Tohru Kawamura

Model calculations and experiments on photoionized plasmas in relevant to laboratory astrophysics

Abstract Analyses of the steady-state rate equations for photoionized Ar and C are presented and the conditions necessary for such plasma formation in laboratory are discussed. The results of calculations are shown for two plasmas with widely varying density but having the same ionization parameter. Further, we discuss an experiment to study low density carbon exposed to a laser-plasma generated X-ray source where time-resolved X-ray spectra were observed. These are the first steps necessary to model astrophysical phenomena relevant to photoionized plasma.

Simulations on laser ablation and its applications

Integrated laser ablation simulation code includes phase transition from liquid to neutral gas to partially ionized plasma, detail laser absorption processes, equation of state, hydrodynamics, and radiation transport, is developed to describe ablation phenomena with phase transition and properties of emission plasmas. For an application of this simulation code, we perform simulations on optimization of laser produced plasmas for extreme ultra violet (EUV) light sources. Because of very low laser intensities (from 1010 W/cm2 to 1011 W/cm2) compared with that in laser fusion cases, it is necessary to include phase transition effects into ablation radiation hydrodynamics code.

Line shapes of Heβ including higher-order satellite lines for Ar ions in dense plasmas

We have calculated the Stark broadening of Heβ of dopant Ar ions including higher-order satellite lines. Since spectator electrons are bound in high-lying orbitals, the higher-order satellite lines are unresolved from the parent lines. The total intensity of the higher-order satellite lines depends on the population of doubly-excited states. The probabilities for the existence of high-lying orbitals in dense plasmas have been considered. In the spectral calculation of the higher-order satellite lines, the Stark broadening is limited by the critical field strength, above which a bound state cannot exist, of the outermost orbital. The critical field strengths for Rydberg spectator orbitals are lower than those of low-lying orbitals. The results show that the higher-order satellite lines modify total line shapes.

Effective statistical weights of Rydberg orbitals of dopant Ar in dense plasmas and their effect on higher order satellite lines

Calculated are the effective statistical weights of Rydberg orbitals of dopant Ar ions in dense hydrogen plasmas. Energy levels are shifted due to the screening of the nuclear Coulomb potential and the Stark effect. The electric microfield distribution is evaluated by using the adjustable-parameter exponential approximation. The Ryberg orbitals are easily ionized not only by the continuum lowering but also by the plasma electric microfield. The statistical weights of high-lying orbitals are strongly influenced by surrounding charged particles. The results of present calculations are compared with More’s empirical formula, and it is found that the effective statistical weights of Rydberg orbitals remain higher than those predicted by More’s formula.

Analysis of highly excited states of dopant Ar in dense hydrogen plasmas and contribution of higher order satellite lines

Abstract We calculated the effective statistical weight of Rydberg orbitals of dopant Ar ions in dense hydrogen plasmas. Energy levels are shifted due to the effect of screened potential and the Stark effect. The electric microfield distribution is evaluated by using the adjustable-parameter exponential (APEX) approximation. The statistical weight of high-lying orbitals is strongly influenced by surrounding charged particles. In order to estimate contribution of higher order satellite lines, the probability for existence of highly excited states is found to be very important. The emission lines of H-like and He-like Ar ions are enhanced by the unresolved satellite lines from the states with a spectator electron with principal quantum number n=4–6 for ion densities higher than 10 23 cm −3 and electron temperature in the range of 0.5–1.0 keV , and the results of present calculations are compared with Mores empirical formula, and it is found that the effective statistical weight of Rydberg orbitals remains higher than that predicted by Mores formula.

Analysis of laser imploded core plasma formation by Li-like satellite lines from argon dopant

Abstract Core plasma formation in a laser-driven fusion pellet is studied by means of time-resolved X-ray spectroscopy. The fusion pellet containing a high pressure deuterium gas doped with a small amount of Ar was irradiated with high-power laser beams from Gekko XII, and the temporal evolution of spatially-averaged electron temperatures and densities was inferred from Ar K-shell emissions. In a previous paper (Y. Ochi et al., JQSRT 2000;65:393), we note that a uniform core plasma was attained only when the uniform irradiation conditions were satisfied. Based on this result, in the present study, an additional low-modal non-uniformity was intentionally imposed on the pellet to investigate its influence on the core plasma formation. Experimental results showed that Li-like satellite lines from Ar became more intense than the He- β resonance line at the later time of implosion. Focusing on this feature, we derive plausible combinations of the electron temperatures and densities of the core plasma by evaluating atomic processes that dominate the line emissions. We suggest, in comparison with one-dimensional hydro-code simulations of the implosion, that the core plasma is quenched probably due to hydrodynamic mixing of the cold dense pusher with the hot core, which is initiated by the drive non-uniformity.

X-ray spectroscopic analysis of the influence of low-modal nonuniformity on the formation of a hot spark

In the direct-drive scheme implosion of the inertial confinement fusion, the hot spark formation is critically affected by laser irradiation non-uniformities and subsequent hydrodynamic instabilities. Influence of the low- modal irradiation non-uniformities on the hot spark formation was investigated by means of the time- and space- resolved x-ray spectroscopic measurements. Experimental results were compared with post-processed hydro-code simulations by the aid of x-ray spectrum analysis code.

Analysis of X-ray spectra of laser-produced Ar plasmas — effects of spectator electrons

A spectrum calculation code including the effects of one spectator electron has been constructed. Assuming that ions with a spectator electron are related with next ionization stage under local-thermal-equilibrium (LTE), the spectator orbitals which are treated in this code are determined by the Griems boundary. Excited levels higher than the boundary satisfy the LTE condition approximately. In this paper, the effects of a spectator electron on the X-ray spectra from Ar ions are discussed.