Fast particle emission from CO2 laser produced plasmas

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OPTICS COMMUNICATIONS

March 1978

FAST PARTICLE EMISSION FROM CO 2 LASER PRODUCED PLASMAS J.L. BOCHER, J. MAT1NEAU and M. RABEAU Commissariat h l'Energie A t o m i q u e , Centre d'Etudes de Lhneil B.P. No. 27, 94190 - Villeneuve-Sablt-Georges, France

Received 21 November 1977 Spatial distributions and spectra of non thermal particles emitted from C O 2 laser aluminum plasmas have been recorded. With laser fluxes greater than 1013 W/cm2 ions with MeV maximum kinetic energy have been detected as well as fast electrons in the range of 50 to 500 keV. The results are discussed in terms of resonant absorption as a function of different parameters such as laser flux and angle of incidence.

1. Introduction Up to present time, experimental results have confirmed that the interaction of intense CO 2 laser radiation with solid targets was strongly non linear at relatively low fluxes [ 1 - 7 ] . Characterization of non thermal particle emission has proven to be an essential information for a precise understanding of the current laser fusion investigations. In this paper, we present some experimental results regarding the interaction of a nanosecond CO 2 laser with solid aluminum targets. Experiments have been performed up to 3 X 1013 W/cm 2 . At this level of flux, we can admit that inverse bremsstrahlung is unimportant compared with resonant absorption [1]. To investigate the absorption of the laser light by this process we have perfomled measurements of fast particles, the effects of angle of irradiation and incident laser flux on their production, kinetic energy and distribution.

allel to the laser beam axis. By using different targets, it was possible to vary the angle of incidence ~ from 0 ° to 20 ° (see fig. 1). With such a geometry, symmetry with

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2. Experimental set up and diagnostics

-@ The laser system provided energy up to 10 J in a 1.7 ns (fwhm) pulse. The energy contained in the laser prepulse did not exceed 10 -3 J, while the contrast ratio was 2 × 105 (in power). The circularly polarized laser beam (70 mm in diameter) was focused onto plane or conical massive aluminum targets by means of a 20 cm focal length NaC1 lens. The measured focal spot size was 120/.tm in diameter. Tile axis of conical targets was par-

j/

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t./"

Fig. 1. Experimental set up, diagnostics and target. 297

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respect to the plane of incidence was preserved lot all the diagnostics. Faraday cups located symmetrically around the target are able to collect ions in dilTerent directions for an angle'y between 0 ° and 30 ° with respect lo rite target normal. A TSN 503 streak camera [8] provided with a Be/Au p h o t o c a t h o d e sensitive to tile X ray spectrum in the energy range 1 10 keV was located either at 45 ° or 90 ° with respec! to the plane of incidence. The X-ray spectrum was filtered with an additional 2 5 / m l Be [oil. The temporal resolution of the camera was better than 100 ps. An 8000 V / c m electrostatic spectrometer recorded fast electron spectra in the energy range 30 to 500 keV. The photographic film chosen for use in tile spectromelet was placed behind a 25 Hm beryllium window. The slit aperture was reclangular (2 X 35 ram). The resolution in energ5, was 40%. l{xperimenlal observations were made in the plane of incidence.

March 1978

c o m p o n e n t (defined in fig. 3) as a function of ~lle angle of incidence for different i , c i d e n l laser fluxes. The plol. ted data were recorded close to 7 -- 12 ° aml 25" with respect to the target normal. For 7 and ez fixed, the kine tic energy of the [astest ion componelll is contintxous[v increasing with Ihe laser flux in the range ~d6 X 1012 to 3 X 1013 W/cm2. This behaviour is valid FOl any angle of incidence.

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l:ig. 3. Typical ion time-of-llight signals. Showing tile so called "fastest ion component" for different laser fluxes, p - 0 °.

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March 1978

keV

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Fig. 4. Spatial energy distribution o f the fastest ion component close to the normal to the target.

b) Above 6 × 1012 W/cm 2 fastest ion components occur and are preferentially emitted close to the target normal. For a given angle o f incidence ~, their spatial energy distribution is weakly anisotropic inside a cone of twenty degree half angle (see fig. 4). For "7 fixed, the kinetic energy o f the fastest ion component is very sensitive to a five degree variation of the angle o f incidence. The curves have a pronounced maximum for ex = 10 °. Moreover, we also note that the spatial distribution in energy corresponding to relatively slow ion component (for ¢ ~ emax/10) is quasi isotropic. Knowing experimentally from X ray line emission the highest number o f charge during the interaction, we can deduce for the AI I1+ ion a maximum kinetic energy o f 180 eV per number of charge (with emax ~ 3 X 1013 W/cm 2 and a = 10°).

4. X ray and fast electron emission For ~ = 10 ° typical densitometers of the X-ray signal obtained with a fast streak camera are presented in fig. 5. The X-ray pulse duration was o f the same order of the laser pulse duration. F o r the measurements made at ")' = 45 ° with respect to the plane o f incidence, we observed a second peak at about 2 ns after the beginn-

With

8000

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Fig. 5. Time resolved X-ray measurements obtained with a TSN 503 streak camera, v = 45 °. ing of the X-ray emission. We have controlled that this second emission was the signature (via photoelectrons) o f fast electrons in the range of 20 to 60 keV. Effectively when adding a 8000 V/cm electrostatic field between the plasma and the entrance slit o f the camera, the second peak disappeared. These two signals coexisted only for incident laser fluxes greater than 1013 W/cm 2. For observations at 90 °, even at maximum flux available onto the target we did not record the second peak. For more extensive informations regarding the fast electron spectra, we have used the simple electrostatic spectrometer described previously. One o f the spectra is reproduced in fig. 6. The electron spectrum appears clearly as non thermal. The recorded spectra seems to be composed by two different populations. The low and high energy distribution are centered at about 50 and 300 keV respectively. However when computing the percentage of transmitted energy through a 25 ~m beryllium window we found that the recorded spectra in the film is strongly modified in particular for low energy electrons (down 100 keV). The poor resolution of the instrument as well as the misappreciation o f the film sensitivity do not 299

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March 1978

sell consistent field and transfer their kinetic energy to them. Tile maximum of tile kinetic energy of tile faslest ion component obtained for cc = 10 ° is related to tile maximum of the well known function q,lr) [91 which occurs tbr r = 0.7 where r is equal to (k 0 L) 1/3 X sin o(, L being the characteric density scale length and oe tile angle of incidence. Under Ibis assumption we can cst imate a density scale length of tile order of 100 Hm. Th rough tlle ponderomotive force some electrons can gain a nlaxinmm kinetic energy given by lhe formula

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e(keV) = 6.8 X 10 -7 X [L 2 (/lm) ~ (Hm)] I/3 X c~ 1/2 (W/cm2).

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Fig. 6. Electron spectrum and computed transmission and back scattering of a 25.urn Be window for fast electrons. allow to give any quantitative value regarding tile number of electrons and consequently the energy carried out by them.

5. Discussion For oblique incidence the electric field component parallel to the density gradient drives Langmuir oscillations whose mnplitude increases as we approach the resonance point at n = n c. In a warm plasma longitudinal waves are generated which carry some part of the energy of the electromagnetic wave. Finally the energy of the plasma waves goes to heat electrons by collisionnal and/or Landau damping. But some others electrons are also accelerated by the ponderomotive force Fp ~ V (E2)/2eo which resuits from the presence of an intense inhomogeneous electric field inside the plasma near the critical density. Thus the longitudinal wave damping as well as the acceleration of electrons through the ponderomotive force contribute to the absorption of the laser light. These two mechanisms are angle dependent. In both cases, most of the electrons pull the ions by means of 300

This relation valid for o~ = 10 ° and L ~ 100 Hm leads to etnax ~ 170 keV with (1/- 3 X 1013 W/tin 2 . Tiffs maximum kinetic energy is comparable to tile maximum kinetic energy of 180 keV per number of charge for A111+ ion. Because fast electrons and fastest ions are intimately related through a sell" consistent field, the corresponding part of tile electron spectra cannot be observed. Then only run-away electrons are recorded as shown in fig. 6. It should be noted that tire characteristic electron density scale length L taken out from fast electron and ion measurements is a time and space average value near and down the critical density. In fact for incident laser fluxes greater Iban 1013 W/cm 2, radiation pressure may become comparable lo kinetic pressure in the vicinity of critical density. Thus the expected profile steepening may be important and characterized by an electron density scale length smaller than 100 #m. In conclusion, we can say that the results bring new arguments in favor of a resonant process for laser light absorption and production of fast particles. But the apparently obvious experinrents of varying the angle of incidence are still difficult because of tile space and time ewHution of the plasma and tile changing irradiation conditions during the interaction [11,121. A more complete study will involve polarization measurement s of incident and reflected laser light as well as a suitable interferometric diagnostic in order to tbllow the density profile near and above-the critical density [131. The authors are indebted to Drs. A Bekiarian, C. Patou, P. Guillaneux and M. Decroisette for helpfull

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discussions. We acknowledge Bayer for numerical calculations regarding the transmission o f the beryllium foil for electrons up to 500 keV incident energy. We would like to thank G. Nierat and M. Rostaing for their participation in the experiments, E. G o e t c h y and M. J o u r d a i n for their technical assistance for laser working.

References [ 1 ] J.L. Bocher, J.P. Elie, J. Martineau, M. Rabeau and C. Patou, Laser hlteraction and related plasma phenomena, Rensselaer Polytechnic Institute. Troy, New-York, Nov. 1976. [2] R.A. Haas, M.J. Boyle, K.R. Manes and J.E. Swain, J. Appl. Phys. 47 (1976) 1318. [3 ] C. Yamabe, E. Setoyama, A. Thein, M. Yokohama and C. Yamanaka, Japn. J. Appl. Phys. 16 (1977) 131.

March 1978

[4] P. Kolodner and E. Yablonovitch, Phys. Rev. Lett. 37 (1976) 1754. [5 ] B. Grek, H. Pepin, T.W. Johnston, H.A. Baldis and J.N. Leboeuf, to be published. [6] D.V. Giovanielli, J.F. Kephard and A.H. Williams, J. Appl. Phys. 47 (1976) 2907. [7] G.D. Enright, N.H. Burnett and M.C. Ridcharson, to be published. [8] J.P. Gex, N. Fleurot and R. Sauneuf, Revue de Phys. Appl. 12 (1977) 1049. [9] V.L. Ginzburg, The propagation of electromagnetic waves in plasmas (Pergamon Press, 1970)p. 217. [10] D.V. Giovanielli and R.P. Godwin, Am. J. Phys. 43 (1975) 808. [11] J.S. Pearlman and M.K. Matzen, Phys. Rev. Lett. 39 (1977) 140. [ 12 J P. W//gli and T.P. Donalson, Phys. Rev. Lett., to be publislied. [ 13 ] H .G. Ahlstrom, 1 lth Eurepean Conf. on Laser interaction with matter, Oxford, England, Sept. 1977.

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