RESUMO
A theoretical model for investigating the radiative transfer of an x-ray free electron laser (XFEL) pulse is developed based on a one-dimensional radiative transfer equation. The population dynamics of energy levels is obtained by rate equation approximation coupling with the Fokker-Planck equation, in which the electron energy distribution function (EEDF) is self-consistently determined. As an illustrative example, XFEL pulse propagation through a solid-density aluminum (Al) is investigated. The characteristics of the temporal evolution of the x-ray pulse shape, level population, and EEDF are demonstrated. The EEDF usually has two parts in XFEL-Al interactions: the near equilibrium part in the lower energy regions and the nonequilibrium part in the higher energy region. The deep gap between the two parts is quickly filled in the solid-density Al plasma. The pulse shape is distorted and the duration shortens as the x-ray pulse propagates through the Al sample. The x-ray transmission spectra were compared with experimental and other theoretical results, and good agreement was found. There are slight discrepancies between the transmission obtained by solving the Fokker-Planck equation and Maxwellian assumptions because nonequilibrium electrons in the higher energy region account for only a small fraction of the total electrons.
RESUMO
A type of low-pass filter devices for soft x rays is investigated by using a microchannel plate (MCP) of small channels with square cross section. The measured transmission spectra on the Beijing Synchrotron Radiation Facility showed that the MCP has excellent bandpass effects below 1.5 keV by grazing incidence and internal multireflections. Combined with filters, the MCP energy bandwidth can be narrowed to 100 eV. In contrast to bandpass made of planar mirrors, the MCP has a much smaller size and better bandpass effects, and can be easily extended to high energy ranges. For low-resolution spectrometer applications of soft x rays, this method allows the monochromator to be replaced by a simple MCP filter and therefore significantly reduces alignment complexity in experiments.
RESUMO
The experimental transmission spectrum of a hot bromine plasma [J. E. Bailey, J. Quant. Spectrosc. Radiat. Transf. 81, 31 (2003)] has been simulated by using a detailed level accounting model (DLA). With assumption of the local thermodynamic equilibrium, the major absorption lines of the experimental spectrum are well reproduced by the present DLA calculation, and the details of the absorption line shapes are used to determine the temperature of the plasma. In contrast to the results of two former statistical models, where the temperature was determined via a global fitting to the experimental data, the present DLA diagnoses the plasma temperature by the line ratios of different charge states in the 2p-->3d transition groups resulting in a temperature of 37 eV . It is shown that a change of 1 eV in temperature could cause perceptible changes in the simulated spectrum. It is also shown that the 2 p1/2 -->3 d3/2 absorptions have been overestimated by the statistical models.
RESUMO
We have calculated the radiative opacities of iron plasma in local thermodynamic equilibrium using a detailed term accounting model. The extensive atomic data are obtained by multiconfiguration Hartree-Fock (MCHF) method, with Breit-Pauli relativistic corrections. Extensive configuration interaction (CI) has been included based on LS coupling to obtain energy levels and the bound-bound transition cross sections. A detailed configuration accounting model is applied to evaluate the bound-free absorption cross sections. We simulate two experimental transmission spectra [G. Winhart et al., Phys. Rev. E 53, R1332 (1996); P. T. Springer et al., J. Quant. Spectrosc. Radiat. Transf. 58, 927 (1997)] to verify our calculation model, one is at a temperature of 22 eV and a density of 10(-2) g/cm(3) and the other is at a temperature of 20 eV and a lower density of 10(-4) g/cm(3). It is shown that the strong CI can effectively change the oscillator strengths in contrast to the single configuration HF method. For both of the two simulated transmission spectra good agreement is obtained between the present MCHF results and the experimental data. Spectrally resolved opacities and Planck and Rosseland mean opacities are also calculated. For the isothermal sequence of T=20 eV, when the density decreases from 10(-2) to 10(-5) g/cm(3), the linewidth also decreases so that the iron transition arrays show more discrete line structures and the linewidth becomes very important to the Rosseland mean opacity.
RESUMO
Simultaneous measurements of the self-emission spectrum, the backlighting source spectrum, and the transmission spectrum in one shot, which reduce the experimental uncertainties from shot-to-shot fluctuation, are essential for precise opacity experiments. In order to achieve precise absorption spectrum of Al plasmas, a special half sample sandwich target was designed and short backlighter was used to provide time- and space-resolving diagnostics on the Shenguang II high power laser facility. In the measurement, a cylindrical cavity with CH foam baffles was used to provide a clean x-ray radiation environment for sample heating. The x-ray source spectrum, the transmission spectrum, and the self-emission spectrum of the soft x-ray heated Al sample were recorded in one shot with a penta-erythritol tetrakis (hydroxymethy) methane C(CH(2)OH)(4) (PET) crystal spectrometer by using the point-projection method. Experimental results have been compared with the calculation results of a detailed level accounting opacity code.