低能He~(2+)与He原子碰撞转移电离出射电子能谱研究

利用反应显微成像谱仪对70和100keV He2+与He原子碰撞转移电离(TI)过程中不同出射角度的电子能谱进行了测量,观测到出射电子能谱具有如下分布特征:出射电子速度分布介于0和入射离子速度vp之间;在不同出射角度电子能谱分布均有一极大值存在,随着出射角度的增大,能谱分布极大值逐渐减小;当电子出射角度等于45°时,多数电子集中在0eV附近。上述特征可由低能离子-原子碰撞"准分子"模型进行定性解释。在100keV He2+-He转移电离出射电子能谱中有靶电子被俘获至散射离子连续态(electron capture to continuum,简称ECC)电子的贡献,这可看做是动力学两步过程的作用。     

Expansion Model of the Neutral Component in Vacuum Arc Spot Plasmas

Plasma generated by cathode spots in vacuum arcs expands towards the anode and the walls. In spite of a high degree of ionization (mostly), this plasma contains also a small but noticeable constituent of neutral atoms. Starting from the known analytical solution of a hydrodynamic model describing the plasma ions and electrons, the corresponding motion of neutrals is treated in a similar way. The results show: 1) The neutrals take part in the general plasma expansion; however, their final kinetic energy is lower (by about 50%) than the final energy of the ions. 2) The temperature of the neutrals also is lower than the ion temperature and decreases during expansion. 3) The acceleration of neutrals is caused partly by the pressure gradient, partly by interaction with the ions (friction); the forces diminish rapidly with increasing distance. Finally, the limitations of this approximate solution are discussed.     

Simulation of helium supersonic molecular beam injection in tokamak plasma

To study helium(He) supersonic molecular beam injection(SMBI) into H-mode tokamak plasma, a simplified multicomponent-plasma model under the assumption of quasi-neutral condition is developed and implemented in the frame of BOUT ++. The simulation results show that He species propagate inwards after He SMBI, and are deposited at the bottom of the pedestal due to intensive ionization and weak spreading speed. It is found that almost all injected helium particles strip off all the bounded electrons. He species interact intensively with background plasma along the injection path during He SMBI, making deuterium ion density profile drop at the He-deposited location and resulting in a large electron temperature decreasing, but deuterium ion temperature decreasing a little at the top of the pedestal.     

ELECTRON MOMENTUM SPECTRA OF EXCITED He(2~1S) AND He(2~3S)

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Local spin polarization at surfaces probed by hollow atoms

The relaxation of hollow atoms produced by slow multiply charged ions impinging on surfaces produces characteristic Auger electron spectra. These spectra, which serve as fingerprints of the interaction, can be used to probe local spin ordering at surfaces by relating changes in the intensities of different spin states to local spin polarization at the surface. The area from which the electrons are captured is of the order of a few Angstrom(2), only. The potential of the method is illustrated by He(2+) and N(6+) ions interacting with a ferromagnetic Ni(110) crystal. From the Auger spectra we determine a spin polarization of approximately 90% at room temperature.     

Observation of conduction electrons in Sb-implanted SnO2 by ultraviolet photoemission spectroscopy

Conduction electrons introduced by 90 keV Sb ion implantation into SnO₂ thin films have been observed in photoemission spectra excited with He(I) (hv = 21.2 eV) radiation. The carrier concentration is smaller than expected from the surface Sb concentration seen in XPS. This is discussed in terms of trapping of electrons in Sb(III)-like lone pair states associated with surface Sb ions.     

Wavelength dispersive measurements of characteristic K X-rays of an ECR krypton plasma

X-rays from different transitions of the K series were used to analyze components of the ion charge state distribution inside a microwave heated krypton plasma. It is shown that X-ray transitions involving outer-shell electrons show spectra with clearly resolvable satellite lines arising from low charged ions. In contrast to the evaluation of K_α spectra this allows to get information about the ion charge state distribution in the low ionization region. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evidence for Pauli exchange leading to excited-state enhancement in electron transfer

Single- and double-electron transfer to autoionizing 1s2l2l(') configurations in fluorine ions have been investigated for 1.1 MeV/u collisions of F7+ and F8+ with He and Ne. The resulting Auger electron emission spectra show anomolously large intensities for the formation of the metastable 1s(2s2p 3P) 4P state compared to the similarly configured 1s(2s2p 3P) 2P- and 1s(2s2p 1P) 2P+ states. The large 4P intensity, which cannot be explained on the basis of spin statistics, is attributed instead to the Pauli exchange of similarly aligned electrons.     

类Li氧等离子体光谱模拟

 在FAC程序包中碰撞辐射模型的基础上,模拟出了L壳层的类Li氧等离子体的X射线辐射光谱,其中包括了单、双和三离子模型。经过分析得出,除了碰撞激发以外,级联效应和其它的动力学过程(如：碰撞电离、双电子复合、辐射复合以及共振激发等)对光谱的贡献都是不能被忽略的。并分析了类Li氧等离子体各动力学过程与温度和光谱相对强度之间的关系,分析结果表明X射线光谱的强度能及时响应等离子体温度的变化。     

A study of density in electron-cyclotron-resonance plasma

A theory is developed for the density profile of low temperature plasmas confined by applied magnetic field and an experiment of the electron-cyclotron-resonance (ECR) plasma is conducted to compare the theoretical prediction and experimental measurements. Due to a large electron mobility along the magnetic field, electrons move quickly out of the system, leaving ions behind and building a space charge potential, which leads to the ambipolar diffusion of ions. In a steady-state condition, the plasma generation by ionization of neutral molecules is in balance with plasma loss due to the diffusion, leading to the electron temperature equation, which is expressed in terms of the plasma size, chamber pressure, and the ionization energy and cross section of neutrals. The power balance condition leads to the plasma density equation, which is also expressed in terms of the electron temperature, the input microwave power and the chamber pressure. It is shown that the plasma density increases, reaches its peak and decreases, as the chamber pressure increases from a small value (0.1 mTorr). These simple expressions of electron temperature and density provide a scaling law of ECR plasma in terms of system parameters. After carrying out an experimental observation, it is concluded that the theoretical predictions of the electron temperature and plasma density agree remarkably well with experimental data     

Ionization Potential Depression in Hot Dense Plasmas Through a Pure Classical Model

The ionization potential of an ion embedded in a plasma, lowered due to the whole of the charged particles (ions and electrons) interacting with this ion, is the so‐called plasma effect. A numerical plasma model based on classical molecular dynamics has been developed recently. It is capable to describe a neutral plasma at equilibrium involving ions of various charge states of the same atom together with electrons. This code is used here to investigate the ionization potential depression (IPD). The study of the IPD is illustrated and discussed for aluminum plasmas at mid and solid density and electron temperatures varying from 50eV to 190eV. The method relies on a sampling of the total potential energy of the electron located at an ion being ionized. The potential energy of such electron results from all of the interacting charged particles interacting with it. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

超短脉冲强激光场中电子剩余能计算

为了控制光场感应电离等离子体的温度以使得基于光场感应电离机制的X射线激光辐射获得更高的增益值,利用准静态隧道电离模型,定量计算了电子剩余能量与激光偏振参量、波长(频率)、激光强度以及电离介质电离能的变化关系。计算结果表明线偏振短波长激光以及高电离能介质产牛的等离子体中电子剩余能比较低,因此适合用作复合机制X射线激光;对于低价离子存偏振度α介于0～1之间的某一处最高,因此,可以通过调整偏振度α,获得某一椭圆偏振光,会更有利于基于光场感应电离电子碰撞机制X射线激光增益的提高。     

X-ray spectra of high temperature tungsten plasma calculated with collisional radiative model

Tungsten is regarded as an important candidate of plasma facing material in international thermonuclear experimental reactor （ITER）, so the determination and modeling of spectra of tungsten plasma, especially the spectra at high temperature were intensely focused on recently. In this work, using the atomic structure code of Cowan, a collisional radiative model （CRM） based on the spin-orbit-split-arrays is developed. Based on this model, the charge state distribution of tungsten ions is determined and the soft X-ray spectra from high charged ions of tungsten at different temperatures are calculated. The results show that both the average ionization charge and line positions are well agreed with others calculations and measurements with discrepancies of less than 0.63% and 1.26%, respectively. The spectra at higher temperatures are also reported and the relationship between ion abundance and temperature is predicted in this work.     

Probing strong field ionization of solids with a Thomson parabola spectrometer

Intense ultrashort laser pulses are known to generate high-density, high-temperature plasma from any substrate. Copious emission of hot electrons, from a solid substrate, results in strong electrostatic field that accelerates the ions with energies ranging from a few eV to MeV. Ion spectrometry from laser–plasma is convolved with multiple atomic systems, several charge states and a broad energy spread. Conventional mass spectrometric techniques have serious limitations to probe this ionization dynamics. We have developed an imaging ion spectrometer that measures charge/mass-resolved ion kinetic energies over the entire range. Microchannel plate (MCP) is used as the position-sensitive detector to perform online and single shot measurements. The well-resolved spectrum even for the low-energy ions, demonstrates that the spectral width is limited by the space-charge repulsion for the ions generated in the hot dense plasma.     

磁绝缘线振荡器内气体碰撞电离数值模拟

针对气体碰撞电离过程,介绍了蒙特卡罗碰撞（MMC）的处理方法,利用MMC方法编写了气体碰撞电离模块,将其移植到3维全电磁粒子模拟程序NEPTUNE之中,模拟了充有He气的磁绝缘线振荡器（MILO）。模拟结果表明:当He气密度较低时,电离的正离子由于较重无法自由移动,形成了正离子通道,可以有效中和电子束空间电荷场,有利于电子束传输和群聚,提高了束波互作用效率,微波输出功率得到了明显提高,起振时间也有所缩短;当进一步增加He气密度时,电离碰撞增强,电子和离子数目会雪崩式增长,电子束由于碰撞增强而导致能散度增大,其负效应已经远大于中和空间电荷场的正效应,反而不利于电子束的群聚和共振,从而导致输出微波功率降低乃至截断,起振时间缩短是由于其在非雪崩阶段的正效应积累所致,但是随着负效应的增强起振功率不能得以维持,二极管最终将闭合。另外,还模拟了MILO填充空气、水蒸气及二氧化碳等多原子、多组分气体的碰撞电离物理过程。模拟结果显示,同压强情况下,填充空气、水蒸气及二氧化碳的脉冲缩短现象要比填充He气等较低原子序数气体的情况严重得多。     

Superhyperfine structure in the EPR spectra and optical spectra of impurity f ions in dielectric crystals: A review

The results of observation and simulation of the superhyperfine (ligand hyperfine) structure (SHFS) of the electron paramagnetic resonance (EPR) spectra of rare-earth and uranium impurity ions in dielectric crystals have been systematized. The resolved SHFS of the EPR spectra of doped cubic crystals (with the fluorite and perovskite structures) has been observed for orientations of a constant magnetic field along the crystallographic axes. Most attention has been paid to tetragonal double fluorides LiRF₄ (R = Y, Lu, Tm), in which the SHFS of the EPR spectra has also been found for intermediate orientations of the magnetic field. For the LiYF₄: Nd³⁺ single crystal, the splitting of optical spectral lines due to the interaction of Nd³⁺ ions with nuclear magnetic moments of the nearest neighbor fluorine ions has been observed for the first time. The Van Vleck paramagnet LiTmF₄: U³⁺ is characterized by the SHFS with clearly distinguishable components due to the interaction of uranium ions both with nuclei of the fluorine ions and with enhanced magnetic moments of the thulium nuclei. The SHFS envelopes of the EPR spectra of Yb³⁺, Ce³⁺, Nd³⁺, and U³⁺ ions in LiYF₄ and LiLuF₄ crystals are well reproduced by numerical calculations based on the microscopic model using only three fitting parameters: the width of transitions between the electron-nuclear sublevels of the complex containing the paramagnetic ion and nuclei of the ligands and two constants of covalent bonding of the f electrons with 2s and 2p electrons of the nearest neighbor fluorine ions.     

Ionization-induced electron trapping in ultrarelativistic plasma wakes

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV/m, in good agreement with an analytical model and PIC simulations.     

Correlation between double and nonresonant single ionization

We performed simultaneous measurements of electrons and ions produced in Xe photoionization driven by an 800-nm, 100-fs laser pulse. The obtained energy and angular resolved electron spectra allow the identification of the electronic states populated during the ionization. Xe2+ ions appear at the same laser intensity as electrons emerging from a nonresonant 9-photon ionization process of Xe. Similar to optical tunneling ionization, the nonresonant ionization delivers low-energy electrons needed for the formation of Xe2+ by a backscattering process.     

Optical amplification of argon excimers in the vacuum ultraviolet spectral region, produced by ultrashort-pulse high-intensity-laser-produced electrons

Optical-field-induced ionization (OFI) electrons were produced in a 30 cm Ar-filled hollow fiber by shining a high-intensity laser into the fiber. By use of such a long plasma with the help of a multilayer dielectric mirror made for 126 nm emission, we observed significant optical amplification of the Ar₂ ^* emission at 126 nm. The optical amplification resulted in a maximum small signal gain coefficient of 0.16 cm^-1 at 126 nm at 10 atm of Ar pressure.