Secondary electrons in a plasma-wall sheath

Langmuir probes are used to monitor secondary electrons in the plasma-wall sheath of a laboratory plasma. Data are presented to show that the secondaries penetrate into the plasma. RF noise measurements in the sheath are also presented.     

激波风洞设施中的等离子体包覆目标电磁散射实验研究

利用JF10高焓激波风洞设施, 进行了等离子鞘包覆目标的电磁散射测量实验. 基于矢量网络仪的步进扫频体制, 在C波段进行实验, 观测到等离子鞘对目标雷达散射截面(radar cross section, RCS)的影响. 并且, 目标散射测量值中直接体现了激波风洞的高速气流状态信息: 气流前段会造成散射回波的剧烈变化且稳定性差, 持续0.5–1 ms; 激发的等离子鞘有效持续时间仅约为2 ms, 衰减了目标RCS回波.     

Electrical characteristics of argon radio frequency glow dischargesin an asymmetric cell

Measurements of the current and voltage at both electrodes of a parallel-plate, capacitively coupled RF discharge cell (the Gaseous Electronics Conference Reference Cell) were combined with measurements of the voltage on a wire inserted into the glow region between the electrodes, for argon discharges at pressures of 1.3-133 Pa and peak-to-peak applied voltages ⩽400 V. Together, these measurements determined the RF voltage, current, impedance, and power of each sheath of the plasma. Simple power laws were found to describe changes in sheath impedances observed as voltage and pressure were varied. An equivalent circuit model for the electrical behavior of the discharge was obtained. The equivalent circuit model can be used to relate the electrical data to plasma properties such as electron densities, ion currents, and sheath widths. The results differ from models previously proposed for asymmetric RF discharges, and the implications of this disagreement are discussed     

Multislit streak camera investigation of plasma focus in thesteady-state rundown phase

The technique of multislit streak photography was applied to investigate the plasma focus in the steady-state rundown phase. This enabled the axial and radial positions, and the rundown velocity of the current-carrying plasma sheath to be measured simultaneously. From these measurements, the radial profile of the plasma sheath in the plasma focus tube could be reconstructed and the plasma current was calculated. These calculated currents represented about 70% of the total circuit current flowing out of the capacitor bank, and these agreed with earlier results which were obtained via computer simulations of the measured voltage and current waveforms     

Gamma-electron dynamics and radio frequency capacitive sheathdiagnostics

Time-resolved measurements of gamma-electrons energy spectra in low-pressure capacitively coupled RF discharge are presented. Time dependence of the sheath voltage is calculated from experimental results and compared with theoretical predictions. Good quantitative agreement is observed for collisional sheath. In low pressure discharge sheath voltage time dependence is close to [1+cos(ωτ)], which qualitatively corresponds to the theoretical results. Asymmetry of electron energy spectra for increasing and decreasing sheath voltage is observed and explained by divergence and convergence of electron trajectories in plasma (klystron-effect)     

Electric probe measurements of high-voltage sheath collapse in cathodic arc plasmas due to surface charging of insulators

High-voltage sheath dynamics near a negatively biased substrate in cathodic arc plasmas are investigated using a biased electrical probe. Since the sheath is devoid of electrons, the sheath boundary can be inferred from the position where a positively biased probe draws no electron current. The extent of the sheath is primarily dependent on the plasma density, the ion velocity and the applied voltage. Using insulating substrates, the sheath boundary eventually retracts due to a dynamic reduction in the applied voltage. This reduction is caused by positive charge accumulation on the insulator surface. The collapse time of the sheath is dependent on the plasma density and the substrate characteristics. We believe this to be the first direct observation of the reduction in the width of the high-voltage sheath when implanting an electrical insulator using plasma-based ion implantation (PBII). This information is important when determining the optimal parameters for plasma-based ion implantation of insulators. Our measurements are compared with theoretical predictions based on the Child-Langmuir equations for high-voltage sheaths. By choosing appropriate values for the secondary electron coefficient the theory could be made to fit the experimental data. A discussion of the validity of the choice of secondary electron coefficients is presented.     

Measurement of the interaction potential of microspheres in the sheath of a rf discharge

The interaction potential of two microspheres that are levitated in the sheath region of a radio frequency (rf) argon discharge is studied experimentally by analyzing their trajectories during head-on collisions. It is shown that the interaction parallel to the sheath boundary can be described by a screened Coulomb potential. Thus, values for an effective charge and a screening length can be obtained. The horizontal part of the interaction potential has been determined for several plasma conditions. There is no evidence for an attractive part in the potential within the accuracy of the present measurements and the given plasma conditions.     

Evolution of a dust void in a radio-frequency plasma sheath

The onset and growth of a dust void are investigated in a radio-frequency (rf) sheath of a capacitively coupled argon plasma. A circularly symmetric void emerges and grows with increasing rf power and pressure in the central region of the dust cloud levitating in the sheath. Experimental measurements of the void diameter are compared with the predictions of a simple phenomenological theory, based on a balance of forces on dust grains.     

The response of a microwave multipolar bucket plasma to ahigh-voltage pulse with finite rise time

A collisional model that describes the response of a microwave multipolar bucket plasma to a high-voltage pulse with finite risetime has been developed for plasma immersion ion implantation (PIII). The agreement between this model and the measurements of the sheath position and target current in a 100 mtorr helium plasma is found to be much improved when the risetime of the pulse and the ion energy distribution during the PIII process is considered     

Current-driven rotating-kink mode in a plasma column with a non-line-tied free end

First experimental measurements are presented for the kink instability in a linear plasma column which is insulated from an axial boundary by finite sheath resistivity. An instability threshold below the classical Kruskal-Shafranov threshold, axially asymmetric mode structure, and rotation are observed. These are accurately reproduced by a recent kink theory, which includes axial plasma flow and one end of the plasma column that is free to move due to a non-line-tied boundary condition.     

On the Boil-Off Time of Probe Surfaces in Plasmas

The time required for the melting of the surface of a glass-insulated probe inserted in a plasma is computed including the influence of plasma mass motion. This effect is relevant for probe measurements in moving current sheaths of electrical gas discharges. For dense plasma focus experiments, it is shown that the time of formation of a transition layer of evaporated material is shorter than the ion relaxation time in the plasma and the transit time of the current sheath by the probe position.     

Structure of a Plasma-Focus Current Sheath from Magnetic and Optical Measurements

Simultaneous measurements of the magnetic-field distribution and the light intensity emitted by the current sheath (CS) generated in a Plasma Focus (PF) device are presented. For operating deuterium pressures in the range 0.55-2.5 mbar, we derive the plasma density and the mean hydrodynamic velocity in a frame of reference fixed to the sheath, where both quantities are in agreement with the ones predicted by the strong ionizing shock theory. The electric field in the quoted frame is also deduced, and the obtained values are consistent with those reported by other authors.     

Langmuir probe in magnetized plasma: Study of the diffusion parameter

In low-temperature magnetized plasma, Langmuir probe measurements must be corrected because of the electron diffusion through the sheath, which is formed around the probe collector. The correction factor, which is called the electron diffusion or electron sink parameter, depends on many other parameters such as the probe geometry, the electron diffusion coefficient, the sheath thickness, or the potential profile through the sheath. Based on a previous work, we determine the values of this parameter under different experimental conditions and we study the effect of the electron energy, of the probe-biased voltage, and of the magnetic field intensity on this parameter. The results are compared with theoretical models published in the literature. An empirical equation is determined to fit the diffusion parameter value versus magnetic field intensity.     

霍尔推进器中磁化二次电子对鞘层特性的影响

为进一步研究霍尔推进器壁面二次电子发射对推进器性能的影响,采用流体模型数值模拟了二次电子磁化效应的等离子体鞘层特性.得到二次电子磁化鞘层的玻姆判据.讨论了不同的磁场强度和方向、二次电子发射系数以及不同种类等离子体推进器的鞘层结构.结果表明:随器壁二次电子发射系数的增大,鞘层中粒子密度增加,器壁电势升高,鞘层厚度减小;鞘层电势及粒子密度随着磁场强度和方位角的增加而增加;而对于不同种类的等离子体,壁面电势和鞘层厚度也不同.这为霍尔推进器的磁安特性实验提供了理论解释. 关键词： 霍尔推进器 磁鞘 二次电子     

Variations of Plasma Parameters and Sheath Thickness Due to the Extraction of Ion and Electron Flux in a Double Plasma Device

Influence of charged particle extraction on plasma parameters and on ion sheath has been investigated in a double plasma device. When ions are extracted from the plasma, the plasma density as well as the positive ion flux into the sheath increases. As a result a sheath contraction takes place. Again, in case of electron extraction, it is found that the plasma density as well as the positive ion flux into the sheath decreases. As a result a sheath expansion takes place. Furthermore, it is observed that the floating potential of a plate can be controlled by extracting charged particles from the plasma. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure of the charged sheath at the plasma-charged body boundary

Based on a hydrodynamic plasma model that incorporates the inertia and temperatures of the plasma components, we have analytically and numerically solved the problem on the structure of the charged sheath at the plasma-charged body boundary. We formulate a criterion for the existence of a stationary charged sheath at the plasma-charged body boundary for both charge signs and with allowance made for the inertia and temperatures of the charged plasma components. We determine the minimum and maximum electric potentials and densities of the charged plasma components in the sheath. We show that the charge profile in the sheath can form a double-layer structure in which the ion-depleted and electron-depleted sublayers are adjacent to the plasma and the charged body, respectively, with the electric potential in the sheath remaining monotonic.     

Determination of sheath parameters by test particles upon local electrode bias and plasma switching

Equilibrating gravitation by electric forces, microparticles can be confined in the plasma sheath above suitably biased local electrodes. Their position depends on the detailed structure of the plasma sheath and on the charge that the particles acquire in the surrounding plasma, that is by the electron and ion currents towards it. Bias switching experiments reveal how the charge and equilibrium position of the microparticle change upon altered sheath conditions. Above a critical bias, the particle is subject to an additional downward acceleration that cannot be explained solely by gravity and ion drag. This acceleration can be attributed to a positive charging of the particle induced by extreme out-of-equilibrium conditions of the plasma sheath in its surroundings: locally the plasma sheath can be completely deprived of electrons by means of the bias. We observe similar particle behaviors also in the afterglow of the discharge for a persisting bias voltage on the electrode: damped oscillation into a new equilibrium or (accelerated) fall according to the bias. The observed particle dynamics in locally tailored plasma sheath environments directly monitors changes in electric field structures and plasma density profiles.     

Investigations on ionic processes and dynamics in the sheath region of helium and hydrogen discharges by laser spectroscopic electric field measurements

Temporally and spatially resolved measurements of the electric field distribution in the sheath region of RF and dc discharges provide a detailed insight into the sheath and ion dynamics. The electric field is directly related to the sheath ion and electron densities, the sheath voltage, and the displacement current density. Under certain assumptions also the electron and ion conduction current densities at the electrode, the ion current density into the sheath from the plasma bulk, the ion energy distribution function, and the power dissipated in the discharge can be inferred. Furthermore, the electric field distribution can give an indication of the collision-induced conversion between different ion species in the sheath. Laser spectroscopic techniques allow the noninvasive in situ measurement of the electric field with high spatial and temporal resolution. These techniques are based on the spectroscopic measurement of the Stark splitting of Rydberg states of helium and hydrogen atoms. Two alternative techniques are applied to RF discharges at 13.56 MHz in helium and hydrogen and a pulsed dc discharge in hydrogen. The measured electric field profiles are analyzed, and the results discussed with respect to the ion densities, currents, energies, temporal dynamics and species composition. Received: 26 July 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

The Double Sheath at the Plasma‐Wall Boundary

The relation between plasma parameters determining a steady‐state charged sheath at a plasma‐wall boundary was derived and analyzed using a fluid model, which accounted for inertia and partial pressure of both plasma components. The relation generalizes the well‐known Bohm criterion and, in particular, shows that a steadystate sheath may be formed in some cases where the Bohm criterion is not satisfied. Conditions allowing formation of a double sheath structure were formulated and analyzed. It was shown that the structure may be formed only if a) the generalized Bohm criterion is satisfied, and b) the Sagdeev potential has a minimum. The double sheath structure consists of two sub‐sheath of different polarity, which depends on the wall potential polarity and the relation between the plasma component temperatures and inertia. For positive wall potential, the sub‐sheath adjacent to the plasma (the sheath edge) is enriched by negative particles, and the sub‐sheath adjacent to the wall is enriched by positive particles, while the sub‐sheath polarities are reversed for negative wall potential. An analytical theory was formulated and illustrated by numerical solutions of the Poisson equation for special cases. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sheath motion in a capacitively coupled radio frequency discharge

The sheath motion in a capacitively coupled RF discharge is highly nonlinear. The voltage waveform on a cylindrical probe placed in the sheath region is measured as a function of position and time. A circuit model of the probe-discharge system relates the observed probe voltage to the sheath motion. The equations derived from this circuit model are solved numerically with varying nonlinear sheath motions; the resulting waveforms are compared with the experimental observations to determine the actual sheath motion. The time-varying plasma potential is also determined, indirectly, from the comparison. The authors also report observation of oscillations related to the plasma frequency, whose peak harmonic component can be calculated from a single plasma model. These oscillations can be a useful plasma diagnostic for determining plasma density. The presence of these high-frequency oscillations may significantly enhance the rate of stochastic heating of electrons