Feilu Wang

X-ray astronomy in the laboratory with a miniature compact object produced by laser-driven implosion

It has been suggested that the extreme states of matter generated by high-intensity lasers could allow conditions similar to those in the vicinity of black holes to be studied in the lab. The observation of striking similarities between the X-ray spectra emitted by a laser-driven laboratory plasma and those measured from two high-mass binary star systems suggests such potential has been realized.

Opacity Studies of Silicon in Radiatively Heated Plasma

Measurements of the opacity of silicon at high temperature and high density are reported. A silicon dioxide foam was heated by eight nanosecond laser beams while a backlighter X-ray source was produced with a picosecond laser. Absorptions of the 1-2 transitions of Si XII through Si VI were observed in the wavelength range from 6.6 to 7.1 A. The experimental results are simulated with theoretical calculations under local thermodynamic equilibrium using a detailed level accounting model and can be reproduced in general when the effects of the oxygen in the SiO2 are taken into account.

Experimental evidence and theoretical analysis of photoionized plasma under x-ray radiation produced by an intense laser

Photoionized plasma was studied experimentally under laboratory conditions by means of high intensity short pulse lasers. The experiment consists of a gold cavity filled with nitrogen gas. Six laser beams were focused on the inner surface of the gold cavity, thereby generating an almost black-body radiation having temperature of 80 eV inside the cavity. This radiation heats the nitrogen gas mainly by means of photoionization. L-shell emissions from N V to N VII have been observed in the wavelength range between 90 and 200 A. A time-dependent Detailed Configuration Accounting computer program has been developed to analyze the experimental spectra. In contrast to standard analysis of astrophysical observations, the evidence for photoionization is inferred from the spectral lines ratios. Comparison between the experimental and simulated line spectra indicates that the radiation heated nitrogen attains temperature of 20-30 eV, much lower than the source radiation temperature. Paradoxically, it is also shown that similar line emissions can be reproduced computationally also when the radiation and plasma temperatures both equal approximately 60 eV. This misleading result indicates that experimental simulation in laboratory is sometimes necessary to avoid misinterpretation of astrophysical spectra

Relativistic electrons produced by reconnecting electric fields in a laser-driven bench-top solar flare

Laboratory experiments have been carried out to model the magnetic reconnection process in a solar flare with powerful lasers. Relativistic electrons with energy up to megaelectronvolts are detected along the magnetic separatrices bounding the reconnection outflow, which exhibit a kappa-like distribution with an effective temperature of ~109 K. The acceleration of non-thermal electrons is found to be more efficient in the case with a guide magnetic field (a component of a magnetic field along the reconnection-induced electric field) than in the case without a guide field. Hardening of the spectrum at energies ≥500 keV is observed in both cases, which remarkably resembles the hardening of hard X-ray and γ-ray spectra observed in many solar flares. This supports a recent proposal that the hardening in the hard X-ray and γ-ray emissions of solar flares is due to a hardening of the source-electron spectrum. We also performed numerical simulations that help examine behaviors of electrons in the reconnection process with the electromagnetic field configurations occurring in the experiments. The trajectories of non-thermal electrons observed in the experiments were well duplicated in the simulations. Our numerical simulations generally reproduce the electron energy spectrum as well, except for the hardening of the electron spectrum. This suggests that other mechanisms such as shock or turbulence may play an important role in the production of the observed energetic electrons.

X-ray and EUV spectroscopy of various astrophysical and laboratory plasmas: Collisional, photoionization and charge-exchange plasmas

Several laboratory facilities were used to benchmark theoretical spectral models that are extensively used by astronomical communities. However, there are still many differences between astrophysical environments and laboratory miniatures that can be archived. Here we setup a spectral analysis system for astrophysical and laboratory plasmas to make a bridge between them, and we investigate the effects from non-thermal electrons and the contributions from a metastable level population on level populations and charge stage distribution for coronal-like, photoionized, and geocoronal plasmas. Test applications to laboratory measurement (i.e., electron beam ion trap plasma) and astrophysical observation (i.e., Comet, Cygnus X-3) are presented. A time evolution of the charge stage and level population are also explored for collisional and photoionized plasmas.

PHOTOIONIZATIONAL PLASMAS. II. COMPUTATIONAL RESULTS

A new computer code, PhiCRE, has been developed to calculate the ionization and population distributions in a photoionizational-collisional-radiative plasma. Comparisons with experiments show that the present code provides rather accurate ionization distributions in photoionized plasmas and show reasonable agreement with other codes. Using this code, we have carried out a systematic study of the behavior of the charge state distributions and the average charge as a function of several parameters of the incident radiation and the plasma parameters.

ELECTRONIC STRUCTURE AND RADIATIVE OPACITY OF THE METALLIC ELEMENTS IN HOT AND DENSE STELLAR MATERIAL

Electronic structures of the metallic elements in hot and dense stellar material are calculated with an average-atom scheme, which is designed to consider the environmental influence in a mixture. It ensures that all kinds of atoms have the same temperature, the same chemical potential, and the same electron density at the boundaries between the atoms, and that the electrical neutrality within each atomic sphere is satisfied by using a self-consistent field calculation. Opacities, which are strongly environmentally dependent, for stellar materials with solar composition and relative abundance have been calculated with the calculated electronic orbitals and excitation cross sections. Comparisons are made between the present opacity results and those of OPAL and LANL (Los Alamos National Laboratory Astrophysical Opacity Library) to show that the environmental influence on the electronic structures of the atoms and ions in a hot and dense mixed material has been adequately taken into account using a completely different method. Agreement is quite good for a variety of temperatures and densities. The results also show that the contributions from both bound-bound and bound-free radiative transition processes of the metallic elements are significant, although their relative abundance is much smaller than hydrogen and helium.

PHOTOIONIZATIONAL PLASMAS. I. THEORY

In this paper, an attempt is made to define the subject of photoionizational plasmas through rigorous mathematical formalism. The central results of this paper are the following. (1) A set of recursive equations is introduced for the computation of the charge state distributions in photoionizational plasmas. (2) Quantitative validity limits are given for both the collisional and the photoionizational domains. (3) A parameter that determines the charge state distribution in the photoionizational regime is introduced. (4) We provide detailed discussion about the most important components of the emission spectrum in the different equilibrium domains of the emitting plasma. Some of these components have not been reported so far as included in the analysis of such plasmas.

Formation and evolution of a pair of collisionless shocks in counter-streaming flows

A pair of collisionless shocks that propagate in the opposite directions are firstly observed in the interactions of laser-produced counter-streaming flows. The flows are generated by irradiating a pair of opposing copper foils with eight laser beams at the Shenguang-II (SG-II) laser facility. The experimental results indicate that the excited shocks are collisionless and electrostatic, in good agreement with the theoretical model of electrostatic shock. The particle-in-cell (PIC) simulations verify that a strong electrostatic field growing from the interaction region contributes to the shocks formation. The evolution is driven by the thermal pressure gradient between the upstream and the downstream. Theoretical analysis indicates that the strength of the shocks is enhanced with the decreasing density ratio during both flows interpenetration. The positive feedback can offset the shock decay process. This is probable the main reason why the electrostatic shocks can keep stable for a longer time in our experiment.

Characteristic measurements of silicon dioxide aerogel plasmas generated in a Planckian radiation environment

The temporally and spatially resolved characteristics of silicon dioxide aerogel plasmas were studied using x-ray spectroscopy. The plasma was generated in the near-Planckian radiation environment within gold hohlraum targets irradiated by laser pulses with a total energy of 2.4 kJ in 1 ns. The contributions of silicon ions at different charge states to the specific components of the measured absorption spectra were also investigated. It was found that each main feature in the absorption spectra of the measured silicon dioxide aerogel plasmas was contributed by two neighboring silicon ionic species.