Yuan Jian-Min

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].

Strengthened Hyper-Raman Lines with the Coherent Superposition State

High-order harmonic generations from a one-dimensional Coulomb potential atom are calculated with the initial state prepared as a coherent superposition between its ground and first excited states. When the energy difference of the two states is small, we can choose proper laser pulse such that the first excited state can be excited only to other bound states instead of being ionized. We show that only the hyper-Raman lines are observable instead of the harmonics. The energy difference of the ground and the first excited state can be deduced from the highest peak of the hyper-Raman lines. We further show that the similar results can be obtained by using a combination of two laser pulses with different frequencies interacting with the atom initially at the ground state.

Temperature diagnostics for iron plasmas by means of transmission spectrum obtained by accurate atomic data

Accurate atomic data such as fine-structure energy levels and oscillator strengths are calculated to obtain the radiative opacity and transmission of iron plasmas. At a temperature of 22 eV and a density of 0.01 g/cm3, the calculated transmission spectrum is in better agreement with the experiment than that by other opacity codes such as OPAL. The transmission spectrum is very sensitive to the temperature of plasmas, meaning that it is an ideal diagnostic tool for the temperature of plasmas. Practical calculations show that the uncertainty of the temperature diagnostics obtained by theoretical predictions is better than ±2 eV.

Opacity of Hot and Dense Plasmas of a Mixture using an Average-Atom Approach

An average-atom model is proposed to calculate the opacities of hot and dense plasmas of a mixture. A self-consistent scheme is used to reach the requirements of the same temperature and chemical potential for all kinds of atoms in the mixtures, the same electron density at the boundaries between the atoms, and the electrical neutrality within each atomic sphere. The orbital energies and wavefunctions for the bound electrons are calculated with the Dirac-Slater equations. The occupation numbers at each orbital of each kind of atom are determined by the Fermi-Dirac distribution with the same chemical potential for all kinds of atoms. As an example, the opacity of the mixture of Au and Cd is calculated at a few temperatures and densities.

Polarization Encoded Quantum Key Distribution over Special Optical Fibres

Employing a polarization compensator, an optical fibre quantum key distribution (QKD) system based on polarization coding has been developed. To obtain the compensator setting parameters, the measurement of the laser pulse polarization is performed with one single photon detector. We obtain a sifted key bit rate of about 2 kbits/s and a qubit error rate lower than 10% within 3.5 h. It is shown that polarization coding can be used for QKD over optical fibres as well. At the same time, the system is simple, easy to operate, practical and user-friendly. It gains more advantages than other systems over optical fibres when used in local area quantum communications and where the functional agility is important.

Near threshold photodetachment cross section of negative atomic oxygen ions

A 40-target state close-coupling calculation for the photodetachment cross section of negative atomic oxygen near threshold is carried out with core–valence electron correlation by using the R-matrix method. It was shown that after considering the excitations of two electrons from the 2s shell, the electron affinity of O− (2s22p5 2Po) agrees with the experimental result much better than that just considering the excitations of electrons only from the 2p shell as well as only one electron from the 2s shell. Total cross section as well as the main contribution of the ionization channels to the partial cross section are illustrated to show the structure near threshold clearly.

Quantum Secret Sharing with Two-Particle Entangled States

We present a new protocol for the quantum secret sharing (QSS) task among multiparties with two-particle entangled states. In our scheme, the secret is split among a number of participating partners and the reconstruction requires collaboration of all the authorized partners. Instead of multiparticle Greenberger–Horne–Zeilinger states, only two-particle entangled states are employed in this scheme. By local operations and individual measurements on either of the two entangled particles, each authorized partner obtains a sequence of secret bits shared with other authorized partners. This protocol can be experimentally realized using only linear optical elements and simple entanglement source. It is scalable in practice.

Orbital Relaxation Effects in the Calculation of Aluminum K α Absorptions

A recent experimental Kα transmission spectrum of an aluminum plasma is theoretically studied by a detailed level accounting model. It is found that the orbital relaxation effects of the K- and L-shell orbitals should be considered to calculate accurate line positions and strengths. To do this the initial and the final radial wave functions of Kα lines are respectively optimized by solving the full relativistic Dirac–Fock equation. Extensive configuration interaction calculations are performed to obtain the energy levels and the oscillator strengths. It is shown that both the line positions and the line strengths agree quite well with experiment when the orbital relaxations are considered.

Interference effects on the photoionization cross sections between two neighbouring atoms: nitrogen as an example

Interference effects on the photoionization cross sections between two neighbouring atoms are considered based on the coherent scattering of the ionized electrons by the two nuclei when their separation is less than or comparable to the de Broglie wave length of the ionized electrons. As an example, the single atomic nitrogen ionization cross section and the total cross sections of two nitrogen atoms with coherently added photoionization amplitudes are calculated from the threshold to about 60 A (1 A = 0.1 nm) of the photon energy. The photoionization cross sections of atomic nitrogen are obtained by using the close-coupling R-matrix method. In the calculation 19 states are included. The ionization energy of the atomic nitrogen and the photoionization cross sections agree well with the experimental results. Based on the R-matrix results of atomic nitrogen, the interference effects between two neighbouring nitrogen atoms are obtained. It is shown that the interference effects are considerable when electrons are ionized just above the threshold, even for the separations between the two atoms are larger than two times of the bond length of N2 molecules. Therefore, in hot and dense samples, effects caused by the coherent interference between the neighbours are expected to be observable for the total photoionization cross sections.

Radiative Transition Line Shape in Warm and Dense Argon Plasma: a Two-Centre Model Study

A two-centre model is employed to study the electronic structure of argon plasma at a density of 1022 cm−3 and a temperature of 5eV. The model takes into account the influence of the nearest neighbour on the electronic structure of the radiator and gives a proper description of the transient molecular behaviour in dense plasmas where the mean interatomic distance can be the order of the orbital spatial extent. The orbital energies of quasi-molecular Ar2 against the interatomic distance are calculated in the framework of density functional theory. It is shown that the orbital energies are not monotonic functions of the distance because of two competitive effects: the repulsive potential between electrons and the attractive potential between electrons and nuclei. The characteristics of this two-centre system can be categorized by three regimes depending on the ratio between the electron wavelength and the interatomic distance: atomic regime, screened atomic regime, and quasi-molecular regime. This classification agrees qualitatively with the conclusion of full quantum-mechanical approach [Yonger S Met al 1988 Phys. Rev. Lett. 61 962].