激光等离子体辐射不透明度的细致模型

细致谱线模型是等离子体辐射不透明度精确计算的理论方法,涉及到很多原子物理过程,需要计算大量的、精确的原子参数.在高温、稠密等离子体环境下,谱线展宽、组态相互作用、电离能下降等效应与不透明度的准确计算密切相关.文章重点介绍了目前作者在辐射不透明度细致谱线模型研究方面取得的进展情况,并且与国内外典型的实验和理论结果进行了对比.     

Spatial characterization of laser-induced sparks in air

Images and emission spectra of sparks produced by laser-induced breakdown in air were investigated as functions of the laser energy and optical configuration. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using an intensified CCD camera and a Cassegrain optics system coupled to an ICCD spectrometer. The results provided information about the first stages of laser-induced spark breakdown. Good reproducibility of the plasma location and shape was observed; these parameters depended largely on the optical configuration and plasma energy absorption rate. The high spatial resolution of the Cassegrain optics system was used to observe different ionization levels in the plasma kernel, which confirmed the electron cascade mechanism for plasma formation. The different ionization levels partially explained the asymmetry of the ignition induced by the plasma generation in gaseous mixture. Backward propagation of the plasma along the laser path was observed using the high spatial and temporal resolution of the experimental apparatus. The propagation was largely due to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout the plasma. This observation was correlated with images obtained using the ICCD camera.     

Beam-Plasma Heating Model

A one-dimensional ionization and heating model is applied to results of several electron-beam-plasma interaction experiments. Beam energy is deposited resistively in the plasma at a rate ?j2, where j is the return current density and ? the plasma resistivity both classical and anomalous due to ion acoustic or e-e-mode turbulence. Principal energy losses include ionization, line radiation, inelastic electron impact excitation, bremsstrahlung and radiative recombination. The level of ionization and plasma heating are computed as a function of neutral gas pressure, beam rise time, pulsewidth and current density, and resistivity model. Plasma dynamics and kinetic effects such as expansion and end loss are not explicitly included in the model.     

Monochromatic and mean radiative properties of astrophysical plasma mixtures in nonlocal thermodynamic equilibrium regime

Radiative-hydrodynamics and radiative transfer simulations of astrophysical plasmas require the determination of radiative properties. However, most of the plasma radiative properties are calculated assuming the plasma in coronal equilibrium or local thermodynamic equilibrium regimes that is often not the case for many scenarios. In this work, we present nonlocal thermodynamic equilibrium calculations of radiative opacities of Fe and S and of an astrophysical plasma mixture for temperatures larger than 100 eV. We also analyze the departure from local thermodynamic equilibrium simulations.     

Radiative opacity of plasmas studied by detailed term (level) accounting approaches

Detailed term and level accounting (DTA and DLA) schemes have been developed to calculate the spectrally resolved and Rosseland and Planck mean opacities of plasmas in local thermodynamic equilibrium. Various physical effects, such as configuration interaction effect (including core-valence electron correlations effect and relativistic effect), detailed line width effect (including the line saturation effect), etc., on the opacity of plasmas have been investigated in detail. Some of these physical effects are less capable or even impossible to be taken into account by statistical models such as unresolved transition arrays, super-transition-array or average atom models. Our detailed model can obtain accurate opacity of plasmas. Using this model, we have systematically investigated the radiative opacities of low, medium and high-Z plasmas under different conditions of temperature and density. For example, for aluminum plasma, in the X-ray region, we demonstrated the effects of autoionization resonance broadening on the opacity for the first time. Furthermore, the relativistic effects play an important role on the opacity as well. Our results are in good agreement with other theoretical ones although better agreement can be obtained after the effects of autoionization resonance broadening and relativity have been considered. Our results also show that the modelling of the opacity is very complicated, since too many physical effects influence the accuracy of opacity. For medium and high-Z plasmas, however, there are systematic discrepancies unexplained so far between the theoretical and experimental opacities. Here, the theoretical opacities are mainly obtained by statistical models. To clarify the discrepancies, efforts from both sides are needed. From the view-point of theory, however, a DLA method, in which various physical effects can be taken into account, should be useful in resolving the difference. Taking gold plasma as an example, we studied in detail the effects of core-valence electron correlation and line width on the opacity. Our DLA results correctly explained, for the first time, the relative intensity of the two strong absorption peaks located near the photon energy of 70 and 80 eV, which was experimentally observed by Eidmann et al. [Europhys. Lett., 1998, 44: 459].     

First observation of two-electron one-photon transitions in single-photon K-shell double ionization

Experimental evidence for the correlated two-electron one-photon transitions (1s(-2)→2s(-1)2p(-1)) following single-photon K-shell double ionization is reported. The double K-shell vacancy states in solid Mg, Al, and Si were produced by means of monochromatized synchrotron radiation, and the two-electron one-photon radiative transitions were observed by using a wavelength dispersive spectrometer. The two-electron one-photon transition energies and the branching ratios of the radiative one-electron to two-electron transitions were determined and compared to available perturbation theory predictions and configuration interaction calculations.     

Elastic scattering of a photon by an atom with an open shell

In the nonrelativistic approximation, the absolute values and the forms of the differential cross sections of the anomalous elastic scattering of a linearly polarized x-ray photon by manganese and copper atoms in the energy region of the ionization threshold of the 1s shell are calculated for the wave functions of one-electron states. We take into account the multiparticle effects of relaxation of atomic shells in the field of core vacancies, multiplet splitting, the configuration interaction, and Auger and radiative decays of vacancies, as well as the processes of double excitation/ionization of the ground state of the atom. The calculation results are in good agreement with the synchrotron experiment.     

Resonance inelastic scattering of a photon by the neon atom in the region of the K and KL 23 ionization thresholds

The influence of multiparticle effects on the shape and absolute values of the double differential cross section of the resonance inelastic scattering of a linearly polarized x-ray photon by the neon atom in the energy region of the K and KL ₂₃ ionization thresholds is studied theoretically. The radial relaxation of electronic shells, the spin-orbit and multiplet splittings, the configuration interaction in the states of double excitation of the atom, and the Auger and radiative decays of vacancies formed are taken into account. The calculation results are predictive in character and agree well with experiment for an incident photon energy equal to 5.41 keV.     

Nonlinear Dynamics of Laser Interaction with High Z Plasma

The formation and evolution of a plasma in the interaction of laser radiation with gold targets are studied by computer simulations using MHD code (ESC-CASTOR). The nonlinear features of the interaction including the absorption of laser radiation, heating of the plasma components, and the deformation of the density profile under the action of the ponderomotive force are discussed. The role of these nonlinear effects together and the influence of the radiative and collisional transport in the evolution of the density profile are discussed based on the numerically obtained two-dimensional dynamical structures of the hydrodynamic parameter (electron density n_e, ion density n_i, electron temperature T_e, ion temperature T_i), radiation temperature T_r, ponderomotive force profile, and the space-time dependencies of the absorption efficiency and the ionization rates. The radiation spectrum is analyzed and the regularities in the motion of the critical density surface are clarified.     

W44+ 离子双电子复合速率系数的理论研究

利用全相对论组态相互作用方法,详细研究了W44+ 离子从基组态3s23p63d104s2俘获一个电子形成双激发态(3s23p63d104s2)nln′l′(n = 4 ~ 6,n′= 4 ~7) 的双电子复合(DR) 过程。通过比较不同壳层电子激发的DR 速率系数,得知4s 电子激发和3d 电子激发的DR 速率系数分别在低温和中高温度时给出了主要贡献,得到了主要的电子激发DR通道。在1 eV~50 keV 温度范围内,计算了n = 4~18 的DR速率系数,并外推到了n= 100,得到总DR 速率系数。比较总DR 速率系数、三体复合(TBR) 以及辐射复合(RR) 速率系数,结果表明DR 速率系数在研究的温度范围内远大于TBR 和RR 速率系数,其将明显地影响ITER 等离子体的电离平衡和离化态布居。Based on the fully relativistic configuration interaction method, theoretical calculations are carried out to research the dielectronic recombination (DR) processes, in which W44+ ions in the ground state 3s23p63d104s2 trap an electron to form doubly excited states (3s23p63d104s2)nln’l’(n =4~6,n′= 4~7). The comparison of the DR rate coefficients of different shells shows that DR approach is as follow: the 4s subshell excitation dominates to DR at low temperature, but 3d subshell excitation attributes to DR at high temperature. Total DR rate coefficients from n=4~18 are evaluated directly, and the results are extrapolated up to n = 100 in the temperature range from 1 to 5×104 eV, and thus get the total DR rate coefficients. Compared total DR rate coefficients to three-body recombination (TBR) rate coefficients and radiative recombination (RR) rate coefficients, it showed that the total DR rate coefficients obviously significantly greater than other two recombination rate coefficients, and thus it obviously influence ionization equilibrium and ionization state population of ITER plasma.     

MCDF程序包和电偶极辐射跃迁几率的相对论计算程序

本文讨论如何利用Grant等多组态Dirac-Fock(MCDF)程序包(1980)中的现成资源,依据相对论辐射跃迁的普遍理论,计入组态相互作用和跃迁中的轨道驰豫效应,在Coulomb规范和长度规范下,续编原子的电偶极辐射跃迁几率的计算程序。     

Re30+离子双电子复合速率系数的理论研究

复杂结构离子的双电子复合速率系数在极紫外光刻光源、核聚变等应用研究的等离子体光谱模拟和诊断中具有重要的应用价值。利用全相对论组态相互作用方法,详细计算了基组态为4p64d9的Re30+离子经双激发态（4p64d9）-1nln'l'（n=4~6,n'=4~23）的双电子复合（DR）过程。研究分析了激发、辐射通道,组态相互作用,级联退激对DR速率系数的影响。其中内壳层4p电子激发的DR速率系数是总DR速率系数的28.2%~44.9%,所以内壳层4p电子激发的贡献不可以忽略。级联退激对DR速率系数的最大贡献为12.9%,也必须要予以考虑。通过对双电子复合、辐射复合、以及三体复合速率系数的比较,辐射复合速率系数的最大值为DR速率系数的22.6%,三体复合速率系数的最大值仅为DR速率系数的0.3%。因此,DR速率系数远远大于辐射复合和三体复合速率系数。该结果表明DR过程对于等离子体离化态分布、能级布居以及光谱模拟都极为重要。为了方便应用,对基态和第一激发态的总DR速率系数进行了参数拟合。该研究结果将为Re激光等离子体的光谱模拟及复杂结构离子DR过程的进一步研究提供参考。Dielectronic recombination (DR) rate coefficients of complex ions are very important in some application research, such as extreme ultraviolet lithography and nuclear fusion. Based on the fully relativistic configuration interaction method, theoretical calculations are carried out to research the DR processes, in which Re30+ ions in the ground state 4p64d9 to (4p64d9)-1nln'l'(n=4~6, n'=4~23). Influence of excitation and radiation channels, configuration interaction, the effect of decays to autoionizing levels possibly followed by radiative cascades (DAC) are analyzed. The contributions through 4p subshell excitations to the total rate coefficient are 28.2%~44.9% in the whole temperature region. Hence the contributions from inner-shell electron excitation are very important. The contributions from the DAC transitions increase smoothly with the increasing temperature and are about 12.9% at 50 000 eV. The contributions of DAC can not be neglected. By means of compared total DR rate coefficients to radiative recombination rate coefficients and three-body recombination rate coefficients, it shows that the maximum value of the radiation recombination rate coefficient is 22.6% of the DR rate coefficient and the maximum value of the three-body recombination rate coefficient is only 0.3% of the DR rate coefficient. The total DR rate coefficient is greater than either the radiative recombination or three-body recombination coefficients in the whole temperature range. The corresponding DR process is very important for plasma ionization distribution, population level and spectrum simulation. In addition to facilitate the application, the total DR rate coefficients for the ground state and the first excited state are fitted to an empirical formula. These results will provide the reference for the further analyses of rhenium laser plasma spectrum simulation and the complex structures ions DR process.     

Charge-state distribution and Doppler effect in an expanding photoionized plasma

The charge state distributions of Fe, Na, and F are determined in a photoionized laboratory plasma using high resolution x-ray spectroscopy. Independent measurements of the density and radiation flux indicate unprecedented values for the ionization parameter xi=20-25 erg cm s(-1) under near steady-state conditions. Line opacities are well fitted by a curve-of-growth analysis which includes the effects of velocity gradients in a one-dimensional expanding plasma. First comparisons of the measured charge state distributions with x-ray photoionization models show reasonable agreement.     

Guided ionization waves: Theory and experiments

This review focuses on one of the fundamental phenomena that occur upon application of sufficiently strong electric fields to gases, namely the formation and propagation of ionization waves–streamers. The dynamics of streamers is controlled by strongly nonlinear coupling, in localized streamer tip regions, between enhanced (due to charge separation) electric field and ionization and transport of charged species in the enhanced field. Streamers appear in nature (as initial stages of sparks and lightning, as huge structures—sprites above thunderclouds), and are also found in numerous technological applications of electrical discharges. Here we discuss the fundamental physics of the guided streamer-like structures—plasma bullets which are produced in cold atmospheric-pressure plasma jets. Plasma bullets are guided ionization waves moving in a thin column of a jet of plasma forming gases (e.g., He or Ar) expanding into ambient air. In contrast to streamers in a free (unbounded) space that propagate in a stochastic manner and often branch, guided ionization waves are repetitive and highly-reproducible and propagate along the same path—the jet axis. This property of guided streamers, in comparison with streamers in a free space, enables many advanced time-resolved experimental studies of ionization waves with nanosecond precision. In particular, experimental studies on manipulation of streamers by external electric fields and streamer interactions are critically examined. This review also introduces the basic theories and recent advances on the experimental and computational studies of guided streamers, in particular related to the propagation dynamics of ionization waves and the various parameters of relevance to plasma streamers. This knowledge is very useful to optimize the efficacy of applications of plasma streamer discharges in various fields ranging from health care and medicine to materials science and nanotechnology.     

The trapping of A1(XII) and (XIII) resonance lines in a laser-heated plasma

Effects of radiative trapping and quenching processes that may be important on a time scale of tens of picoseconds are examined in connection with the escape of A1(XII) and (XIII) resonance-line photons from a laser-heated aluminum plasma. For the A1(XII) line, radiative trapping by itself would introduce significant delays on this time scale. However, collisional ionization from the excited state is more important and dominates the transfer of this line. Unless the effective optical depth is reduced by rapid internal motions, the time dependence of the A1(XII) line will reflect the source distribution near the surface, rather than the state of the plasma in the interior. For the A1(XIII) line, the delay due to radiative trapping is smaller and collisional ionization is competitive, but does not completely dominate.     

Effect of dielectronic recombination on the charge-state distribution and soft X-ray line intensity of laser-produced carbon plasma

The effect of dielectronic recombination in determining charge-state distribution and radiative emission from a laser-produced carbon plasma has been investigated in the collisional radiative ionization equilibrium. It is observed that the relative abundances of different ions in the plasma, and soft X-ray emission intensity get significantly altered when dielectronic recombination is included. Theoretical estimates of the relative population of CVI to CV ions and ratio of line intensity emitted from them for two representative formulations of dielectronic recombination are presented.     

Radiative opacity of low-Z plasma using screened hydrogenic model including l-splitting

Radiative opacities of low-Z plasma are computed using average atom model. Screened Hydrogenic Model including l-splitting (SHML) is used as the atomic model to obtain the internal structure of ions embedded in plasma. The phenomenon of pressure ionization of levels is described by a Gaussian density dependent degeneracy function. Use of the most recent compilation of the screening constants makes it possible to include l-splitting in a direct manner. The average ionization in the plasma is obtained by self-consistently solving the non-linear set of coupled SHML equations along with the charge neutrality condition of Wigner–Seitz cell. The frequency dependent opacity of Aluminum plasma is compared with the data obtained from Los Alamos (LANL) opacity library and a reasonable agreement is seen. The computed values of Rosseland and Planck opacity of C, Al and Fe plasma are also in good agreement with the LANL opacity database value for different plasma density and temperature.     

Role of laser-induced plasma in ultradeep drilling of materials by nanosecond laser pulses

Radiative effects of the laser-induced ablative plasma on the heating and ablation dynamics of materials irradiated by nanosecond laser pulses are studied by the example of graphite ablation. On the basis of combined thermal and gas dynamic modeling, the laser-induced plasma plume is shown to be a controlling factor responsible for ultradeep laser drilling due to plasma radiation, both bremsstrahlung and recombinative. We demonstrate that plasma radiative heating of the target considerably deepens the molten layer, thus explaining the observed crater depths.     

A numerical model for multigroup radiation hydrodynamics

We present in this paper a multigroup model for radiation hydrodynamics to account for variations of the gas opacity as a function of frequency. The entropy closure model (M₁) is applied to multigroup radiation transfer in a radiation hydrodynamics code. In difference from the previous grey model, we are able to reproduce the crucial effects of frequency-variable gas opacities, a situation omnipresent in physics and astrophysics. We also account for the energy exchange between neighbouring groups which is important in flows with strong velocity divergence. These terms were computed using a finite volume method in the frequency domain. The radiative transfer aspect of the method was first tested separately for global consistency (reversion to grey model) and against a well-established kinetic model through Marshak wave tests with frequency-dependent opacities. Very good agreement between the multigroup M₁ and kinetic models was observed in all tests. The successful coupling of the multigroup radiative transfer to the hydrodynamics was then confirmed through a second series of tests. Finally, the model was linked to a database of opacities for a Xe gas in order to simulate realistic multigroup radiative shocks in Xe. The differences with the previous grey models are discussed.     

Ne、Ar、Kr、Xe等离子体Planck和Rosseland平均不透明度的近似计算

根据平均原子模型和类氢光吸收系数，近似地计算了Ne、Ar、Kr、Xe等惰性元素高温、高密度等离子体的Planck和Rosseland不透明度。在计算线谱吸收过程中，采用了唯象方法处理线谱演化成谱带时的加宽效应及谱带重叠效应。将不透明度数据拟合成温度和密度的幂函数形式，它能够用于辐射体的数值拟合。