EXPERIMENTAL STUDY OF POPULATION INVERSION IN LASER PLASMAS

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EXPERIMENTAL STUDY OF POPULATION INVERSION IN LASER PLASMAS

J. Bocher, M. Busquet, P. Combis, J. Le Breton, M. Louis-Jacquet, J.

Mexmain, M. Naudy

To cite this version:

J. Bocher, M. Busquet, P. Combis, J. Le Breton, M. Louis-Jacquet, et al.. EXPERIMENTAL STUDY

OF POPULATION INVERSION IN LASER PLASMAS. Journal de Physique Colloques, 1986, 47

(C6), pp.C6-253-C6-257. �10.1051/jphyscol:1986632�. �jpa-00225875�

JOURNAL DE PHYSIQUE,

Colloque C6, suppl6ment au n o 10, Tome 47, octobre 1 9 8 6

EXPERIMENTAL STUDY OF POPULATION INVERSION IN LASER PLASMAS

J.L. BOCHER, M. BUSQUET, P. COMBIS, J.P. LE BRETON, M. LOUIS-JACQUET, J.M. MEXMAIN and M. NAUDY

Commissariat A 1'Energie Atomique, Centre dlEtudes de Limeil-Valenton, B.P. 27, F-94190 Villeneuve-Saint-Georges, France

Au moyen du laser OCTAL, aux longueurs d'onde de 1,06 p et 0,35 pm, nous avons irradiB ces cibles minces composges de diffgrents matsriaux (AB, CH, Au) sur une tache focale soit circulaire, soit lin6aire et 3 des Bclairements compris entre 5 . 1 0 ~ ~ w/cm2 et 5 . 1 0 ~ ~ w/cm2. Les spectres X enregistrgs dans les garnmes 5

-

^{8 A et 15}

-

^{300 A} ont permis de mettre en Bvidence des inversions de population dans des plasmas d'aluminiurn et de carbone.

Thin and thick composite targets of various materials (Al, CH, Au) have been irradiated on both circular and linear focal spots with the OCTAL laser facility at wavelengths 1.06 and 0.35 p and laser irradiances 5 1012 w/cm2 to 5 1014 w/cm2.

X ray spectra in the range 5

-

^{8 A and 15}

-

^{300 A} recorded respectively by means of flat TlaP and grazing incidence spectrographs gave some experi- mental evidence of population inversion in aluminum plasmas.

FIG 1 Circular focal spot

(BL -

1014 w/cm2)

H.1 H.?

low density

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986632

JOURNAL D E PHYSIQUE

In several previous experiments performed with layered targets irradiated at 1.06 and 0.35 p, x-ray spectra recorded in the short wavelength range (5

-

⁸

^A)

had given some evidence of population inversion : in the case of aluminum coated gold target, the intensity ratio of ls2

-

ls3p and ls2

-

ls4p lines from He-like Al ions appeared to be close to one in the low density plasma blow-off (Fig 1).

With gold coated aluminum microdiscs, the intensity ratio of 1s

-

^{4p and}

Is

-

5p lines from H-like Al ions was found greater than 1 in the high density region (Fig 2)

FIG 2

FIG 3

-

high density

X U V S P E C T R O G R A P H

-

CYLINDRICAL L E N S Sub K e V

STREAK_CAMERA

A more detailed analysis has been carried out with targets typically composed of 1500

A

plastic substrate coated with 1000 A of aluminum.

A simplified schematic representation of the experimental arrangement is shown in fig 3. Four of the eight beams of the laser OCTAL have been focused onto targets by both cylindrical and spherical lenses.

The two main diagnostics were a grazing incidence spectrograph (GIS) with a toroXdal collection mirror (Fig 4) and a flat TlaP crystal spectrograph (FCS) with a spatial resolution slit (100 pm width). Both have a high spectral resolution : respectively

a f f

^{= 2 1 0 3} for GIS and 400 for FCS. X-ray spectra recording is performed with film : SC5 plates for GIS and Kodak SB2 for FCS. In addition we imaged the plasma in the 1

-

^1.5 keV X-ray energy range using a pinhole camera

(30 pm in diameter).

FIG 4

-

E L E C T R O H l C

L A S E R B E A M I U A D l N O O E Y l C E

l N C l D E H C E 87.10.

SPECTRAL RANGE 10 t o 3 7 0 i

RESOLVING POWER 1.1 1 0 ' ( 1 8 0 ~ )

For this diagnostic a typical result is shown in fig 5 for an aluminurn target irradiated at 0.35 pm laser wavelength and 5 1012 w/cm2 incident intensity (cylindrical focusing). The transverse dimension of the plasma was approximately 180 pm and a good superimposition of the four linear focal spots was achieved.

FIG 5

-

9,

.

^{5.10" Il/c.'}

A

-

^{0.35 m}

C6-256 JOURNAL DE PHYSIQUE

Fig 6 a,b shows a small part of same spectra recorded over the 20

a -

³⁸⁰

a

wavelength range, for two types of target structures : first an extended plane target is deposited on a brass ring, second a same foil is deposited on a support provided with a wall located at 200 pn from the rear side of the target. No detectable difference was observed between the two spectra.

FIG 6 a,b

-

EXTENDED P U N E TARCEZ

P L 8 T t C F O I L ( 1 S 0 0 1 ) C O A T E D W I T H L1 1 0 0 0 1

WALL-SIDED TARCET

P A S T I C FOIL ( I 6 O D A I COATED W l T H &I l 0 0 0 1

In these experiments the expected mechanism of inversion is recombination and as it is shown in Fig 7 the target of second type seemed favorable to increase the temperature of the plasma owing to its collision with the copper wall, followed by its rapid decrease enhanced by radiative cooling. The unfruitful results were probably due to the too high density of the aluminum plasma during its expansion.

At last, targets of the first type were irradiated with the same laser at 1.06 p. For two plasma lengths (4 and 8 mm) XUV spectra were recorded in the same wavelength range as previously (Fig 8).

A significant increasing of the intensity of the 5f

-

3d line of lithium-like ions of aluminum versus the plasma length was observed. This might be the result of a X-ray amplification as it has been demonstrated in similar experiments by

P. JAEGLE at lower laser intensity.

FIG 7

-

TARGET STRUCTURE

1; : r

^{-:ER PULSE}

=

^r

\ 1

SPATIAL VARIATION OF ELECTRON

TEUPERATURE AND D E N S I N OF THE CENTRAL RESH OF THE ALURINUM

4 fl 0 . 5 1 . 3 z.5 ^{F O I L}

T I R E (8s)

FIG 8

I I

I I

-- LA- ^I

^I