Dans le but d'etudier un sch6ma de laser X dans les nickelol'des, on a besoin d'un modele pour .... R.W. Lee, B.L. Whitten, and R.E. Stout, J.Q.S.R.T.,. 32, p.
JOURNAL DE PHYSIQUE Colloque C1, Suppl6ment au n03, Tome 49, Mars 1988
DIAGNOSTICS OF GOLD LASER PLASMAS
J.C. GAUTHIER, J.P. GEINDRE, P. MONIER, C. CHENAIS-POPOVICS, N. TRAGIN, M. KLAPISCH' and A. BAR-SHALOM" Laboratoire de Physique des Milieux Ionisks and GRECO Interaction Laser Matiere, Ecole Polytechnique, F-91128 Palaiseau Cedex, France ' ~ a c a hInstitute of Physics, Hebrew University, Givat Ram, IL-91904 Jerusalem, Israel Nuclear Research Center of Neguev, Beer-Sheva, Israel
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RESUME Dans le but d'etudier un sch6ma d e laser X dans les nickelol'des, on a besoin d'un modele pour dCterminer les parametres qui decrivent les plasmas d'ions lourds Le sujet de ce travail est d'etudier les plasmas d'or crehs par fortement ionisCs. laser et de comparer les resultats expdrirnentaux aux predictions d'un modele collisionnel-radiatif dans les nickeloTdes. La tempbrature et la densit6 hlectronique traceur d'aluminium, et sont sont mesurCes B partir des raies d'emission d'un comparCes aux valeurs obtenues avec le modele collisionnel-radiatif pour un plasma d'or pur. Les resultats montrent que l'on peut estimer la temperature et la densite electronique dans un plasma d'or pur.
ABSTRACT In order to achieve a nickel-like X ray laser scheme we need a tool to determine the parameters which characterise the high-Z plasma. The aim of this work is to study gold laser plasmas and to compare experimental results to a collisional-radiative model which describes nickel-like ions. The electronic temperature and density are measured by the emission of an aluminium tracer. They are compared to the predictions o f the nickel-like model for pure gold. The results show that the density and temperature can be estimated in a pure gold plasma.
1. EXPERIMENTAL CONDITIONS 0
We have recorded X ray spectra o f gold and aluminium plasmas in the range 3-7A. The plasma is obtained by focusing a laser beam o n a solid target at 0.53 pm with a 600 ps pulse. The laser intensity o n the target was about lo'% W/cmZ. The targets consisted of mixtures of gold and aluminium& The percentage gold/aluminium in the target have been varied : 0. 3, 8 . 14, 4 0 and 100 % of gold. The targets have been overlayed with plastic in order to reduce the reabsorption of X ray in the plasma which is then confined on the center of the focal spot.
2
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DIAGNOSTIC TOOLS
We have diagnosed the plasma from the aluminium emission lines. The electronic temperature was measured from hydrogen-like and helium-like line intensity ratios (cf. Fig. 1 ) ; and the electronic density was obtained from Stark broadening of the 4 -> n = 1 line width : Hex (cf. fig. 2). helium-like n
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The parameters have been derived by comparison of experimental results to theoretical spectra. These spectra have been computed with the collisional-radiative code RATION coupled to SPECTRA which takes into account Stark broadened spectral profiles and radiative transfert.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988177
JOURNAL DE PHYSIQUE
Cl-358 experimental spectrum
best theoretical adiustment
2.1
1.3 energy ( k e V )
Figure 1
:
Diagnostic of temperature for a plasma with 8 % of gold
->
KT- 575 eV.
Figure 2 : Diagnostic of density for a plasma with 8 % of gold - r Ne = 7.5 102' Full curves : theoretical profiles. Points : experimental profile.
ern-=.
3. EXPERIMENTAL RESULTS Varying the gold and aluminium percentages we have essentially changed the to 7 . 5 1 0 ' ~ m - ~ ) . the temperature remaining constant (about density (from 10" 5 7 5 eV) in the probed region of the plasma. The electronic density decreases as soon as the gold percentage was more than 7 %. If the gold percentage is less than 40 %, the nickel-like ion density is very low : the ratio of nickel-like ion density to the total ionic density is less than and only 3 line intensities are measurable : 3d9=,, 4f,,, (J-1) -> 3dt0 (J-0) 3 %, labelled 4f2, 3d9,, 4p3,, (J-1) -> 3d'O (JPO) labelled 4p1, and 4plI2 (J=l) -> 3dto ( J 4 ) labelled 4p2. In a pure gold plasma, the nickel-like 3d9,,, ion density is 5 % of the total ionic density. This percentage is sufficient to rise 7 line intensities in a measurable range. 4
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COMPARISON WITH COLRAD THEORY
Experimental measurements have been compared to the predictions of the collisional radiative model of gold COLRAD. This code describes the 3~~ 4e nickel-like excited levels ( e varying from 0 to 3). The relativistic distorted wave method has been developed to calculate the excitation cross-sections~2'. This method is guided by two central ideas :
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Analytic factorisation of the collision excitation cross section formulas into a radial part common to many transitions, and an angular part which is specific for each transition but is easily calculated. The radial part depends indirectly on the transition energy AE. This dependence is weak, allowing linear approximation.
For the gold-aluminium mixtures, we have compared the experimental nickel-like line intensity ratios (4f2/4pl and 4f2/4p2) to the theoretical values calculated with the temperature and density determined from the aluminium emission lines (cf. Table I). The results are in close agreement. and density Electronic temperature (400 I T < 600 eV) (9.10" < ne (5.10" ~ m - ~ values ) have been de:iied for a pure gold target by comparison of nickel-like line intensity ratios with the COLRAD model (cf. Fig. 3). The obtained values agree with the extrapolation of the mixed targets values (cf. Table 1).
Ne (cm-=)
Te (eV)
% of Ni-Like
100
9.10'~1
