Plasma modes in low-dimensional electron system over surface of liquid helium

The collective plasma modes in a quasi-two-dimensional (Q2D) electron system located over the free surface of liquid helium are studied theoretically within many-body dielectric formalism. The dispersion of modes is considered both over bulk liquid and over helium film where the essential modification of interelectron interaction occurs due to screening effects in the substrate with a large value of dielectric constant. It is shown that the plasma spectrum consists of longitudinal and transverse branches which dispersion laws depend on the values of the dielectric constant of helium and the film thickness. For the helium film covering metal, the longitudinal mode is acoustic differing of that for the surface electron (SE) system over bulk helium.     

Electron-ion recombination in a helium plasma produced by laser

Experimental investigation of the recombination coefficients in a helium plasma produced by laser is reported. The relative contribution of electronic collision processes and of atomic collision processes on the values of the recombination coefficients are pointed out according to the value of the neutral pressure of helium.     

Influence of electron energy distribution on helium recombining plasma diagnostics using line emissions

The influence of electron energy distribution on helium recombining plasma diagnostics is investigated using a helium collisional‐radiative model. The population densities of excited helium atoms are calculated for Maxwellian and non‐Maxwellian distribution plasma cases. In the case of the Maxwellian distribution plasma, the electron temperature and electron density determined by the Boltzmann plot method agree well with the input plasma parameters. On the other hand, it is indicated that the electron temperature and electron density are significantly underestimated in the bi‐Maxwellian distribution plasma case, even though the density of the hot electron components is three orders smaller than that of the bulk electrons. This result indicates that in a non‐Maxwellian helium recombining plasma, evaluation of the particle balance based on line emissions from excited helium atoms would be difficult because the reaction rate of atomic and molecular processes is strongly dependent on the electron temperature and density.     

利用介质阻挡探针法测量大气压氦等离子体射流电子密度

分别利用电子的漂移速度和等离子体的传播速度计算了大气压下氦等离子体射流的电子密度。     

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The electron densities in the atmospheric pressure helium plasma were calculated by means of electron drift velocity and the jet velocity respectively. The electron velocity and jet velocity can be calculated by means of helium plasma jet current measured by a dielectric probe and plasma discharge current signal measured by voltage probes. The results show that the estimated electron densities of the helium plasma jet calculated from electron drift velocity and the jet velocity are in the order of 10 ¹¹ cm ^-3 and they increase with applied voltage. There is a little fluctuation in the value of the electron density along the jet axis of the plasma. This result is the same as the measured electron density in atmospheric pressure helium non-thermal plasma jet by using a Rogowski coil and a Langmuir probe. This is in one order lower than the electron density measured by microwave antenna.     

Probe Measurements of Wave Fluctuations Excited in Glow Discharges

Fluctuations of the electric field, the charged particle density, the electron temperature and the plasma potential are simultaneously measured by a probe in the positive column of helium glow discharge at a few torr gas pressure by exciting a small amplitude ionization wave. It is proposed that these values can be determined by analyzing the probe current.     

Probe Measurements of Wave Fluctuations Excited in Glow Discharges

Fluctuations of the electric field, the charged particle density; the electron temperature and the plasma potential are simultaneously measured by a probe in the positive column of helium glow discharge at a few torr gas pressure by exciting a small amplitude ionization wave. It is proposed that these values can be determined by analyzing the probe current.     

Spherically confined two-electron atoms immersed in Debye plasma

The effect of spatial confinement in the presence of plasma environment on the dipole allowed transition properties for the first five helium-like atoms (He through C⁴⁺) is studied. The Debye screening describes the effect of plasma. Additionally, the effect of a spatial confinement (as, for example, it is in the case of atoms surrounded by liquid helium) is described by locating the atom in the center of a spherical box. Time-dependent variation perturbation theory is applied to estimate the orbital energies, polarizabilities, transition energies, oscillator strengths, and transition probabilities. The effect of spatial confinement in the plasma environment shows interesting features. The decrease of the radius of confinement exerts pressure within the system and produces instability in the energy values ultimately ionizing the system. The ionization pressures at different plasma screenings are estimated. The ionization pressure at zero Debye screening for helium agrees with existing data.     

Plasmon production in a plasma by a Raman process. Threshold of stimulation of the plasmons. Application to a helium plasma

An investigation of the plasmon production by Raman conversion of laser light is reported. The threshold laser power for plasmon amplification is derived from a balance the gain process (Raman conversion) and the loss processes (Landau and collisional dampings). Application to a helium plasma provides threshold values for visible and infrared laser radiation near some He(I) forbidden transitions.     

Some notes on the role of meta-stable excited state of helium atom in laser-induced helium gas breakdown spectroscopy

A study of the experimental results on the plasma emissions of water and ethanol vapor samples, induced by Nd:YAG laser in ambient helium and nitrogen gases at atmospheric pressure, is presented here. The result reveals distinct geometrical and spectral characteristics of the plasma emissions generated in the helium gas when compared to those observed from nitrogen gas plasma. Most remarkable is the narrow line width and low continuum background exhibited by emission lines of the analyte atoms from helium plasma, including the hydrogen emission line which is known to suffer from notorious broadening effects in conventional laser induced breakdown spectroscopy (LIBS). It is further shown on the basis of the measured spatial distributions and time profiles of the emission intensities, that the excellent spectral quality is attained by taking advantage of the meta-stable excited state of helium atoms for the delayed excitation of the hydrogen and other analyte atoms, this allows the detection of those atomic emissions to be performed under more favorable conditions. The result of this study has thus demonstrated the feasibility of achieving high-quality spectrochemical analysis, including hydrogen analysis with laser-induced helium gas breakdown spectroscopy. PACS 52.38.Mf     

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采用另加偏压的单阴极弧氦放电直线等离子体装置对氦等离子体的基本特性进行了研究.对氦轴向输运规律做了描述并与光谱测量数据做了定性地比较.实验结果表明,氦等离子体的电子温度与电子密度均随放电电流、约束磁场的增加而增加.氦原子与氦离子的辐射光谱随放电电流、偏压、磁场的变化规律进行了测量分析,同时氦离子对钨靶积分辐照效应进行了观察.这些结果不但提供了氦等离子体的基本特性,对于研究氦离子与面向等离子材料相互作用导致产生气泡、肿胀、脆化损伤等的评估,特别是对将来伴有(n,α)反应时具有一定的参考价值.     

大气压低温氦等离子体射流的诊断研究

为了加快低温氦气等离子体射流的工程化进程,通过自主设计的同轴式介质阻挡放电等离子体射流发生器,在放电频率10 kHz,一个大气压条件下产生了稳定的氦气等离子体射流。通过分析不同工况下的电压电流波形可以发现单纯增加氦气体积流量只能小幅的增加电流脉冲幅值,而对放电时间、电流脉冲数的影响不大。增加放电峰值电压时电流脉冲幅值会得到较大幅度增加。通过发射光谱法对大气压氦气等离子射流的活性粒子种类、电子激发温度、电子密度进行了诊断。结果表明,大气压氦气等离子体射流中的主要活性粒子为He Ⅰ原子、N₂第二正带系、N+₂的第一负带系、羟基（OH）,H原子的巴尔末线系（H_α和H_β）与O原子,这表明虽然该试验中使用的氦气纯度已达99.99%,但其中仍残留有少量的空气,同时放电时大气中的空气会被卷吸到放电空间发生电离。还可以发现,主要活性粒子的相对光谱强度随氦气体积流量的增加及放电峰值电压的增大均呈现上涨的趋势。选用He Ⅰ原子的四条谱线对不同试验工况下的电子激发温度进行了计算,得到大气压氦气等离子体射流的电子激发温度在3 500~6 300 K之间,电子激发温度随放电峰值电压与氦气体积流量的增大总体上呈现上升的趋势。但由于反向电场的存在,某些峰值电压可能会出现电子激发温度下降的情况;根据Stark展宽原理对大气压氦气等离子体射流的电子密度进行了计算,发现电子密度的数量级可达1015 cm-3,同时增大峰值电压与氦气体积流量均可有效的提高射流中的电子密度。这些参数的研究对氦气等离子体射流在工程实际中的应用具有重要意义。     

Long‐Time Evolution of a Low Pressure Laboratory Plasma after Application of Transient high Voltage Positive Pulses

The long‐time evolution of weakly‐collisional plasma with application of high voltage positive pulses to an electrode immersed in plasma, with pulse widths less than as well as more than ion plasma periods, is studied. The plasma is produced by electron impact ionization of argon or helium gas, where electrons are coming out from dc biased hot thoriated tungsten filaments. It is observed that during the temporal evolution of argon plasma, a beam component exists along with temporal bulk electrons giving rise to a double hump profile of transient Electron Distribution Function (EDF). However, in the case of temporal evolution of helium plasma, only a bulk electron population is present. The obtained results are explained by understanding the role played by thermionically emitted electrons during the plasma evolution, the role of the difference of ionization rates of helium and argon, and the higher temporal plasma potential. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

级联弧放电氦等离子体特性研究

采用另加偏压的单阴极弧氦放电直线等离子体装置对氦等离子体的基本特性进行了研究。对氦轴向输运规律做了描述并与光谱测量数据做了定性地比较。实验结果表明,氦等离子体的电子温度与电子密度均随放电电流、约束磁场的增加而增加。氦原子与氦离子的辐射光谱随放电电流、偏压、磁场的变化规律进行了测量分析,同时氦离子对钨靶积分辐照效应进行了观察。这些结果不但提供了氦等离子体的基本特性,对于研究氦离子与面向等离子材料相互作用导致产生气泡、肿胀、脆化损伤等的评估,特别是对将来伴有(n, α)反应时具有一定的参考价值。     

同轴空心阴极氦等离子体的电子激发温度研究

利用同轴空心阴极放电装置,产生氦低温等离子体。通过对等离子体的发射光谱进行测量和计算,研究放电功率以及氦气压强对等离子体的电子激发温度的影响。结果表明:氦低温等离子体的发射光谱主要由连续谱和原子谱线构成,放电功率和压强对谱线的强度具有明显影响。压强的变化不仅影响电子从电场中获得的能量,还会影响电子与原子的碰撞频率,从而导致电子激发温度随着氦气压强的增大,出现先上升后下降的变化趋势。     

Generation and investigation of a parallel-plate DBD driven at 1.6 MHz with flowing helium

The development of a new driven circuit that can drive a parallel-plate dielectric barrier discharge with flowing helium at atmospheric pressure is presented. The operating principle is based on the capability of a Tesla coil to generate high voltages with high oscillating frequencies. The discharge is generated inside a one open end parallelepipedic plasma chamber made of glass, using two outside parallel-plate electrodes. Depending on the helium flow-rate the plasma has three developing stages. In the homogeneous developing stage (1 l/min helium flow-rate) the plasma is characterized by using an electric model and based on its emission spectrum.     

环境气氛对激光微等离子体辐射强度的影响

采用YJG-Ⅱ激光微区分析仪、组合式多功能光栅光谱仪和CCD数据采集处理系统构成的激光微等离子体光谱分析系统,以国家标准土壤样品(BGW07411)为样品,在Ar,He和不同He-Ar混合的环境气氛下,以Ca Ⅱ 393.367 nm,Ca Ⅱ 396.847 nm为分析线,实验研究了土壤激光微等离子体辐射强度。研究结果表明,He-Ar混合气氛环境等离子体发光时间、辐射强度均好于单一He,Ar环境气氛。当He-Ar混合气体分别为：He 66.7%,Ar 33.3%时,等离子体辐射强度明显增强,并在此条件下研究了辅助电极高度对激光微等离子体辐射强度的影响。当辅助电极高度为3 mm时,激光微等离子体的辐射强度达到最佳。     

The impact of the Ramsauer effect on the frequency of elastic collisions in inductive RF discharges in inert gases

This paper presents the results of investigating the power absorption mechanism of an inductive RF discharge plasma. Dependences of the frequency of elastic electron collisions with inert gas atoms (helium, neon, argon, and krypton) on the pressure are given. In the frequency range of 3 × 10⁶–3 × 10⁷ s^?1, an equivalent plasma resistance and the power input into the plasma are determined by the values of collision frequency and electron density within a skin layer and do not depend on the type of gas within the limits of experimental error. Upon reaching the electron temperature of ～1 eV, the energy of the main part of electrons lies in the range of Ramsauer’s minimum for elastic cross section. This leads to a decreasing elastic-collision frequency in heavy inert gases as compared to helium.     

Temporal evolution of plasma from a highly ionized helium capillarydischarge

The generation of a highly ionized helium capillary plasma and the study of its temporal evolution are discussed. A 30-cm-long and 1-mm-diameter helium plasma was created with well-terminated kiloamp current pulses of 90-ns full width at half-maximum (FWHM). Emission spectroscopy was used to study the recombination of totally stripped ions into hydrogenic helium ions and to measure the evolution of the plasma density from the Stark broadening of HeII transitions. A 1.2-kA discharge current pulse was observed to create a plasma density of 8×10¹⁶ cm^-3 in 1 torr of helium. The maximum intensity of HeII transitions occurs in the afterglow of the discharge pulse, following the collisional recombination of totally stripped ions with plasma electrons when the plasma cools. The study is of interest in relation to the possibility of obtaining amplification in the 164.0-nm line of HeII in a capillary discharge     

氦气对MPCVD沉积金刚石的影响

为了确定添加氦气对微波等离子体化学气相沉积(MPCVD)金刚石膜的影响,采用发射光谱法(OES)在线诊断了CH₄-H₂-He等离子体的发射光谱特性,研究了He对等离子体内基团空间分布的影响;并利用扫描电子显微镜(SEM)和拉曼(Raman)光谱对不同He体积分数下沉积出的金刚石膜进行了表征。结果表明：随着He体积分数的增加,等离子体内H_α, H_β, H_γ, CH和C₂基团的谱线强度均呈上升趋势,其中H_α基团的谱线强度增加最大。光谱空间诊断发现He的加入导致等离子体中各基团的空间分布均匀性变差,造成沉积出的金刚石膜厚度极不均匀。沉积速率测试表明,He的加入导致碳源基团相对浓度增加,有利于提高薄膜的沉积速率,当He体积分数由0 vol.%增加至4.7 vol.%时,沉积速率提高了24%。SEM测试结果表明,随着He体积分数的增加,金刚石膜表面形貌由(111)晶面取向向晶面取向混杂转变,孪晶生长明显。高He(4.7 vol.%)体积分数下由于C₂基团的相对浓度较高,导致二次形核密度增加。此外,由于基片台受到等离子体的刻蚀和溅射作用,导致薄膜沉积过程中引入了金属杂质原子。二次形核和杂质原子的存在使得孪晶大量的产生,薄膜呈现出压应力。