Z箍缩内爆的MARED程序数值模拟分析

MARED程序是模拟Z箍缩内爆过程的二维三温辐射磁流体力学程序,它适用于不同装置条件和不同负载参数.利用MARED程序对Z箍缩内爆进行模拟,结合丝阵Z箍缩实验分析表明:相同负载质量条件下,钨丝阵内爆产生的X射线辐射功率远大于铝丝阵产生的X射线功率;相同负载电流条件下,负载质量越大,计算得到的X射线功率越低;X射线功率随着负载电流增加而增加.MARED程序能够较好地反映Z箍缩内爆动力学过程,特别是不稳定性发展的二维图像,能够给出与不稳定性简化模型的理论分析及实验结果定性一致的演化规律.MARED程序模拟丝阵填充泡沫形成辐射场的初步计算得到了与Sandia实验室模拟Z装置上丝阵填充泡沫定性一致的结果.     

Quasi-adiabatic compression of a plasma column by a longitudinal magnetic field

Approximate 1.5-dimensional MHD equations are derived that describe the quasi-adiabatic compression of a thin plasma column by a longitudinal magnetic field. The parameters of the compressed plasma are obtained analytically as functions of the initial conditions and longitudinal field. The stability of plasma compression against the Rayleigh-Taylor instability is investigated. It is shown that, in the Z-Θ-pinch geometry, increasing the longitudinal magnetic field makes it possible to achieve radial compression ratios of 20–30 without violating the cylindrical symmetry of the column. The possibility of thermonuclear ignition in a thin plasma column in a Z-Θ-pinch configuration is studied. The ranges of the initial plasma densities and temperatures and the initial lengths of the plasma column that are needed to achieve ignition in a plasma compressed by a factor of 20–30 are determined. The parameters of the electromagnetic energy source required to achieve such a high plasma compression are estimated.     

An analytic Voigt profile escape probability approximation

An efficient analytic Voigt profile escape probability approximation is presented and compared to exact numerical calculations. Using this analytic approximation, the two-level-atom source function is also computed for optically thick media, which have been previously evaluated exactly by Avrett and Hummer. Comparison with a more realistic laboratory plasma case also validates the usefulness of the approximation for economical modeling of radiation transport in optically thick plasmas.     

The prospects for high yield ICF with a Z -pinch driven dynamic hohlraum

Recent success with the Sandia Z machine has renewed interest in utilizing fast Z -pinches for ICF. One promising concept places the ICF capsule internal to the imploding Z -pinch. At machine parameters relevant to achieving high yield, the imploding Z -pinch mass has sufficient opacity to trap radiation giving rise to a `dynamic hohlraum'. Our concept utilizes a 12 MJ, 54 MA Z -pinch driver producing a capsule drive temperature exceeding 300 eV to realize a 550 MJ thermonuclear yield. We present the current high-yield design and its development that supports high-yield ICF with a Z -pinch driven dynamic hohlraum.     

Calculation of radiative heat transfer in SF6 arcplasmas

Radiation transport in SF₆ are plasmas have been treated for pressures of 0.101, 0.5, and 1.0 MPa. We have investigated the use of an approximate method of partial characteristics. We have calculated spectral absorptivities for SF₆ plasmas for temperatures from 300 to 35 000 K, and have used these absorptivities to calculate the two functions designated ΔI and ΔSim. These functions are integrals over all radiation frequencies for given line segments with a linear variation of temperature along the line. The ΔI and ΔSim values are functions of the end temperatures and the length of the line, and are used to evaluate radiation transfer from line segments in the plasma where the temperature variation for each line segment is approximated as being linear. The validity of this method of partial characteristics has been demonstrated by comparing exact calculations with the approximate calculations to evaluate radiation intensities, radiation fluxes, and divergence of radiation fluxes for specified temperature profiles. The method of partial characteristics has been applied to one- and three-dimensional calculations of radiative heat transfer in simplified temperature profiles. Agreement up to 10% is obtained with exact calculations, but with a large reduction of computation time     

Photon escape probabilities for stark-broadened lyman series lines

A simple formula is derived for the escape probability of photons in optically thick resonance lines of hydrogenic ions. The result is valid whenever quasi-static Stark broadening predominates in the line wings, a condition that is satisfied in most laser-produced plasmas. In contrast to impact (Lorentzian) broadening, Stark broadening tends to decrease the probability of photon escape as the plasma density increases.     

Escape factors in spherically symmetric plasmas

In this paper we present explicit formulas for the escape factors of strong spectral lines in spherical-symmetric plasmas. The plasma does not necessarily have a homogeneous density. The calculations include two new elements: the first incorporates an accurate calculation of the angle-averaged areal density of ions from any internal point to the edge of the sphere. The second is the use of a saddle point method to carry out the integration over the line profile. Approximate expressions for the asymptotic behavior of the escape factor are given for Gaussian, Lorentzian and Holtsmarkian line shapes.     

Cylindrical <Emphasis Type="Italic">Z</Emphasis>-pinch in gas. I. Determination of dependences between discharge integral characteristics

The previously unknown dependences between integral characteristics of the cylindrical Z-pinch in argon were experimentally studied at the stage of its current shell formation and at the initial stage of its plasma pinch compression. Based on these dependences, the power balance at the above stages was determined taking into account the discharge magnetic field energy, electrodynamic force work, gas ionization energy, Joule and shock heating of plasma.     

Dimensionless similarity invariants and criteria of cylindrical Z-pinch compression regimes in a gas

A dimensionless equation containing two dimensionless complexes Z and d ₀ was derived from the consideration of the energy balance at the early stage of cylindrical Z-pinch development in a gas. The requirement of the equation invariance under the transformation of stretching (compression) of coordinates and time imply that the Ξ and d ₀ complexes are dimensionless similarity invariants of the indicated discharge. These invariants were shown to be quantitative criteria of realization of single, double, triple, etc. cylindrical Z-pinch plasma column compression regimes.     

Escape factors for laser-plasmas

A method of parameterizing escape factors (transmission factors and net radiative brackets) for conditions typical of laser-produced plasmas is introduced. The assumptions of planar geometry, exponentially decreasing emissivity and absorption coefficient with distance with a step rise at a particular point, and spatially constant Doppler broadened line profiles have been made. The effect of velocity gradients in spectrally shifting the absorption and emission line profiles relative to each other is taken into account assuming linear velocity gradients with distance. A parameter R representing the ratio of the spatial scale-length of the absorption coefficient to the Doppler decoupling length is introduced. Fitting formulae for transmission factors and net radiative brackets are given which are valid for all R and all optical depths. In the limit of small R (large Doppler decoupling length), the escape factors asymptotically approach formulae developed originally by, for example, Holstein assuming negligible plasma velocities. For large R (small Doppler decoupling length), the escape factors have been shown to asymptotically approach the Sobolev approximation. The parameterized net radiative bracket has been used in the hydrodynamic and atomic physics code ‘EHYBRID’ for the calculation of the effect of radiation trapping on population densities in laser-produced plasmas. The output of the modified EHYBRID code has then been post processed using the parameterised transmission factors to simulate 123 Ne-like and 399 F-like germanium resonance line intensities emitted in typical X-ray laser experiments. We obtain an agreement between the simulated and experimental spectra.     

Prospects of “water-window” X-ray emission from subpicosecond laser plasmas

Soft-X-radiation in the “water-window” region (23.3–43.6 ?) mainly from carbon laser plasmas generated by subpicosecond (700 fs) 0.248-μm laser pulses is studied as a function of angle of incidence and intensity (up to 10¹⁸ W/cm²) for p-polarized laser light. Furthermore, comparison is made between plasmas generated from massive and foil targets. Numerical calculations are performed using a hydrocode coupled to X-ray line and continuum emission calculations including radiation transport. The optimized conditions to achieve maximum water-window X-ray emissivity and, in particular, carbon Lyman-α line emission are investigated. In addition, analytical scalings are presented. These theoretical results are essentially confirmed by previous experiments. It is found that at optimized conditions, picosecond or subpicosecond laser plasma X-ray sources with a power of the order of 1–10 GW in a spectral window of 1 ? could be developed. Received: 6 August 1998 / Final version: 6 August 1999 / Published online: 30 November 1999     

Predictions of arc temperature profiles using approximate emission coefficients for radiation losses

Coefficients are calculated for the net emission of radiation per cc from the center of cylindrical isothermal plasmas of various temperatures and radii. Coefficients are given as a function of temperature and radius for the D lines of sodium vapor at 250 torr and for the continuum radiation of air at 1 and 30 atm. These coefficients are used in the Elenbaas Heller equation to derive temperature profiles of wall stabilized arcs e.g. for 4·5A with a radius of 0·35 cm in sodium vapor at 250 torr and 10,000 and 20,000A with a radius of 1 cm for air at 30 atm. Comparisons are made with calculated temperature profiles where self absorption effects and the radiation transfer are treated exactly, but still assuming thermodynamic equilibrium. It is found that the approximation using the net coefficients yields central arc temperatures and electric field strengths for a given arc current accurate to 10 per cent. Computation time for the approximate calculation is more than an order of magnitude less than for the calculation where radiation transfer is treated exactly. Thus the approximation facilitiates an account of line radiation for arc plasmas having complex spectra.     

Calculation of the escape probabilities of Fe XVII resonance lines for the Voigt profile

Using the Voigt profile we obtained, we calculate the escape probabilities of Fe XVII resonance lines at 15.02, 13.28, 12.12, 11.13, 11.02 and 10.12 ? for optically thick plasma, both for slab and cylindrical geometry. The oscillator strength, the number density of the absorbing atoms in the ground state, and the optical depth in the line center are discussed in this calculation. Results show that the escape probabilities for the slab geometry are larger than that for the cylindrical geometry. This calculation is useful for the study of the Fe XVII resonance lines.      

Calculation of the escape probabilities of Fe XVII resonance lines for the Voigt profile

Using the Voigt profile we obtained, we calculate the escape probabilities of Fe XVII resonance lines at 15.02, 13.28, 12.12, 11.13, 11.02 and 10.12 Å for optically thick plasma, both for slab and cylindrical geometry. The oscillator strength, the number density of the absorbing atoms in the ground state, and the optical depth in the line center are discussed in this calculation. Results show that the escape probabilities for the slab geometry are larger than that for the cylindrical geometry. This calculation is useful for the study of the Fe XVII resonance lines.     

Self-absorption quantification in the case of SF6–N2 thermal plasma mixture

A simple and accurate method is presented to estimate quantitatively the self-absorption effect for the emission radiations from nitrogen in the case of SF₆–N₂ plasma mixture. The self-absorption phenomena of optically thick lines are treated parametrically on the basis of escape factor. The resonance escape factors for bound–bound transitions for the lines emitted by atomic nitrogen at 113.5, 119.96, 124.32, 149.26 and 174.53 nm are calculated assuming Voigt line shape. The escape factors for free-bound (continuum) radiation which have been the object of fewer investigations are also considered. The escape factors are calculated as a result of the solutions of the equation of radiative transfer which is independent from any special geometry. Assuming a constant plasma size the dependence of escape factors on the temperature, pressure and the nitrogen proportion in SF₆ plasma are also taken into account. These calculations may be useful in plasma modeling and diagnostics as well as the application of laser-induced plasma spectroscopy (LIPS) used in the quantitative analysis of elemental compositions.     

Coherence effects on photon absorption in optically thick plasmas

A kinetic photon transport model that accounts for spatial coherence is applied to line radiation in optically thick plasmas. It is shown that the photon emission and absorption processes are delocalized in space, which alters the global plasma opacity to spectral lines. Based on this analysis, we demonstrate that spectral profiles and escape factors can be much larger than expected from usual formulas.     

Multiparameter dimensionless complex determining the compression dynamics of a cylindrically symmetric Z-pinch in a gas

For the initial compression stage of a cylindrically symmetric Z-pinch in a gas, on the basis of the energy balance of the discharge we derived a dimensionless equation of the relative radius of its plasma cord, containing a generalized variable, i.e., a multiparameter complex Ξ which determines the discharge compression dynamics and is a result of generalization of experimental data. It is shown that the experimental dependence of the relative plasma cord maximum temperature on the complex Ξ has a tendency to saturation, which is due to the rapid increase in Z-pinch radiation intensity in deuterium upon cord compression.     

Calculation of radiative heat transfer in argon arc plasmas

Radiation emission and absorption in arc plasmas are important energy transfer processes. Exact calculations, though possible in principle, are usually impossible in practice because of the need to treat a large number of spectral lines and also the continuum radiation in the whole spectrum range. Recently, we have used an approximate method of partial characteristics to evaluate the radiation intensities, radiation fluxes and the divergence of radiation fluxes for SF₆ arc plasma with cylindrical symmetry. In this paper, we have extended our calculations toargon arc plasmas for the plasma pressures of 0.1, 0.5 and 1.0 MPa. We have calculated the coefficients of absorption for Ar plasmas at temperatures from 300 to 35 000 K, and have used these coefficients to calculate the partial characteristics. Both the continuum and the line spectra have been included in calculations. We have taken into account the radiative photo-recombination and bremsstrahlung for the continuous spectrum, and over 500 spectral lines for the discrete spectra.The method of partial characteristics has been applied to three-dimensional calculations of radiative heat transfer — i.e. radiation intensity, radiation flux and its divergence — in simplified temperature profiles. Conclusions have been made concerning validity and utilization of the method of partial characteristics in general gas dynamics problems.     

Opacities of high temperature highZ plasmas

A procedure is described for computation of opacities for highZ plasmas. Bound-bound, bound-free, free-free and scattering processes are considered. The inputs for these have been obtained by solving IEEOS form of Saha’s equation. Detailed calculations of opacities have been done for tungsten and uranium up to 10 keV of temperature. Preliminary results have been presented at the Nucl. Phys. and Solid State Phys. Symp., Dept. of Atom. Energy, held at Ahmedabad, India (December 1976).