed and altitude. A flight condition corresponds to a unique distribution of dielectric properties. For the conical cylindrical, the results show that flight speed is generally negatively correlated with the transmissivity of the plasma sheath. The reflection characteristics of electromagnetic waves at the L-band and Ku-band when obliquely incident to the plasma sheath show a downward trend. When the frequency is increased to Ku-band, the propagation characteristics of electromagnetic waves in the plasma sheath are related to the position of the sheath.

Numerical investigation of sheath characteristics for electronegative magnetized plasma and dust charging

We have studied the effects of the magnetic field on the active electronegative plasma sheath properties and dust charging process in the sheath region for two different collisional models: constant ion mean free path and constant ion mobility using 1d3v fluid hydrodynamics model. It is found that the magnetic field strength and choice of collisional models have a significant effect on the active plasma sheath characteristics and charging of an isolated dust grain. The sheath criterion for an active electronegative magnetized plasma for both collisional models has been extended, and the effects of neutral gas pressure, source frequency, obliqueness of magnetic field, and initial electric field at sheath edge are graphically illustrated. There are two distinct regions observed in the sheath region: magnetic field and electric field dominant regions. The spatial distribution of plasma sheath parameters is systematically presented. It is found that the evolution of dust surface potential is affected by the magnitude of the magnetic field and collisional models. The stable levitation of dust grains in the sheath region is close to the sheath entrance. Moreover, the total force experienced by an isolated dust grain in the sheath region rapidly increases close to the material surface, and the magnitude of force is higher for larger dust grain.

Electronegative magnetized plasma sheath properties in the presence of non-Maxwellian electrons with a homogeneous ion source

The three-fluid model was employed to study electronegative magnetized plasma sheath properties in the presence of non-Maxwellian electrons. The Bohm condition for the electronegative magnetized plasma was extended in the presence of a homogeneous ionization source term and an ion-neutral collision force. The effect of the following physical parameters: the degree of non-Maxwellian electrons, ion-neutral collisions, obliqueness of the magnetic field and source frequency on the plasma sheath parameters are numerically analyzed and presented. It was found that the magnitude of the potential increased in all cases and hence the velocity of the ions increased towards the wall. The plasma sheath parameters were not significantly varied about 2λ De from the presheath—sheath interface, but after that, the sheath parameters abruptly changed towards the wall. The effect of ion-neutral collisions and the obliqueness of the magnetic field on the potential distribution, net charge density and drift velocity of ions are discussed. Furthermore, a comparison of plasma sheath characteristics with and without the ion source term is systematically presented.

Numerical investigation of dynamic properties of plasma sheath with pitching motion

Research on the dynamic properties of a plasma sheath coupled with pitching motion of the vehicle has great significance in solving the problem of communication interruption in the process of vehicle reentry. This paper investigates the dynamic properties of the plasma sheath by using the simplified conventional Burnett (SCB) equations and the Navier-Stokes (NS) equations with the thermochemical non-equilibrium effect. The eleven-species chemical kinetic models are applied to the comparison and there is verification of a dynamic plasma sheath simulation for the first time. After the introduction of vehicle pitching motion, the dynamic results are more consistent with the experimental data than the simulated results when treating it as static state. The plasma sheath characteristic parameters show periodic properties, whose changing period is the same as the pitching motion period. However, because of different velocities of the pitching motion, phase shifts exist in different positions of the vehicle. The enhancement of the rarefied effect weakens the disturbance to the plasma sheath. This research reveals the distribution and regularities of the dynamic plasma sheath. It is significant in solving the ionization blackout problem and the design of the reentry vehicle, and provides reliable data for further research on the dynamic plasma sheath.

Magnetized plasma sheath properties in the presence of Maxwellian low-temperature and non-Maxwellian high-temperature electrons

The characteristics of a magnetized plasma sheath in the presence of two-groups of electrons, namely, low-temperature electrons (LTEs) and high-temperature electrons (HTEs), have been studied using the fluid model. The LTEs are considered to obey the Boltzmann distribution, whereas the HTEs follow the so-called q-nonextensive distribution. The Bohm-sheath criterion which gets modified is developed for such plasmas, and the compiled fluid equations are solved numerically. The obtained results signify that the q-nonextensive parameter and concentration ratio have a significant effect on the magnetized plasma sheath parameters. The magnitude of sheath potential monotonically increases toward the wall consistent with Debye shielding. The temperature ratio of HTE to LTE also affects the electrostatic sheath potential. Although the q-parameter and concentration ratio affect the electron and ion density profiles as both decrease toward the wall, the electron density decreases much faster than ions. In addition, the LTE density decreases at the sheath entrance, whereas the HTE density increases for the increasing concentration ratio.

Influence of peak current on substrate plasma sheath properties of Ti films deposited by high-power pulsed magnetron sputtering

For film deposition, the substrate sheath properties, such as the plasma density, ion-to-atom ratios around the substrate, are more important for the film structure. In this paper, titanium thin films were deposited on grounded substrates by high-power pulsed magnetron sputtering (HPPMS) with the peak current in the range of 113–185 A. A simple and new equivalent circuit model of the sheath was established to study the plasma density around the substrate sheath. The Ti ion-to-atom ratios near substrate were studied by optical emission spectroscopy (OES), and the film structure was detected by transmission electron microscopy (TEM). The results showed that the calculated plasma density was from 0.8 × 10[Formula: see text] to 1.4 × 10[Formula: see text] m[Formula: see text] at different peak current. These were consistent with the results measured by a modified one-grid ion collector using saturation current probe method, which proved our proposed equivalent circuit model was correct. The Ti ion-to-atom ratios around the substrate were estimated at about 24%–62%. The plasma density and ion to atom ratio around the substrate increased with the peak current, and this could lead to a higher film crystallization and preference growth on Ti (101) and (100).

Effect of surface protrusion on plasma sheath properties in atmospheric microdischarges

The electric field enhancement due to the presence of cathode surface protrusion is investigated in atmospheric microdischarges with the goal of identifying the plasma sheath properties (such as cathode sheath thickness and electric field distortion). The electric field enhancement caused by surface protrusion is examined by adjusting the aspect ratio and the protrusion size. It is found that the cathode electric field enhancement depends strongly (weakly) on the aspect ratio (size) of the protrusion when it is much smaller than the discharge gap distance. In particular, the axial electric field in both vacuum and discharges becomes nonlinear with the protrusion on the cathode. The cathode sheath thicknesses obtained by two different methods are compared. With the same axial (or radial) protrusion dimension, increasing the aspect ratio will result in a significant decrease in the sheath thickness, whereas increasing the axial protrusion size with an unchanged aspect ratio will only lead to a slight decrease in the sheath thickness. The results contribute to predicting the relative plasma sheath properties from the geometrical parameter of the surface protrusion in atmospheric microdischarges.

Plasma sheath properties in a magnetic field parallel to the wall

Particle in cell simulations were carried out with a plasma bounded by two absorbing walls and a magnetic field applied parallel to them. Both the sheath extent and the potential drop in it were derived from simulations for different plasma parameters, such as the electron and ion temperature Ti, particle density, and ion mass. Both of them exhibit a power law dependent on the Larmor to plasma ion pulsation ratio Ωi. For increasing values of the magnetic field, the potential drop within the sheath decreases from a few Ti/e down to zero, where e stands for the electron charge. The space charge extent increases with Ωi and saturates to 2.15 ion Larmor radius. A simple model of sheath formation in such a magnetic field configuration is presented. Assuming strongly magnetized electrons, and neglecting collisions and ionizations, a new typical length is evidenced, which depends on the ratio Ωi. The charge separation sheath width is theoretically found to increase from a combination of the electron gyroradius and the ion Debye length for low Ωi ratios up to several ion gyroradii for strongly magnetized ions. Both the calculated sheath extent and plasma potential show a fair agreement with the numerical simulations.

Hypersonic Vehicle Telemetry Blackout Analysis

During certain hypersonic flight regimes, shock heating of air creates a plasma sheath, resulting in telemetry attenuation or blackout. The severity of the signal attenuation is dependent on vehicle configuration and orientation, flight trajectory, and transmission frequency. With the promise of airbreathing hypersonic vehicles looming on the horizon, telemetry solutions must be found to address safety considerations for flight testing (that is, flight termination and/or catastrophe analysis). This attenuation phenomena are investigated with a focus on the nonequilibrium plasma sheath properties (electron concentration, plasma frequency, collision frequency, and temperature) for a range of flight conditions and vehicle design considerations. Trajectory and transmission frequency requirements for airbreathing hypersonic vehicle design are then addressed, with comparisons made to both shuttle orbiter and Radio Attenuation Measurements C-II reentry flights. The effects of an applied magnetic field normal to the plasma interface are shown over a range of flight conditions. It is shown that a combination of vehicle shaping effects, high transmission frequency, and an applied right-hand circularly polarized magnetic field (less than 1 T) show promise toward mitigating the telemetry blackout.

Study on the transmission properties of plasma sheath based on transfer matrix method

引入分层模型传输矩阵法，分析高速临近空间飞行器等离子体鞘套传输特性.通过对典型飞行器模型进行纳维-斯托克斯方程组数值求解，仿真了等离子体鞘套的流场电子密度分布，分析了电磁波在等离子体鞘套内传输时的衰减常数，给出信号传输的反射系数和透射系数.基于传输矩阵法的仿真结果与理论分析结果和时域有限差分法仿真分析结果有良好的一致性，同时，该方法有效降低了计算复杂度，适用于任意电子密度分布和厚度的等离子体鞘套传输特性的数值求解.

Potential contour shaping and sheath behavior with wall electrodes and near-wall magnetic fields in Hall thrusters

Graphite electrodes are embedded within the discharge channel of a Hall effect thruster to focus ions for improved performance. Cusp-shaped magnetic fields are added around the electrodes to shield the electrodes from high electron current. Internal plasma potential measurements inside the discharge channel show that the presence of floating graphite does not significantly affect the potential contours at 150 V anode potential. Creation of closed contour pockets are observed with the electrodes biased 10 and 30 V above the anode potential. The electrodes also cause a compression of the acceleration region in the thruster. The cause of the changes in the potential contours is attributed to a shifting of discharge electrode from the anode to the electrodes and an expansion of the near-wall plasma sheath. The presence of the cusp magnetic fields is shown to affect the current collected by the electrodes, a behavior associated with modification of the plasma sheath properties due to magnetization of electrons.

Properties of plasma sheath with ion temperature in magnetic fusion devices

The plasma sheath properties in a strong magnetic field are investigated in this work using a steady state two-fluid model. The motion of ions is affected heavily by the strong magnetic field in fusion devices; meanwhile, the effect of ion temperature cannot be neglected for the plasma in such devices. A criterion for the plasma sheath in a strong magnetic field, which differs from the well-known Bohm criterion for low temperature plasma sheath, is established theoretically with a fluid model. The fluid model is then solved numerically to obtain detailed sheath information under different ion temperatures, plasma densities, and magnetic field strengths.

Equilibrium properties of the plasma sheath with a magnetic field parallel to the wall

Motivated by the Magnetized Target Fusion (MTF), a systematic investigation of the equilibrium properties of a 1D plasma sheath with a magnetic field parallel to the wall was carried out using analytical theory and kinetic simulations. Initially uniform full Maxwellian plasma consisting of equal temperature collisionless electrons and ions is allowed to interact with a perfectly absorbing conducting wall, which charges positively due to large ions gyro-radii. The analysis of the steady-state plasma and field profiles reveals the importance of the relation between electron and ion thermal Larmor radii and plasma Debye length. In particular, the sheath width scaling, the details of the particle flows and the break-down of force balance components exhibit different behaviors in three possible regimes. Despite our primary motivation, the results in this paper can also be applicable to the divertor and the first wall of tokamaks.

Sheath formation in a collisional electronegative magnetized discharge

The plasma sheath structure in an electronegative magnetized discharge is investigated. The profiles of the positive ion density and flux velocity as well as those of the plasma potential and sheath thickness are obtained under the influence of various parameters such as collision frequency, negative ion concentration and negative ion temperature. In addition, the influences of these parameters on the profiles of the electron and negative ion density are obtained. The results are compared with the electropositive as well as the collisionless electronegative sheath. There is competition between the effects of collisions and the magnetic field, which determines the plasma sheath properties.

Plasma sheath properties calculated using measured secondary electron emission coefficients

Measurements of secondary electron emission (SEE) are presented for graphite, hydrogen-implanted graphite and an amorphous graphite (a-C:H) carbon layer, over a wide range of energies and angles of incidence. These data have been integrated over a Maxwellian velocity distribution to simulate the effect of SEE due to plasma electrons. The SEE yields have then been used to calculate plasma sheath potentials, sheath heat transmission coefficients and sputtering yields for a range of ions as a function of plasma temperature. The increased SEE yield with temperature reduces the normalized sheath potential and increases the sheath transmission coefficient markedly. The role of space charge in the sheath is discussed.

Experimental investigation of plasma sheath properties of a re-entry vehicle

Experimental investigation of plasma sheath properties of a re-entry vehicle