Relaxation to nonequilibrium in expanding ultracold neutral plasmas

We investigate the strongly correlated ion dynamics and the degree of coupling achievable in the evolution of freely expanding ultracold neutral plasmas. We demonstrate that the ionic Coulomb coupling parameter Gamma(i) increases considerably in later stages of the expansion, reaching the strongly coupled regime despite the well known initial drop of Gamma(i) to order unity due to disorder-induced heating. Furthermore, we formulate a suitable measure of correlation and show that Gamma(i) calculated from the ionic temperature and density reflects the degree of order in the system if it is sufficiently close to a quasisteady state. At later times, however, the expansion of the plasma cloud becomes faster than the relaxation of correlations, and the system does not reach thermodynamic equilibrium anymore.     

超越平均原子模型模拟高温高密等离子体离子布居

从实际应用的角度出发，常采用平均原子(AA)模型进行简化处理.为了能更精确计算高温高密等离子体的离子布居，提出了一套超越AA模型的方法.该方法能够较好地处理局域热动平衡(LTE)等离子体离子布居，也可有效地处理非局域热动平衡(n-LTE)等离子体离子布居. 关键词： 平均原子(AA)模型 离子布居 非局域热动平衡(n-LTE)     

Experimental realization of an exact solution to the Vlasov equations for an expanding plasma

We study the expansion of ultracold neutral plasmas in the regime in which inelastic collisions are negligible. The plasma expands due to the thermal pressure of the electrons, and for an initial spherically symmetric Gaussian density profile, the expansion is self-similar. Measurements of the plasma size and ion kinetic energy using fluorescence imaging and spectroscopy show that the expansion follows an analytic solution of the Vlasov equations for an adiabatically expanding plasma.     

Electron and ion mobilities in argon plasma with fluorine as additive

Results of the numerical evaluation of the electron and ion mobilities in plasmas from mixtures of argon and fluorine are given. The temperature dependence of the number densities of the components and their mobilities as a function of temperature for low-pressure (from 0.1 kPa to 1.0 kPa) and low-temperature (from 500 K to 5000 K) argon plasmas with 20% and 30% of added fluorine are evaluated. It is assumed that the system is kept under constant pressure and that a corresponding state of local themodynamical equilibrium is attained in it. The previously derived expression for the modified Debye radius, offering the possibility to treat the plasmas as weakly non-ideal in the whole temperature range, is used and the cut-off at the Landau length rather than at the smallest ionic radius is introduced. This alteration in the evaluation procedure gives different changes in the final numerical results for all the relevant quantities. It was found that the equilibrium plasma composition depends considerably on the presence of fluorine in the whole temperature range for both the pressures; the addition of fluorine significantly enhances the number densities of all relevant charged constituents. This fact influences the electron and ion collision frequencies as well as their mobilities.     

Effective statistical weights of bound states in plasmas

Based on recent advances in the study of the statistics of interparticle distances and angles in plasmas, we develop an approach for the determination of the effective statistical weights of atomic (ionic) quantum states in ideal and nonideal plasmas. This approach allows one to account naturally for the effects of both the perturbation of the bound states by the neighboring ions and for the binding energy reduction due to the screening of the Coulomb interaction. We analyze the roles of tunneling and overbarrier escape of the optical electron from the parent ion potential well. The effects of neighbor ions and free electrons on these processes, and the simultaneous presence of several perturber ion species in the plasma are treated. We show that the present approach offers significantly more accurate effective-statistical-weight values in comparison to the existing theoretical treatments, and yields physical expressions for the empirical factors of the existing theories. Examples of calculations of effective statistical weights are given. The effects of the atomic (ionic) states collectivization on the collisional-radiative kinetics of dense plasmas are discussed. Received 10 August 2001     

The onset of rotational motion of dusty plasma structures in strata of a glow discharge in a magnetic field

An interpretation is given to the previously observed action of a magnetic field on the state of a dusty plasma structure in strata of a glow discharge. The conditions of previous experiments are analyzed, in which a nonuniform rotation and a change in the degree of order of a dusty plasma structure (the translational order), as well as a correlation between them, were revealed. Based on this analysis and on data in the literature on dusty plasmas in a magnetic field, a hypothesis is made that the reason for the rotation of the structure is the ion drag force. Additional experiments on the observation of the onset of rotational motion of a structure in “weak” and “strong” magnetic fields are conducted. It is shown that rotation reversal (and rearrangement of the order of the structure) is caused by changes in the direction of ionic flows—from internal regions of the structure to its periphery and vice versa—in the weak and strong magnetic fields. The results obtained agree qualitatively with the hypothesis adopted, as well as with the data of the two-dimensional theory of strata.     

稀疏膨胀过程中几何位形对于电子非局域热传导的影响

发展了考虑一维柱对称、球对称位型下流体演化的Fokker-Planck程序, 在流体力学极限下对程序进行了校验. 利用程序模拟研究了球对称位型、平板位型下等离子体在自由稀疏演化过程中电子热流的非局域热输运行为, 分析了几何位型对电子非局域热传导的影响. 非局域卷积理论的计算研究发现, 稀疏过程中空间的几何效应会减小外向电子热输运的非局域性.     

Time-dependent nested-well plasma trap

When arranged in a nested-well configuration, plasma traps of the Penning/Malmberg type become potentially suitable for confining neutral plasmas. The nested-well configuration allows confinement of overlapping electron and ion plasmas. With an ion plasma overlapped by an equal-charge-density electron plasma, the ion density can exceed the severe ion density limit which occurs within traditional single-well plasma traps. With a nested-well configuration which is time dependent, overlapping electron and singly charged-ion plasmas can be confined at the same temperature. Possible uses of thermal neutral plasmas confined within time-dependent nested-well plasma traps include basic plasma science studies and plasma processing applications     

Comparative Study on Excitation Mechanism of Chromium Emission Lines in Argon Radio‐Frequency Inductively‐Coupled Plasma,Nitrogen Microwave Induced Plasma,and Argon or Nitrogen Glow Discharge Plasmas

Boltzmann plots of both atomic and ionic chromium emission lines are investigated to compare the excitation mechanisms in four different plasmas: an argon inductively‐coupled plasma (Ar‐ICP), a nitrogen high‐power microwave induced plasma (N₂‐MIP), an argon glow discharge plasma (Ar‐GDP), and a nitrogen glow discharge plasma (N₂‐GDP). The plots of the atomic lines and the ionic lines give both linear relationships as well as similar excitation temperatures in the case of the Ar‐ICP, the N₂‐MIP, and the N₂‐GDP. It implies that a thermodynamic process such as electron collision would control their excitations. However, only in the case of the ionic‐line plot in the Ar‐GDP, a departure from linear relationship is observed and the estimated excitation temperature is rather higher than that with the atomic lines, meaning that a specific excitation mechanism exists in the Ar‐GDP. A possible explanation for these results is that a charge‐transfer collision between chromium atom and argon ion plays a dominant role in exciting highly‐lying energy levels of chromium ion, especially in the Ar‐GDP.     

Influence of ion-beam plasma on ion extraction efficiency in a single-grid ion source

The influence of ion-beam plasma on ion extraction efficiency in a single-grid ICP ion source is researched. The single-grid ion source is considered as a system with two plasmas, ion-beam plasma and the source plasma, separated by an extraction grid. Results of experimental measurements of the potentials of the two plasmas and the ion beam current dependence on these potentials are presented. It is shown that the ion extraction efficiency depends equally on both the acceleration potential and on the potential of the ion-beam plasma. The obtained results demonstrate the key role of the ion-beam plasma in the ion source operation, which is important in technological applications and space thrusters.     

Fe and Fe+2%Si targets as ion sources via UV laser ablation plasma

In the last years the ion component of a laser-produced plasma has been considered and studied as an object to provide high-density ion sources, which can be applied in many fields such as laser-induced implantation. In this work a KrF laser beam of 10⁸ W/cm² irradiance was focused onto single-crystalline Fe and single-crystalline Fe with 2% of Si targets and the characteristics of both free expanding laser-produced plasmas were compared. The time-of-flight (TOF) method was applied to determine the ion charge yield at various laser fluences and the ion angular spread. The analyses of TOF spectra showed a synergetic effect of the silicon admixture in target material on the Fe ions production. Besides, this admixture was also responsible of the increasing of the plasma temperature which corresponds in turn to the increasing of the average kinetic energy of the particles as well as of the more collimated ion distribution.     

Implementation and assessment of an extended hydrogenic molecular model in UEDGE

Two sets of pure deuterium plasmas are simulated using UEDGE, one including atoms and molecules and the other including atoms only as separate fluid species. Simulation results in one and two dimensions are reported to assess the role of molecules in tokamak plasmas. It is shown that thermal coupling of the molecules to the plasma can be relevant for the onset of detachment under highly collisional conditions in simple geometries, but play a modest role at low collisionality. Ion-molecule equipartition presents an additional ion energy loss channel, dissipating ion energy that would otherwise heat the electrons by equipartition. The resulting reduced electron temperature increases the ion-electron recombination by an order of magnitude as the plasma density is increased, yielding earlier and deeper detachment.     

Ion charge state distributions of alloy-cathode vacuum arc plasmas

Ion charge state distributions (CSD's) of alloy-cathode vacuum arc plasmas have been calculated under the assumption that thermodynamic equilibrium is valid in the vicinity of the cathode spot, and equilibrium CSD's “freeze” when the plasma is rapidly expanding. In this way, experimental data of titanium-hafnium alloy-cathode plasmas have been simulated using a system of Saha equations generalized for multiple elements. It was found that the CSD's of titanium and hafnium freeze approximately at the same plasma temperature and density. The freezing parameters depend slightly on the plasma composition. For the example considered, freezing occurs at temperature of T=3.0-3.8 eV and a total heavy particle density of order 10²⁵m^-3     

Recombination effects during the laser-produced plasma expansion

The precise characterization of plasmas generated by laser irradiation is needed for the development of ion sources. As the characteristic parameters of the expanding plasma vary with both the distance and the time, an experimental study of their evolution is appropriate for a deeper knowledge of the plasma. The purpose of this work is to study the same characteristics of laser plasma produced by ablation of a pure Cu target as a function of the distance from the target along the propagation axis of the plasma plume. We irradiated the target by a KrF laser and a lens of 15 cm focal length. As the diagnostic system, a small Faraday cup array and an axial Faraday cup were utilized to study the spatial variation in the total charge carried by plasma ions. Charge loss during the plasma expansion was observed due to the recombination of charged species, which occurs within a critical distance, relatively close to the target, where the plasma density is high enough. The critical distance was determined for different laser fluences; beyond the critical distance, the collisions among plasma particles are negligible and the ion charge remains frozen. It was observed that the critical distance increases as the laser fluence increases.     

Diagnosis of laser produced plasma with charge collectors

Laser produced expanding plasmas exhibit appreciable spatial charge separations as measured by a low impedance Faraday cup. This can result from energetic electrons drifting with the positive ions. A Faraday cup collector with a magnetic filter in front is shown to measure the correct ion component as compared to other charge collector configurations.     

Advance in diagnostics for high-temperature plasmas based on the analytical result for the ion dynamical broadening of hydrogen spectral lines

It is well known that the ion dynamics is very important for Stark broadening of spectral lines in high-T plasmas. However, it is usually assumed that with the increase of the plasma density N and/or of the principal quantum number n of the upper level of the radiator, the ionic contribution to the impact width (ICIW) tends to zero. In distinction to that paradigm, by finding an analytical result for the ion dynamical broadening of hydrogen spectral lines, we show here that with the increase of N and/or n, the ICIW does not decrease. Moreover, for practically important ranges of T, N, and n, this "residual" ICIW, being virtually independent of n, can be comparable to the standard electron impact width. This result leads to: (i) a substantial revision of the past of diagnostic conclusions for a variety of high-T plasma experiments; (ii) a much better possibility to deduce from experimental, Stark-broadened line profiles not only the plasma density but also the plasma temperature; (iii) a significant enhancement of the accuracy of N and T obtained from experimental line profiles; and (iv) a substantial revision of simulation models for the ion dynamical contribution that were based on a wrong assumption that the latter vanishes under increasing N and/or n. The consequences are especially important for tokamak plasmas, where the diagnostics based on experimental profiles of high Balmer and Paschen lines is frequently used.     

共振线俘获对碰撞激发X光激光增益特性的影响

以类氖-锗离子为例,用逃逸概率方法研究了共振线俘获效应对电子碰撞激发X光激光增益特性的影响。对波长为19.6nm,23.2nm和23.6nm等三条激光线,讨论了增益特性对增益区宽度ΔR和介质速度梯度dV/dZ的依赖关系.     

短密度梯度尺度等离子体中的软X射线激光

通过数值求解等离子体流体力学方程组，研究了短脉冲激光打靶结束后形成的短密度梯度尺度等离子体的冷降温过程，讨论了热传导，膨胀作功，辐射损失三种冷却机制对降温过程的贡献，并用解流体力学方程组得到的等离子参数计算ＭｇＸＩ等离子体中波长小于２０ｎｍ的所有可能跃迁的自发辐射放大增益系数，分析了它们与环境条件的关系，指出了在ＭｇＸＩ等离子体中获取水窗附近乃至水窗波段软Ｘ射线激光的可能性。     

Mean-field theory of critical phenomenon for mutually repelling particles in complex plasmas

A mean-field theory of criticality for charged particles in complex plasmas is proposed. It is shown that the existence of the critical point and the liquid-vapor coexistence is fully consistent with a purely repulsive potential between particles; the cohesive field due to the plasma background drives these. The critical exponents, calculated by expanding the free energy near the critical point, are found to be classical. The phase coexistence curve, obtained by minimizing Gibbs potential, is similar to that of other mean-field models, e.g., van der Waals fluids, ionic fluids, etc. These results lend support to the concept of "universality" in widely different systems.     

Neutron Generation and Kinetic Energy of Expanding Laser Plasmas

We investigate the kinetic energy of expanding plasma of a solid target heated by a ultra-short and ultra-intense laser pulse and the efllciency of energy coupling between the ultra-intense laser pulse and the solid target, in order to increase the utilization ratio of laser energy and to raise the neutron generation farther. Some new ideas about improving the energy utilization by head-on collision~, between the expanding plasmas are proposed. The significance is the raise of generation of shorter duration neutron, of the order of picoseconds, which allows for an increase of energy resolution in time-of-flight experiments and also for the investigation of the dynamics of nuclear processes with high temporal resolution.