Spectrum of X-ray emission from nsec CO2-laser produced (CH2)n and A1 plasmas

Volume 53A, number 6

PHYSICS LETTERS

28 July 1975

SPECTRUM OF X-RAY EMISSION FROM nsec CO2 -LASER PRODUCED (CH2 )~AND Al PLASMAS H. P~PINand H.A. BALDIS INRS-Energie, Universitédu Québec,

CF.

1020, Varennes, Québec, Canada

Received 16 May 1975 Studies of the X-ray emission from nsec CO2 laser produced plasmas indicate a stronger deviation of the electron distribution from equilibrium for a (CH2)~plasma than for an Al plasma. The lowest spectral temperature measured is 300 eV at the maximum flux of 5 x 1012 W•cm~.

In this paper we investigate the soft X-ray emission spectrum in short pulse CO2 laser produced Al and (CH2)~plasmas using the absorber method. The resuits indicate an appreciable deviation from a Maxwellian distribution of the electron energies in the case of the (CH2)~plasma and somewhat less for the Al plasma. The lowest spectral temperature measured with an appropriate combination of very thin absorbers is also reported. These results are compared with other work done using short pulse CO2 lasers and (CH2)~target [I]. Electron temperatures measured from X-ray emission of long pulse CO2 lasers produced plasmas [2—6]are also compared with our results. The laser system used for these measurements is described elsewhere [7]. A 4 J, 1.8nsec pulse (FWHM) at 10.6 pm wavelength is focussed with an f/2 off-axis parabolic mirror on the targets. The focal spot diameter was2.250pm giving for a maximum flux of The targets the experiments 5reported X 1012 were W. cm 120pm thick (CH 2)~films and 5Opm thick Al sheets. The results given here were taken with the target normal to the laser beam axis. The X-ray radiation was recorded with two identical detectors using a 0.6 mm thick Ne 102 scintifiator and a RCA 8645 photomultiplier tube. The two detectors were viewing the plasma from approximately the same angle at 45°from the laser beam axis. Movable frames supporting the various absorbers enabled us to change the absorbers between shots. Various lead pinholes could be placed in front of the detectors. In order to operate the photomultiplier tube in the linear regime various neutral filters could be placed in front of the photomultiplier. The signals were displayed on a Tektronix 7904 oscilloscope. The

veiyllium absorbers used varied in thickness from 12 pm to 800pm. In order to measure low temperature 300 eV we used the thin foils combination of 12pm Be in one channel and 2.5pm Al + 12 pm (CH2)~in the other channel which gives the temperature in the spectral interval around 1 keV. The ratio of the intensities of the radiation transmitted through the various absorbers was given by

i I

2 Ec (key)3 I I

I

5 I

-

2000eV

-

-

-

-~

0.1

-~‘ ~‘

1400eV:-

--

: -

-

-

1000eV-

-

-

\\

~.oi

“s

:

:

400~~ •

-

800eV

200ev )

-

2 fl

-

~

-

.00~

~

i~o

400

800

Be Absorber thickness (sm) Fig. 1. X-ray spectrum of ( CH2)~and Al plasmas.

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Volume 53A, number 6

Table 1 Spectral temperature derived with the foil combination 12 ~m Be/2.5 ~tm Al + 12 ~m (CH2)~. 2)

Ø(W

5 X 1O~~

1012

300 ~ 100 350 x 100

180 ~ 50 200 x 50

.m

(CH2)n

Te(eV)

Al

Te(eV)

the ratio of the amplitudes of the signals produced by the two detectors. The calibration of the detectors was checked during each run. The X-ray emission at the maximum flux from the Al and (cH2)0 plasmas is shown in fig. 1. The relative intensities transmitted through six different absorbers are plotted versus the cutoff energies E~of the absorbers (Ec is defined by K(E~).d = 1, K = absorption coefficient of the absorber, d = absorber thickness). The solid curves are calculated under the assumption of thermal bremsstrahlung and recombination emission. The curvature of the spectra of (CH2)0 indicates a considerable deviation from the Maxwellian distribution for the electron energies. This behaviour is in agreement with results of ref. [11which are related to a higher energy range of the X-ray spectra. Refs. [2 o] report also high temperature measurements with low Z plasmas and thick absorbers which indicate nonequilibrium electron distribution. In the case of the Al plasma the various combinations of absorber thickness indicate a quasi-maxwellian distribution for energies lower than 2.5 keV. A small deviation is seen for higher energies. The different X-ray emission spectra for the low Z plasma [(CH2)~] and the higher z piasma [Al] is in agreement with measurements taken at 106pm radiation on (CD2)0 and Fe plasmas [8]. The deviation of the distribution function from the Maxwellian form can be well explained on the basis of plasmas instabilities [9], the thresholds of which are exceeded in this experiment [61. The plasma oscillations induced by the instability provide a mechanism for electron acceleration which affects primarily high energy electrons for which the Coulomb collisions are less effective. The (CH2)~spectrum is more affected by the instabilities than the Al one. This is consistent with the fact that the higher energy part of the spectrum involves ions for which the ionic charge z is higher in the Al plasma than in the (CH2)0 plasma. Due to the higherZ number the development and the effect of the instabilities are less important with Al 494

28 July 1975

PHYSICS LETTERS

plasmas. It is to be noted that the degree of deviation of the distribution function from the Maxwellian form decreases with flux. Table 1 gives values of the spectral temperature T~ obtained for Al and (CH 2)~plasmas for two different fluxes using the 12pm Be and 2.5 pm Al + 12pm (CH2)0 absorbers. Due to the nonequilibrium electron distribution the spectral temperature calculated depends on the thickness of the absorbers used. In order to get a spectral temperature which is fairly close to the temperature of the bulk of the electrons it is necessary to use thin foils which sample relatively low energy X-rays. Beryllium is not a suitable type of filter for good discrimination in a relatively short spectral interval. The combination 12pm Be/25 pm Be does not give the lowest spectral temperature when the Xray radiation is strongly non-thermal. The thin K edge Al filter acts more as a true filter around the K edge value and so the combination 12pm Be/2.5 pm Al + 12pm (CH2)0 enables us to reach a low spectral temperature. The small thickness of (CH2)~used with the Al filter is necessary to render it effectively opaque in the transmission window associate with the L absorption edges. This work has been possible thanks to a fruitful collaboration between INRS-Energie and the Defense Research Establishment of Valcartier. The authors acknowledge gratefully the assistance of J. Gauthier, A. Thibaudeau, C. Trapanier and J.G. Vallee.

References [1] iF.

Kephart, R.P. Godwin, G.H. McCall, Appi. Phys. Lett,

25 (1974) 108.

[21 RE. Beverly, Phys. Lett. 44A (1973) 321. 131 P.E. Dyer, Di. James, S.A. Ramsden, M.A.

Skipper, App. Phys. Lett. 24 (1974) 316. 141 J. Martineau, S. Repoux, M. Rabeau, G. Nierat and M. Rostating, Opt. Comm. 12 (1974). [5] C. Yamabe, E. Setoyama, M. Yokoyama, C. Yamanaka, Lett. 50A (1975) 349. [61 Phys. K. Dick and H. Pepin, Opt. Comm. 13 (1975) 289. [7] F. Rheault, J.L. Lachambre, P. Lavigne, H. Pepin and H.A. Baldis, to be published in Rev. Sci. Inst. (1975); Defense Research Establishment of Valcartier Report no.

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N.G. Basov et a!., Soy. i. Quant. Elect. 3 (1974) 444. W.L. Kruer and J.M. Dawson, Phys. Fluids 15 (1975) 446.