HAL Id: jpa-00219358
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Submitted on 1 Jan 1979
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CARBON SPECTRA FROM CO2 LASER-PRODUCED PLASMAS
R. Bleach, K. Whitney, J. Sandelin, T. Finn, D. Nagel
To cite this version:
R. Bleach, K. Whitney, J. Sandelin, T. Finn, D. Nagel. CARBON SPECTRA FROM CO2 LASER-PRODUCED PLASMAS. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-749-C7-750.
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JOURNAL DE PHYSIQUE CoZZoque C7, suppldment au n07, Tome 40, J u i l l e t 1979, page C7- 749
CARBON SPECTRA FROM C 0 2 LASER-PRODUCED PLASMAS
R.D. Bleach, K.G. Whitney, J.W. Sandelin, T.G. F i n n and D.J. Nagel.
Naval Research Laboratory, Washington, D.C. 20375.
Ultraviolet spectroscopy is a useful technique for the study of plasmas heated by intermediate laser power densities
(%lo8 W/cm 2 ) . In this work, the emission
of solid carbon targets irradiated with 10.6 pm laser pulses was measured and calculated.
The gain-switched pulses consisted of a 70 ns FWHM spike containing 0.4 of the energy followed by a 1 us tail. The focal area for normal incidence on planar gra- phite targets in vacuo was 1 cm 2
.
Averagepower densities were near 5 X lo8 W/cm 2
.
The plasmas were observed parallel to the target plane with an open shutter camera and a 1 meter near-normal-incidence gra- ting spectrograph operating in the 30- 300 nm range. The time- and space- integrated spectra were recorded on Kodak 101 film. Figure 1 shows data from part of the spectral range. Strong line radiation is evident.
Standard wavelength tables were em- ployed to identify the observed lines.
Radiation from CI through CIV was nobed, with most lines from the two highest ion-
ization stages. For example, many lines from CIII triplet systems (with principal quantum numbers n5( and An52) and from CIII singlet systems (n55 and An(3) were observed. CIV doublet-system lines
(1-155 and An=l) were also identified. The most intense is the CIV line at 155 nm.
The CIV 4d-5f and 4f-5g transitions, at 252.4 and 253 nm respectively, suggest that electrons recombined with CV (He-like) ions. The existence of CV ions implies temperatures near 10 K. 5
A stationary, local-thermodynamic- equilibrium (LTE) model was developed to compute radiation emitted from the plasma.
The electron density in the excitation region (%lo1' el./cm2 for C02 lasers) and the temperatures (>lo 4 K)justify the use
of an LTE model. The effects of thermal conduction and radiative cooling were in- cluded, but hydrodynamic flow and radiation transport (opacity) were not computed.
The time history of the total line output was calculated for each ionization stage, as shown in Figure 2 for an irradiance near
8 2
2 X 10 W/cm
.
During the spike of the laser pulse, a large fraction of the ions exist as CV, and the CIV radiation de- creases. It recovers as the plasma cools and recombination occurs. CIII and CII emission appear sequentially. CI emission, not shown in Fig. 2, occurs late in the pulse. The computed relative intensities agree qualitatively with the measured spectra, that is, CIII and CIV are predominant.Absorption lengths were estimated to be less than 100 um for CIII lines (2p-3d at 57.4 nm and 2s-2p at 97.7 nm). This in- dicates the need for more complex calcula- tions, including radiation transport, in order to use line ratio techniques for temperature determinations. The analysis of the measured carbon spectrum will be discussed from the point of view of these opacity considerations and comparisons made against calculated emission spectra.
"Science Applications, Inc.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797362
C Target C IV Vacuum
F i g u r e 1: Densitometer t r a c e s o f c a r b o n u l t r a v i o l e t s p e c t r a e x c i t e d by C 0 2 l a s e r p u l s e s . Contamination l i n e s from oxygen and n i t r o g e n o c c u r above a b o u t 12 eV.
t [psecl
F i g u r e 2 : Computed r a d i a t i v e power f o r t h e i n d i c a t e d i o n i z a t i o n s t a g e s . See t e x t .
