Positive Ion Temperature Research Articles

Effect of Positive-Ion Temperature in the Sheaths Surrounding Cylindrical and Spherical Probes in Electronegative Plasmas

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper analyzes the effect of positive-ion temperature on the sheath that surrounds a cylindrical or spherical probe in an electronegative plasma. We developed a theoretical model that considers a hydrodynamic approach for positive-ion motion. In order to introduce the positive-ion temperature into the model, we consider that positive ions flow adiabatically toward the probe. This model allows us to obtain the electrical potential profile surrounding the probe and the current-to-voltage characteristic curves. These <formula formulatype="inline"> <tex Notation="TeX">$I$</tex></formula>–<formula formulatype="inline"><tex Notation="TeX">$V$</tex></formula> curves were fitted as functions of the probe radius, the probe biasing potential, and the positive-ion temperature. Finally, these expressions allow us to obtain the floating potential of the probe for different electronegativities and positive-ion temperatures. </para>

Ion thermal double layers in a pair-ion warm magnetized plasma containing charged dust impurities

In this paper the formation and the dynamics of ion thermal double layers (ITDLs) in a magnetized plasma, composed of positive and negative ions as well as a fraction of stationary charged (positive or negative) dust impurities have been studied. Using plasma hydrodynamics and Poisson equations for the two ion species, a modified Zakharov–Kuznetsov equation has been derived. The effects of the external magnetic field, the concentration of charged dust impurities, and the negative to positive ion temperature ratio on the ITDLs structure are investigated.

Numerical investigation of the magnetized plasma sheath characteristics in the presence of negative ions

In this work, a planar and low-pressure discharge in a mixture of a noble gas (argon) and an electronegative gas (oxygen) is considered. It is assumed that the produced plasma consists of electrons, two species of positive ions, and one species of negative ion. The behavior of the density distribution and kinetic energy of these charged particles in the sheath region are studied. Also, it is assumed that a weak external magnetic field which is nearly perpendicular to the wall is exerted to the sheath region. The positive ion species are considered as a cold, collisionless fluid while both electron and negative ion densities obey the Boltzmann distribution. By using a hydrodynamic approach and ignoring ionization and recombination, it is shown numerically that by increasing the density of the negative ions in the plasma the density distribution of both positive ion species and the kinetic energy of these ion species decreases and increases, respectively. Also, it is shown that in the presence of the negative ions the normalized electrostatic potential of the sheath region changes and by increasing the negative ion densities the normalized electrostatic potential in the sheath increases. In addition, the effect of the density ratio of both positive ion species, density and temperature of the negative ions, and the magnitude of the external magnetic field are studied on the net density distribution of the charged particles in the sheath region. The obtained numerical results show that by decreasing the temperature of the negative ions the amplitude of the fluctuations of the net density distribution of the charged particles in the sheath region increases and the position of these fluctuations shifts toward the sheath edge. Finally, it is seen that in the absence of negative ions the net density distribution of charged particles in the sheath region is monotonic while in the presence of negative ions it is nonmonotonic.

Sheath analysis in collisional electronegative plasmas with finite temperature of positive ions

The sheath which occurs in front of a plane metallic surface immersed in an electronegative plasma is analysed taking into account the positive ion collisions and thermal motion. Collisions do not affect the equations of the sheath but have a significant influence on the initial conditions, which are obtained from the plasma solution. The combined influence of collisions and positive ion thermal motion on the sheath, the sheath thickness, the positive ion saturation current and the floating potential is analysed.

Effects of collisions and finite ion temperature on the sheath structure of cylindrical probes in low-pressure electronegative discharges

The spatial distributions of electric potential and velocity and density of positive ions are calculated in the surroundings of a negatively biased cylindrical probe immersed in electronegative plasmas. The model equations are solved on the scale of the electron Debye length. The solutions provide the variation of plasma variables along the distance from the plasma bulk region to the probe surface. The control parameters are the ratio of the negative ion density to the electron density, the ratios of the electron temperature to the positive and negative ion temperatures, and the ratio of the rate coefficient for the momentum transfer collision to that for the ionization. Especially, the effects of collision and finite temperature of positive ions are investigated. As the positive ion temperature increases, the sheath width decreases and the positive ion current collected by the probe increases. As the ratio of the rate coefficient for the momentum transfer collision to that for the ionization increases, the sheath edge approaches the plasma region, and the positive ion current to the probe decreases.

Theoretical model of cylindrical Langmuir probe for low-pressure electronegative discharges

(3) e0∇ · E = e(n+ − ne − n−), E =−∇V, where e0 is the permittivity of the vacuum, n− is the negative ion density, e is the electron charge, and V is the electric potential. We assume that electrons and negative ions follow the Boltzmann energy distribution. The equations are developed for cylindrical probe with radial motion of positive ions towards the probe. We have the dimensionless functions and parameters; ξ = r/Λ, n= n+/ne0, u = v+/cs , η = −eV /kTe , α0 = n−0/ne0, γ− = Te/T−, γ+ = Te/T+, q = λD/Λ, δ = νm/νiz, where cs is the Bohm velocity (= √kTe/m+ ), λD is the Debye length and Λ = cs/νiz is the ionization length. Te, T+, and T− are the temperature of electrons, positive ions, and negative ions, respectively, k is the Boltzmann constant, the subscript 0 indicates the value at the plasma bulk region. The equations are solved by using the fourth-order Runge– Kutta method with the boundary conditions at the bulk region

Deduction of the absolute negative ion density by using planar and cylindrical electric probes simultaneously

Analytic formulas are derived for the deduction of the absolute density of negative ions by using the measured electron temperatures and saturation currents of positive ions and negative charges from current-voltage curves taken by planar and cylindrical probes at two different pressures without assuming the sheath potential, sheath velocity, temperatures of positive and negative ions, and effective masses of positive ions. Ratios of ion and electron saturation currents and electron temperatures of two probes and sheath areas of a long thin cylindrical probe at two different pressures are incorporated into two equations with two unknowns for the negative ion density. The procedure to deduce the absolute negative ion density is given.

Influence of the positive ion temperature in cold plasma diagnosis

The influence of the positive ion temperature in cold plasma diagnosis by using Langmuir probes is analyzed. The positive ion zone of the I-V characteristic is used. This zone is distinguished because the charge drained from the plasma is small, diminishing the perturbation due to the measurement process. Nevertheless, it is much affected by the positive ion temperature, thus the traditional methods give inaccurate values for the electron density. Moreover, for an accurate measurement of that current, a good calibration of the instrument used must be ensured. The authors propose the floating potential as the proper parameter to control that calibration.

Sheath formation criterion for negatively charged particles

The criterion for the sheath formation for negatively charged particles is investigated for the cases where the positive ion temperature is higher than that of negatively charged particles. Such situations recently came up, for example, in the boundary of tokamak fusion plasmas, the ion beam cooled by cold electrons (electron-cooling plasma) and dusty plasmas when cold negatively charged dust floats within hotter positive ions. The paper is concerned with the sheath formation around a probe/electrode positively biased with respect to the plasma potential.

A novel bioflocculant produced by Enterobacter aerogenes and its use in defecating the trona suspension

In this paper, a bioflocculant-producing bacterium, named W-23, was isolated from soil and identified as Enterobacter aerogenes. The bioflocculant (named WF-1) produced by W-23 was an acidic polysaccharide composed mainly of uronic acid (13.2%), pyruvic acid (7.4%) and acetic acid (1.6%). The three components sugars of WF-1 were glucose, fructose and manose, and the molar ratio was 10.3:5.4:1 for glucose:fructose:manose. The influence of factors like temperature, bioflocculant dosage and concentration of positive ions on the flocculation were studied. The optimal conditions were as follows: temperature 45 °C, WF-1 90 mg/L and Zn 2+ 0.03 mol/L. As a result, the sedimentation rate was 2.96 × 10 −4 m/s, and the OD of supernatant reached 0.05. The efficiency of the bioflocculant was better than that of conventional chemically synthesized flocculants, such as anionic polyacrylamide or non-ionic polyacrylamide.

On the higher-order solution of the dust-acoustic solitary waves in a warm magnetized dusty plasma with dust charge variation

The higher-order contribution in reductive perturbation theory is studied for small-butfinite-amplitude dust-acoustic solitary waves in warm magnetized three-component dusty plasmas comprised of variational charged dust grains, isothermal ions, and electrons. The basic set of fluid equations is reduced to the Zakharov–Kuznetsov equation for the first-order perturbed potential and a linear inhomogeneous Zakharov–Kuznetsov-type equation for the second-order perturbed potential. Stationary solutions of both equations are obtained using a renormalization method. The effects of the higher-order contribution, external magnetic field, dust charge variation, dust grain temperature, ratios of temperature and density of positive ions to electrons, and directional cosine of the wave vector k along the x axis on the nature of the solitary waves are investigated.

Sheath structure in electronegative plasmas with finite positive ion temperature

An earlier theoretical work, concerning the sheath structure in electronegative plasmas, is extended to include the effect of the positive ion thermal motion. A significant change is observed in the quantities characterizing the sheath with respect to the cold ion assumption. The sheath is contracted when the positive ion thermal motion is considered causing a decrease in the sheath thickness. The ion saturation current and the floating potential are shown to be distinguished quantities in plasma diagnosis of electronegative plasmas by using plane Langmuir probes.

Analytical fit of the I–V probe characteristic for finite ion temperature values: Justification of the radial model applicability

This article deals with an extension of the radial model for the sheath that surrounds a cylindrical or spherical Langmuir probe immersed in a plasma in which positive ion thermal motion is taken into account. The dependence of the electric potential profile and of the I–V characteristic on the positive ion temperature is obtained. Moreover, a different parameterization is established from a numerical fit of the I–V characteristic as a function of the probe radius, the biasing potential, and the ratio between the positive ion temperature and the electron temperature. This parameterization allows us to obtain an analytical approximation to the potential profile and the sheath edge. Finally, it also gives us a means of justifying under what conditions the radial model can be applied in plasmas with finite positive ion temperature.

Ion temperature increase during MHD events on the TST-2 spherical tokamak

Various types of MHD events including internal reconnection events are studied on the TST-2 spherical tokamak. In weak MHD events no positive current spike was observed, but in strong MHD events with positive current spikes, a rapid and significant impurity ion temperature increase was observed. The decrease in the poloidal magnetic energy is the most probable energy source for ion heating. The plasma current shows a stepwise change. The magnitude of this step correlates with the temperature increase and is found to be a good indicator of the strength of each event.

Negative chlorine ions from multicusp radio frequency ion source for heavy ion fusion applications

Use of high mass atomic neutral beams produced from negative ions as drivers for inertial confinement fusion has been suggested recently. Best candidates for the negative ions would be bromine and iodine with sufficiently high mass and electron affinity. These materials require a heated vapor ion source. Chlorine was selected for initial testing because it has similar electron affinity to those of bromine and iodine, and is available in gaseous form. An experiment was set up by the Plasma and Ion Source Technology Group in Lawrence Berkeley National Laboratory to measure achievable current densities and other beam parameters by using a rf driven multicusp ion source [K. N. Leung, Rev. Sci. Instrum. 65, 1165 (1994); Q. Ji et al., Rev. Sci. Instrum. 73, 822 (2002)]. Current density of 45 mA/cm2 was achieved with 99.5% of the beam as atomic negative chlorine at 2.2 kW of rf power. An electron to negative ion ratio as low as 7 to 1 was observed, while the ratio of positive and negative chlorine ion currents was 1.3. This in addition to the fact that the front plate biasing had almost no effect to the negative chlorine ion and electron currents indicates that a very high percentage of the negative charge in the extraction area of the ion source was in form of Cl− ions. A comparison of positive and negative chlorine ion temperatures was conducted with the pepper pot emittance measurement technique and very similar transverse temperature values were obtained for positive and negative chlorine ions.

Intrinsic sheath edge conditions for sheath instability in low-pressure electronegative plasmas

The conditions at the sheath edge that could lead to instability in the plasma–wall boundary in low-pressure electronegative plasmas are considered in a comprehensive manner. The relevant range of the ratio of the negative ion density to the electron density at the sheath edge was derived on the basis of a fluid model and it was clarified how the parameter range are affected by the positive and negative ion temperatures, by the geometries of plasmas (plane-parallel, cylindrical, and spherical), by the directions of the positive ion flow (converging or diverging), and by the effects of ionization and momentum transfer collisions. It was found that in any geometry the instability would be induced in conditions that could be realized without difficulty in low-pressure plasmas.

Experimental observation of dominant propagation of the ion-acoustic slow mode in a negative ion plasma and its application

Different characteristics of ion acoustic waves were experimentally observed in two types of Xe+–F− double plasmas at different electron temperatures. For the lower electron temperature (around 0.15 eV), the slow mode, which had been considered not to dominate the wave propagation, was found to be dominant rather than the fast mode, which was observed to be dominant for the higher electron temperature (around 1.5 eV). According to the previous numerical investigation [Phys. Plasmas 8, 4275 (2001)], the new wave characteristic appeared when the ratio of negative ion mass to positive ion mass and to the ratio of electron temperature to ion temperature are lower than certain critical values. Further, a method of evaluating both the positive ion temperature and the negative ion temperature in a negative ion plasma by observing the dominant slow mode is described. Using this method, the positive and negative ion temperatures in the former plasma were estimated to be 0.075 eV at the highest and 0.1 eV at the lowest, respectively.

Plasma perturbation induced by laser photodetachment

The plasma dynamics arising from laser photodetachment is discussed herein theoretically and experimentally. The hybrid fluid-kinetic model, where the positive ions and electrons are treated by the fluid theory and the negative ions are treated within the ballistic approximation, is extended and applied to the analysis of densities perturbed by laser photodetachment. The agreement between the theory and measured data confirms the validity of the considered plasma dynamics model. This model, including the positive ion perturbation, shows a good agreement with the time evolution and the spatial distribution of perturbed electron densities which are measured by a Langmuir probe inside and outside the laser beam. From the overshoot in the time evolution of perturbed electron current in the center of the laser beam, the positive ion temperature was found to be in the range 0.1-0.25 eV, while the electron temperature changes from 0.3 to 3.2 eV.

Are the oscillations found in electronegative plasmasat low pressure an artefact?

Computations are reported of the plasma-sheath inelectronegative plasmas at low pressure using the fluid model in theparameter region in terms of negative ion density/electron density andelectron temperature/negative ion temperature where oscillations inpotential, electric field and charged particle density were previouslyfound. In this paper, in contrast, the positive ion temperature has notbeen arbitrarily set to zero but has been allowed to vary, to becomecomparable to the negative ion temperature. It is concluded that theoscillations arise from the unphysical assumption of a significantdifference between the ion temperatures, which suppressed positive iondiffusion as a mechanism to smooth out density variations. Thus, theoscillations are not intrinsic to the fluid model but rather they arisefrom arbitrarily setting the positive ion temperature Tp = 0.

Ion Acoustic Shocks Formed in a Collisionless Plasma with Negative Ions

Ion acoustic shocks are formed in a plasma with negative ions, where ${T}_{e}\ensuremath{\ge}{T}_{+}\ensuremath{\gg}{T}_{\ensuremath{-}}$ ( ${T}_{e}$, ${T}_{+}$, ${T}_{\ensuremath{-}}$: temperatures of electrons, positive ions, and negative ions, respectively). Depending on the ratio $\ensuremath{\varepsilon}$ of negative to positive density, a steepening of positive or negative density jumps is observed. For $\ensuremath{\varepsilon}l{\ensuremath{\varepsilon}}_{c}$ ( $\ensuremath{\approx}0.65$, critical value), positive density jumps evolve into compressive shocks. For $\ensuremath{\varepsilon}g{\ensuremath{\varepsilon}}_{c}$, however, negative density jumps evolve into rarefactive shocks. The ratio ${\ensuremath{\varepsilon}}_{c}$ observed is well explained on the basis of Korteweg\char21{}de Vries equation.