Charge screening in a plasma with an external ionization source

An asymptotic theory for the screening of the electric field of a dust particle or a spherical probe in a plasma with an external steady and/or internal (proportional to the electron density) gas ionization source has been developed for the first time. It has been established that the screening of the charge of a spherical body adsorbing the charge of the incident plasma particles is described by a superposition of two exponentials with different screening constants. The two exponentials are retained even in the absence of nonequilibrium fluxes on the macroparticle and only in the special case of an isothermal plasma does the screening become Debye one. The screening length is determined by the ratio of the electron-ion, β_ei, and Langevin, β_L = 4πeμ_i (where μ_i is the ion mobility), recombination coefficients. If β_L ? β_ei, then it is much larger than the electron Debye length. The ions in an isothermal plasma have been found to give the same contribution to the screening as the electrons if the electron-ion recombination coefficient exceeds the Langevin ion recombination coefficient by a factor of 2 or more, β_ei ≥ 2β_L. The Vlasov equation is used to analyze the asymptotic behavior of the macroparticle potential in a collisionless plasma.     

等离子体中氖输运及辐射特性模拟

通过假定一定的温度和密度分布,数值求解了一维等离子体连续性方程,获得了杂质氖电离态分布及辐射功率随空间位置和时间的变化.计算结果显示,在杂质注入时间较短时,由于离子输运及各种损失机制,总杂质密度在空间分布尚未达到平衡,电离态离子主要分布在等离子体周边,完全电离离子所占份额很小.当时间达到0.2s时,氖在等离子体中完全达到平衡状态,体积辐射功率趋于一个稳定的数值.达到平衡状态后,芯部一个很大范围内氖杂质主要以Ne 10离子型态存在,辐射功率以轫致辐射为主,因此辐射功率较小.辐射功率在空间的分布随时间变化较小,主要分布在等离子体周边及边界层一个狭小的辐射带内,说明氖引起的辐射主要由低电离态离子引起.     

Coherent microwave rayleigh scattering from resonance-enhanced multiphoton ionization in argon

Multiphoton ionization and electron recombination processes are studied in argon using coherent microwave Rayleigh scattering from a localized, resonance-enhanced multiphoton ionization produced plasma. A time dependent one-dimensional plasma dynamic model is developed to predict the time evolution of the microwave scattering from the plasma. Experimental results of the argon ionization spectrum and electron recombination rates are in good agreement with the model predictions.     

氩气压力对螺旋波放电影响的发射光谱诊断及仿真研究

螺旋波等离子体源以其高电离效率与高密度优势受到多个领域的青睐。螺旋波放电高电离效率的机理或者功率耦合模式,一直是困扰该领域学者的难点之一,对于放电过程与特性的诊断则是揭示其物理机制的重要途径。光谱诊断能够克服介入式诊断手段对等离子体的干扰同时受等离子体烧蚀等弊端,且响应速度快、操作灵活。为研究螺旋波等离子体的放电特性以及气体压力的影响,开展了以氩气为工质气体的光谱实验研究,并针对实验开展了Helic程序数值模拟。通过改变光纤探头焦距调整径向诊断位置,得到谱线强度的径向分布。由氩原子4p-4s能级跃迁产生的谱线主要集中在740～920 nm区间,谱线相对强度较离子激发谱线较强。实验研究发现,在较低氩气压力范围（0.2 Pa<P_Ar<1.0 Pa）,随着压力增加,放电光强迅速增加,但是当压力增加到大于1.0 Pa之后,光强增长的趋势变缓,甚至部分谱线的相对强度不再增长,达到类饱和状态,朗缪尔探针测量得到离子密度变化趋势与其相似。光强分布在靠近径向边界处（r≈4 cm）存在凸起,且随压力增加,该凸起分布更为明显。通过对电子温度的计算发现,压力增加到一定程度将影响放电均匀性。仿真结果显示,增大压力,功率沉积密度的径向分布逐渐向径向边界处积累,与实验观察到的谱线强度径向凸起相一致,螺旋波与TG波的耦合效率增加。随着气体压力的增加,E_r的径向边界峰值降低,原因是波所受阻尼增强,TG波被有效地局限于径向较窄的边界处。电流密度轴向分量J_z在等离子体内部和边界处的峰值呈显著的减小趋势,可见,虽然压力增加一定程度上提高了等离子体密度,但却相应的减小了电离率,导致轴向电流密度受限。但是径向电流密度J_r却呈现先减小后增大的趋势,且增长幅度明显,综合来看,放电效率有所提高。可见适当增加气体压力,有助于提高放电的功率耦合效率和强度,增加等离子体密度。光强比值法是针对线性谱线参数计算的典型方法,Helic程序亦是专业领域内认可度很高的计算工具,结果可靠,分析方法具有可借鉴性。实验及仿真结果对于提高氩气工质下的螺旋波放电强度提供了一定的参考价值。     

强流电子束碰撞电离背景气体研究

利用数值计算与粒子模拟两种方法,结合实际的实验数据,对高功率微波二极管中相对论电子束与背景气体相互作用碰撞产生的等离子体密度进行了研究.研究结果表明：碰撞产生的等离子体密度数值计算结果与粒子模拟结果基本一致,背景气压在0.01 Pa—0.05 Pa时,碰撞产生的等离子体密度在4—12×10⁹cm^-3,即便在考虑电子离子复合的情况下,数值计算结果与粒子模拟结果依然符合得很好.另外,粒子模拟结果表明：随着气压的增加,等离子体密度呈现先增大再减小然后又逐渐增大的过程, 关键词： 相对论电子束 等离子体 数值计算 粒子模拟     

Recombination properties of Rydberg nonequilibrium plasma created by dye laser

Ultracold Rydberg plasma formed by a dye laser has been studied theoretically. Recombination properties in the presence of three-body collisions in hydrogen-like plasma have been investigated. We have shown that the existing calculation models for three-body recombination (Thomson model, diffusion approximation, and modified diffusion approximation) work only in the high-electron-temperature region—more than or equal to 1 eV. When the electron temperature is substantially less 1 eV, we have found the strong dependence of process probability on the energy of free particle. As a result, there is a drastic slowdown recombination in the region, where the three-body recombination dominates. Another reason for this slowdown of the recombination deals with the increment nonideality parameter. Effect of slowdown of recombination with free-body collisions can be reduced to the situation when the thermal equilibrium is set earlier than the ionization equilibrium, and, hence, the formation of metastable structure is possible in the plasma. Also this effect can cause the inverse population on Rydberg levels.     

Field-induced electron-Ion recombination: A novel route towards neutral (Anti-)matter

We present a novel approach to the preparation of neutral (anti-)matter. The scheme is based on recombination of a free electron and an ion, and can be considered as the inverse of pulsed field ionization. We have obtained promising results on rubidium: at low densities already efficiencies of 0.3% were obtained. This is orders of magnitude more than has been achieved in previous recombination studies.     

Complete numerical solution of electron-hydrogen model collision problem above the ionization threshold

Benchmark results are presented for electrons colliding with hydrogen atoms in the S-wave (Temkin-Poet) model collision problem, which neglects angular momentum. Complete results (elastic, inelastic, and ionization), accurate to 1%, are obtained by numerically integrating Schrodinger's equation subject to correct asymptotic boundary conditions. This marks the first time direct matching to asymptotic boundary conditions has been shown to yield convergent ionization amplitudes for a Coulomb three-body problem. Results are presented for impact energies of 54.4 and 40.8 eV, where comparison with other theories is available.     

The space-time-averaging procedure and modeling of the RF discharge

An approach to modeling RF discharges and the ensuing analysis of fast electron and ion motions for the case of electrode sheaths in the high-pressure RF discharge is discussed. Time-averaging over fast electron motions with the applied voltage frequency gives analytic expressions for the average electric field and average ionization density. The resulting relatively simple equations for the ion density profile describe drift, diffusion, ionization, and recombination processes. The simple scaling rules, the approximate expressions for the density profile in various regions, the sheath length, the ion density at the plasma-sheath boundary, and the dimensionless criteria for various discharge regimes can be deduced. For the non-self-sustained discharge, it is demonstrated that the ion drag towards the electrode and the diffusion results in significant lowering of the ion density in the sheath compared with the positive column at not too high a pressure. The analytic transition criterion from α to γ forms of the self-sustained discharge is obtained. The numerical solution of the averaged ion equations yields the results which nearly coincide with the results of full-scale modeling     

Mechanism of branching in negative ionization fronts

When a strong electric field is applied to nonconducting matter, narrow channels of plasma called streamers may form. Branchlike patterns of streamers have been observed in anode directed discharges. We explain a mechanism for branching as the result of a balance between the destabilizing effect of impact ionization and the stabilizing effect of electron diffusion on ionization fronts. The dispersion relation for transversal perturbation of a planar negative front is obtained analytically when the ratio D between the electron diffusion coefficient and the intensity of the externally imposed electric field is small. We estimate the spacing lambda between streamers and deduce a scaling law lambda approximately D(1/3).     

Generation of short electron bunches by a laser pulse crossing a sharp boundary of inhomogeneous plasma

The formation of short electron bunches during the passage of a laser pulse of relativistic intensity through a sharp boundary of semi-bounded plasma has been analytically studied. It is shown in one-dimensional geometry that one physical mechanism that is responsible for the generation of electron bunches is their self-injection into the wake field of a laser pulse, which occurs due to the mixing of electrons during the action of the laser pulse on plasma. Simple analytic relationships are obtained that can be used for estimating the length and charge of an electron bunch and the spread of electron energies in the bunch. The results of the analytical investigation are confirmed by data from numerical simulations.     

An overview of atomic and molecular processes in critical velocityionization

The authors present an overview of the time development of some atomic and molecular processes in critical ionization velocity (CIV). In the preonset stage, metastable states play an important role: they provide an energy pooling mechanism allowing low-energy electrons to participate in the ionization process; they may explain the low energy threshold was well as the fast time scale in the onset of CIV. For a sustaining CIV to occur, Townsend's criterion has to be satisfied. The kinetic energies of the neutrals are transformed to plasma wave energies via beam-plasma instabilities, and the plasma waves that heat the electron result in a tail formation. Excitation of neutrals with subsequent radiation is an important energy loss mechanism. Finite size also limits the instability growth rate. In the propagation of CIV, ion-molecule reactions and molecular dissociative recombination are important     

Simplified calculations of a thermionic converter in the ignited mode

The charged particle transport in the plasma of a thermionic converter in the ignited mode is treated to be due to the charged particle density gradient and the electric field. The corresponding coefficients as well as the numerical treatment of the ionization and recombination processes are taken from the literature, the latter one in a suitable approximation. The electron temperature is assumed to be uniform within the gap. Taking account of the boundary conditions for the electron and ion currents and for the flux of the kinetic electron energy analytical solutions are found whose numerical evaluations can easily be performed. To make allowance for the Schottky effect and a double sheath at the emitter surface is shown to be necessary and possible with moderate effort for the calculation of I-V curves. The validity limit of the model is discussed.     

Collisional-radiative ionization and recombination coefficients for quasi-stationary homogeneous hydrogen and hydrogenic ion plasmas

The recombination and ionization coefficients have been re-calculated on the basis of the collisional-radiative model. The numerical results obtained deviate from those published byBates, Kingston, andMcWhirter. The deviations reach a factor of about four for the recombination and a factor ten for the ionization coefficients. The smallest deviations occur in the recombination coefficients when the plasma is assumed to be optically thin in all transitions. The conditions for the applicability of the collisional-radiative model are discussed.     

Effect of electron emission on the charge and shielding of a dust grain in a plasma: A continuum theory

The continuum approximation is used to analyze the effect of electron emission from the surface of a spherical dust grain immersed in a plasma on the grain charge by assuming negligible ionization and recombination in the disturbed plasma region around the grain. A parameter is introduced that quantifies the emission intensity regardless of the emission mechanism (secondary, photoelectric, or thermionic emission). An analytical expression for the grain charge Z _d is derived, and a criterion for change in the charge sign is obtained. The case of thermionic emission is examined in some detail. It is shown that the long-distance asymptotic behavior of the grain potential follows the Coulomb law with a negative effective charge Z _eff, regardless of the sign of Z _d. Thus, the potential changes sign and has a minimum if Z _d > 0, which implies that attraction is possible between positively charged dust grains.     

Influence of atom-atom collisions on the collisional-radiative ionization and recombination coefficients of helium plasmas

Coefficients for volume recombination and ionization have been calculated for a dense helium plasma of low degree of ionization. The calculations are based on a collisional-radiative model in which electron-atom, electron-electron-ion, atom-atom, and electron-atom-ion collisions intervene. Molecular species such as He ₂ ^* and He ₂ ⁺ have not been taken into account. The essential results are: At low temperatures and high neutral gas densities the recombination coefficient is proportional to the number density of neutral helium atoms. At high temperatures the presence of neutral particles practically does not influence the recombination process compared to pure ion-electron-electron recombination. At high neutral particle densities, high atom temperatures and low electron densities the ionization process is mainly due to atom-atom collisions. In this point our calculations are in relatively good agreement with recent shock tube measurements of Kalra and Measures (Phys. Fluids14, 2544 (1971)). It is emphasized that the simple two-step model for ionization by shock waves in the noble gases should be replaced by a more general collisional-radiative model in which the atomic level structure intervenes in more detail.     

Strong Enhancement of Electron–Ion Recombination Owing to Free–Bound and Bound–Bound Resonance Transitions

The effect of free–bound and bound–bound resonance nonadiabatic transitions of an electron on electron–ion recombination rates in the plasma of a Ne/Xe and Ar/Xe inert gas mixture has been studied. A kinetic model of recombination has been proposed including energy relaxation in collisions with electrons, resonant electron capture to Rydberg states through three-body collisions of Xe⁺ ions with Ne or Ar atoms and dissociative recombination of NeXe⁺ or ArXe⁺ ions, and n → n' resonance transitions. It has been shown that effective resonance processes occurring in quasimolecular systems sharply increase both the recombination coefficient and the effect of collisions with neutral particles even at quite high degrees of ionization of the plasma.     

Single and double ionization of neon 2s- and 2p-subshells by proton and electron impact

Absolute single and double ionization cross sections of neon 2s- and 2p-subshells for proton (40–900 keV) and electron impact (0.2–10 keV) have been measured using photon spectroscopy in the spectral range of the vacuum ultraviolet. Cross sections for double ionization decrease more rapidly with increasing impact energy than cross sections for single ionization. No definite asymptotic energy dependence of a Bethe-Fano-plot could be found for double ionization in contrast to single ionization. The experimental results are compared with theoretical predictions of the shake-off model and Gryzinski's classical binary encounter theory. Better agreement is found with the latter, indicating that successive binary collisions have to be considered as a strong mechanism for double ionization by protons or electrons of the investigated energy range. Comparison is made with other experimental results for double ionization by photon impact or capture ionization by proton impact.     

微波瑞利散射法测定空气电火花激波等离子体射流的时变电子密度

大气压空气电火花激波等离子体射流的电子密度在亚微秒时间尺度上瞬变,其电子密度的测定很难.基于微波瑞利散射原理,本文测量了空气电火花冲击波流注放电等离子体射流的时变电子密度.实验结果表明:测量系统的标定参数A为1.04 × 105 V·Ω·m–2;空气流注放电等离子体射流的电子密度与等离子体射流的半径和长度有关,结合高速放电影像展示的等离子体射流的等效半径和等效长度,测定的电子密度在1020 m–3的量级,且随时间先快速增长至峰值再成指数衰减.此外,本文还探讨了等离子体射流的不同等效尺度对测定结果的影响;分析结果表明,采用时变等效半径和时变等效长度的计算结果最有效,且第1个快速波峰是由光电离的电离波导致的.