Langmuir Probe Diagnostics of a Low Temperature Non-Isothermal Plasma in a Weak Magnetic Field

This article presents measurements by a cylindrical Langmuir probe in the plasma of a DC cylindrical magnetron discharge át the pressure 1.5 Pa that aim at the experimental assessment of the influence of a weak magnetic field to the estimation of the electron density when using conventional methods of probe data interpretation. The probe data was obtained under the presence of a weak magnetic field in the range 1.10^?2?5.10^?2 T. The influence of the magnetic field on the electron probe current is experimentally assessed for two cylindrical probes with different radii, 50 μm and 21 μm. This assessment is based on comparison of the values of the electron density estimated from the electron current part with the values of the positive ion density estimated from the positive ion current part of the probe characteristic respectively by assuming that at the magnetic field strengths used in the present study the probe positive ion currents are possible to be assumed as uninfluenced by the magnetic field. For interpretation of the probe positive ion current two theories are used and compared to each other: the radial motion model by Allen, Boyd and Reynolds [10] and Chen [11] and the model that accounts for the collisions of positive ions with neutrals in the probe space charge sheath that we call Chen-Talbot model [8]. At lower magnetic field 3 · 10^?2 T the positive ion density values interpreted by using the Chen-Talbot model [8] are in better agreement with the values of electron density compared to those obtained by using the theory [10,11]; therefore the model [8] is used for calculation of the positive ion density from the probe data at higher magnetic fields. The comparison of the positive ion and electron density values calculated from the same probe data at higher magnetic fields shows that up to the magnetic field strength 4 . 10^?2 T with the probe 100 μm and up to 5 . 10^?2 T with the probe 42 μm in diameter respectively the decrease of the magnitude of the electron current at the space potential due to the magnetic field does not exceed the error limits that are usual for Langmuir probe measurements (absolute error ±20%).     

A Collisional Model of the Positive Ion Collection by a Cylindrical Langmuir Probe

This article presents a new theoretical model of ion collection by a cylindrical Langmuir probe at medium and higher pressures which we call the “modified Talbot and Chou model”. The model makes use of the following theories; (a) the kinetic theory by Chou, Talbot and Willis [6, 7] and (b) the theory by Zakrzewski and Kopiczynski [10, 11]. The basic idea is to calculate the decrease of probe ion current due to collisions with neutrals according [6, 7] and the increase of the ion current due to destruction of an orbital motion according [10, 11]. The computed results are presented in the form of graphs suitable for probe data interpretation at medium and higher pressures. The applicability of the results at particular plasma conditions is also discussed.     

Plasma-wall interactions in DC discharges and sheath of Langmuir probes

The plasma-wall interactions in various DC discharges and sheath of Langmuir probe are analyzed and discussed. The methods of their investigations are discussed including fluid and PIC MC. Various assumptions used in fluid models e.g. plasma neutrality, Bohm criterion, Boltzmann electrons approximation, etc. are analyzed. Ion heating and electron cooling effect at the DC plasma wall is discussed and explained. Langmuir probes measurements in high-temperature and ion thruster plasma are analyzed. The secondary electron emission influences the IV characteristic of Langmuir probe especially at positive voltages. However, only elastic reflection processes really contribute significantly to the probe current. The elastic SEE processes reflect electrons from probe with the same relatively high speed. It was observed that the axial magnetic field influences probe characteristics and floating potential more significantly than radial field. The axial field deflects all electrons approaching probe.     

Comment on determination of electron temperature from Langmuir probe data in tokamak edge plasma

Langmuir probe measurements of electron temperature in a plasma in the limiter shadow of a tokamak are presented together with a method of probe data analysis which takes into account the influence of the ion current vs voltage dependence in the determination of electron temperature, The method is based on the transformation of a single into a double probe characteristic. Values of the electron temperature calculated using this method are compared with the values estimated from single probe characteristic data.     

A contribution to the assessment of the influence of collisions on the measurements with Langmuir probes in the thick sheath working regime

In the present work we investigate theoretically and experimentally the influence of elastic collisions in a probe sheath on a cylindrical Langmuir probe. The analysed probe working regime covers conditions under which the following probe characteristic parameters are comparable: the probe radius, the Debye length and both the ion and electron mean free paths.The preliminary investigations under almost collisionless conditions show good agreement between theoretical and experimental values of the ion saturation currents and of the floating potentials only when the ion currents for the studied working regime of the cylindrical Langmuir probe are calculated according to the theory of Chen (Plasma Phys.7 (1965)47). These collisionless currents form the basis for the calculation of the collision-corrected probe characteristics according to the presented procedure by Talbot and Chou (Rarefied Gas Dynamics, Academic Press, New York, 1966, p. 1723).The applied theoretical analysis covers the influence of the collisions on the electron and ion current of the single probe characteristic and on the estimation of the space potential. The results of the calculations are presented in graphical overviews for the series of cases of practical importance. The other working regimes can be covered using the calculating procedure presented.For comparison of the calculated collision-corrected characteristics with those from an experiment we used the positive column plasma of the He glow discharge where the electron density is known and the space potential can be experimentally estimated from the lowest excitation potential of He. The comparison was carried out for the ion and electron currents, the floating potential and the zero-cross of the probe characteristic second derivative.The estimation of the secondary electron current contribution to the total probe current shows that it limits the applicability of the collision-corrected probe characteristic to the plasma diagnostic in the transition to the collision-determined working regime.     

Langmuir Probe Determination of Charged Particles Density in an rf Discharge

The article presents results of probe measurements of the electron and ion densities in an argon rf discharge. The electron density is determined from the electron probe current at the space potential which is estimated by extrapolation of the linear part of the probe electron current dependence to the plasma potential. The plasma potential is calculated from the floating potential. The positive ion density is determined from the probe ion saturated current according to collisional KOPICZYNSKI and ZAKREWSKI model [1]. The agreement between the electron and ion densities is better than in the work of HOPKINS and GRAHAM [2], where the collisionless Lafram-boise theory is used under similar conditions.     

Langmuir probe characteristics in a high pressure plasma in thepresence of convection and ionization

The current to a Langmuir probe operating in a high pressure flowing plasma is calculated for the case where convection of electrons or ions into the sheath adjacent to the probe and thermal generation of electrons and ions within the sheath can both be significant factors in determining probe current. In addition to generating explicit relations for planar, spherical and cylindrical probes for the asymptotic case in which only generation need be considered, a solution is also obtained for the extensive transition between this state and the purely convective state, for which explicit solutions already exist. The theory, which enables the electron/ion density to be calculated from the probe current is applicable to both positive and negative probes     

Monte Carlo Simulations of the Influence of Ion‐Neutral Collisions on the Ion Currents Collected by Electrostatic Probes

We have carried out Monte Carlo simulation of the motion of Ar⁺ ions in the space charge sheath surrounding a cylindrical Langmuir probe. The ion currents to the probe have been calculated from these simulations and the percentages of ions crossing the sheath boundary that are collected by the probe have been determined. It has been shown that the collisions of ions with neutral helium gas atoms in the sheath increase the percentage of ions collected by the probe above that predicted by collisionless orbital motion limited current (OMLC) theory at lower helium pressure and decrease this percentage below the OMLC theory prediction at higher helium pressure. It has been shown also that the ion current almost does not depend on probe radius at higher helium pressures. The results of the simulations have been compared with recent Langmuir probe measurements made in flowing afterglow plasmas and with other probe theories.     

New Applications of Langmuir Probes

In this work, two new possibilities for standard probe diagnostics are described. The first one can be used to study isotropic, collisionless low-pressure plasma in which the electron energy distribution function is close to a Maxwellian one. In such plasmas, the Boltzmann law, Bohm effect, and 3/2 power law are valid. Use of corresponding system of equations for cylindrical Langmuir probes allowed for measurements of probe sheath thicknesses and the mean ion mass. The solution of this task was provided by accurate probe diagnostics of inductive xenon plasma at pressure p = 2 mTorr that resulted in the determination of the Bohm coefficient C_BCyl = 1.22. The second possibility of probe diagnostics includes a method and device for evaluation of ion current density to a wall under a floating potential using a radially movable plane wall Langmuir probe simulator. This measurement in the same xenon plasma served as the basis for development of an ion source in which the given wall was represented by an ion extracting electrode of the ion extraction grid system.     

Diagnosis of a low pressure capacitively coupled argon plasma by using a simple collisional-radiative model

This paper proposes a simple collisional-radiative model to characterise capacitively coupled argon plasmas driven by conventional radio frequency in combination with optical emission spectroscopy and Langmuir probe measurements. Two major processes are considered in this model, electron-impact excitation and the spontaneous radiative decay. The diffusion loss term, which is found to be important for the two metastable states (4s[3/2]₂, 4s'[1/2]₀), is also taken into account. Behaviours of representative metastable and radiative states are discussed. Two emission lines (located at 696.5 nm and 750.4 nm) are selected and intensities are measured to obtain populated densities of the corresponding radiative states in the argon plasma. The calculated results agree well with that measured by Langmuir probe, indicating that the current model combined with optical emission spectroscopy is a candidate tool for electron density and temperature measurement in radio frequency capacitively coupled discharges.     

The Influence of Ion – Neutral Collisions in the Plasma Sheath on the Ion Current to an Electrostatic Probe: Monte Carlo Simulation

We have carried out Monte Carlo simulation of the motion of Ar⁺ ions in the space charge sheath surrounding a cylindrical Langmuir probe. From these simulations the percentage of ions crossing the sheath boundary that are collected by the probe have been determined and thus the ion currents to the probe have been calculated. It is shown that the collisions of ions with neutral helium gas atoms in the sheath increase the percentage of ions collected by the probe above that predicted by collisionless orbital motion limited current (OMLC) theory and that the exponent, χ, of the power law dependence, i₊～U, of the ion current, i₊, on the probe voltage, U_p, increases above the value 0.5 predicted by OMLC theory. The results of the simulations are compared with recent Langmuir probe measurements made in flowing afterglow plasmas.     

Energy distribution of ions in plasma formed by laser ablation of metallic Nb and Ta targets

The laser plasma plume expansion induced by UV nanosecond irradiation from metallic Nb and Ta targets has been studied using Langmuir probe time-of-flight measurements. The ion current shape was measured as a function of the laser radiation flux at various probe-to-target distances. The ion velocity distribution and the ion energy spectra have been determined by measuring the time of flight of ions reaching the Langmuir probe. A multi-mode velocity distribution of ions has been revealed. The dependence of the amplitudes of the plume energy components on the plume scatter time and on the laser radiation flux has been investigated. The experimental time-of-flight data have been compared with a Maxwell velocity distribution. The threshold values of the laser intensity were determined for different distribution modes in the ion energy spectrum.     

Transient characteristic of a nonstationary cylindrical probe with a self-consistent negative electric charge in the ionosphere plasma

The time dependence of the ion current density for a cylindrical probe with a negative self-consistent nonstationary charge in the ionosphere plasma is determined. This dependence is a transient characteristic of this probe, which is analogous to the current-voltage characteristic of the Langmuir probe.     

Collisionless Currents for Cylindric Probes in a Plasma with non-Maxwellian Distribution of the Electron Energy

The radial model developed by Allen, Boyd, Reynolds and Chen [1, 2] for the ion collection of Langmuir probes is extended to describe the case of a non-Maxwellian distribution of the electron energy. Assuming standard distributions, showing with respect to the Maxwell distribution an increasing deficite of fast electrons as the parameter k of the distribution increases, the electron-retarded current characteristic, the potential distribution and the density profiles for electrons and ions in the probe sheath, ion-current characteristics as well as graphs for the ion-density determination are calculated for different k. It is shown, that the application of the Chen model (k = 1) in plasma with k > 1 leads only to a small error in the ion-density measurement.     

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分析了朗缪尔探针在等离子体测量过程中的功率沉积密度,并初步计算了探针的温升,为强流离子源的探针诊断提供指导。     

强流离子源探针能量沉积研究

分析了朗缪尔探针在等离子体测量过程中的功率沉积密度,并初步计算了探针的温升,为强流离子源的探针诊断提供指导。     

Propagation and Diagnostic of Ion Acoustic Waves in a Low Pressure Argon Discharge

In a dc argon discharge column of pressure 5 × 10^?4 torr, ion acoustic waves are studied experimentally. Plasma parameters obtained from the phase velocity of waves are compared with the Langmuir probe measurements. The wave diagnostic method yields the electron temperature lower than the probe method by a factor of 3…?5, if the atomic argon ion is assumed as a dominant ion species. The axial profiles of ion drift velocity show the presence of ions flowing toward the anode along the potential drop in flont of it. Also it is found that the surface condition of the cathode sensitively affects the propagation of ion acoustic waves through the changes in discharge parameters.     

Experimental Study of the Creation of a Fire‐rod I: Temporal Development of the Electron Energy Distribution Function

Positive voltage steps are applied to a planar electrode (collector) immersed in a magnetized discharge plasma column with its surface perpendicular to the magnetic field lines. If the voltage step and the neutral gas pressure are high enough additional ionization occurs in front of the collector and a fire‐rod is created. Time resolved measurements of the plasma response are performed by a one‐sided plane Langmuir probe using standard boxcar technique. The temporal development of the electron energy distribution function after the application of a positive voltage step to the collector is measured by a one‐sided plane Langmuir probe. In a magnetized plasma the electron energy distribution function is proportional to the first derivative of a plane Langmuir probe characteristics. It is found that immediately (∼1 μs) after the application of a positive voltage step to the collector a short lifetime electron population is created. This electron population disappears in approximately 2 μs. It is related to the anomalously large initial electron current collected by the collector “current overshoot”. When the initial current overshoot to the collector is terminated, a high potential (anode) plasma starts to form in front of the collector if the voltage step and the pressure are high enough. The formation of the anode plasma electron population is followed experimentally.     

Excitation of the Potential Relaxation Instability in a Weakly Magnetized Discharge Plasma by a Voltage Step

An instability is triggered in a weakly magnetized discharge plasma by the application of a positive voltage step to a planar collector immersed into the plasma column with its surface perpendicular to the magnetic field lines. Time developement of the plasma density after the application of the pulse is measured by a Langmuir probe. Radial and axial velocity of the plasma density perturbation are measured. Radial velocity is consistent with the increase of the plasma potential in the current channel. Axial velocity is very high. It is interpreted as phase velocity of radial quasiperiodic motion of the plasma in and out of the current channel. Response time of the collector current to the applied voltage step is measured versus different parameters. Experimental results are in agreement with a qualitative model presented in previous work [1] where the observed instability is modeled as a two dimensional potential relaxation instability (PRI). Minor improvements of the previous model are proposed. A rarefaction pulse that moves towards the collector is found as the initial stage of the instability.     

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用线圈电流控制非平衡磁场,用汤森放电击穿形成深度自触发放电,用磁阱捕获放电形成的二次电子和导致漂移电流,形成了高功率非平衡磁控溅射放电。采用偏压为-100V相对磁控靶放置的圆形平面电极收集饱和离子电流;在距离磁控靶14cm的位置由Langmuir探针测量浮置电位;示波器测量磁控靶的脉冲电压、电流、浮置电位和饱和离子电流信号。装置的放电脉冲功率达到0.9MW,脉冲频率最大值为40Hz左右,空间电荷限制条件是控制电子电流和离子电流的主要机制。