S15-5
Atomic process and radiation transport in laser-produced plasmas
レーザープラズマ中の原子過程と輻射輸送
Atsushi Sunahara
砂原	  淳Institute for Laser Technology
2-6 Yamadaoka Suita Osaka Japan 5654-0871
公益財団法人レーザー技術総合研究所 〒565-0871 吹田市山田丘２−６

Abstract
In simulating the laser-produced plasmas, atomic physics is very important. In order to solve thehydrodynamic motion correctly, we have to conduct the radiation hydrodynamic simulation with the accurate
atomic physics. However, the atomic physics in laser-produced plasmas are not easily solved. Here, thenon-local thermal equilibrium should be solved frequently. Also, the radiation transport is not easy to besolved. In this talk, I would like to discuss some example of the problem of the atomic physics inlaser-produced plasmas.
1. Introduction
In the laser-produced plasmas, radiation process
plays an important role on the energy transport.
Especially for the applications, such as theinteraction of ultra-intense with high-Z materials,
the indirect radiation drive implosion in the inertial
confinement fusion, the extreme ultraviolet lightsource, and the laser ablation of metals, high-Zmaterials are initially heated by the laser, and thenthe heated and ionized region starts to emit theradiation. A part of emitted photon propagating into
the material can be easily absorbed in the material.
This energy deposition by radiation can heat thematerial above the melting temperature and drive
the motion of the material.
2. Non-LTE
In the absorption of photon, there are threetransitions can occur; free-free transition such as
inverse-bremsstrahlung, bound-free such as
photo-ionization, bound-bound such as thephoto-excitation, respectively. Competition ofphoton process and collisional one determines thepopulation density. If collisional process isdominant, the plasma condition is in the local
thermal equilibrium (LTE). In LTE, the electron
population density can be obtained by Boltzmann
distribution. However, as the plasma density
decreases and plasma temperature increases, thecollisionality of the electron in the plasma decreases
compared to the photon process. In this region
collisional radiative (CR) model is adopted toobtain the electron population density. In CR model,
the rate equation considering both collisional andradiation processes are solved. Therefore, theaccuracy of the rate coefficients of those transitions
can significantly affect the numerical solution of theelectron population density. Resultantly theionization degree that is very important parameterfor the atomic physics and electron and ion thermal
conduction in the plasma, can be affected by theaccuracy of the rate coefficients. In this talk, I willshow some radiation process in laser-produced
plasmas. In simulating the hydrodynamic motion of
the laser-produced high-Z materials correctly,
accurate coefficients of atomic physics, such as therate coefficients are required. Furthermore, electrontransport is coupled with the radiation transport. In
which the nonlocal transport of electron should betaken into account, because the velocity distributionfunction of electrons in laser-produced plasmas is
not necessary Maxwell distribution function.
3. Radiation transport
Radiation transport is also very important issue incalculating the dynamics of the laser-produced
plasmas. Due to the radiation transport, internalenergy at different position can interact and couple.
In which, the accuracy of calculating the radiationtransport is important as well as the accuracy ofemissivity and opacity at each position. Radiationtransport can be solved by many schemes. However,
the radiation transfer equation is equivalent tosix-dimensional Boltzmann equation and it isdifficult to be solved in the multi-dimensional. In
ordertocombine
thecalculation
ofmulti-dimensionalradiationtransport
andhydrodynamic simulation, the moment method has
been widely used. In this moment method, theradiation energy density E, radiation flux F, andhigher order moment of photon spectral intensity
such as photon pressure P= (f E) are used asindependent variables. Here f is the Eddingtontensor. Then the moment equations are described
as;

(1)
(2)	 
here, zeroth and first-order moment equations areshown. Assuming radiation pressure P to be (f E),
two equations can be solved because theindependent variables are only E and F. ηis theemissivity and χ is absorption coefficient forphoton energy hν. Here, h is the Planck constantandνis the oscillation frequency of photon. Full setof equation (1) and (2) are solved by M-1
method[1]. In the talk, the solution by M-1 method
is compared with other method such as theflux-limited diffusion approximation(FLD). M-1
method shows better quality of the radiationtransport. However, computational time andnumerical stability are better for the FLD method.
References[1] M. Gonz´alez., E. Audit, and P. Huyunh, A&A

464（2007）429-435.