UDC 533.916

PHYSICS

Academician N. G. BASOV, V. A. BOIKO, Yu. A. DROZHIN,
S. M. ZAKHAROV, O. N. KROKHIN, G. V. SKLIZKOV, V. A. YAKOVLEV

INVESTIGATION OF THE INITIAL STAGE OF THE GAS-DYNAMIC EXPANSION OF LASER-PLUME PLASMA

1. The gas-dynamic motion of plasma accompanying the high-temperature heating of condensed matter by focused laser radiation (¹) has been investigated in a number of works (see, for example, (²)), where the principal gas-dynamic parameters of the plasma were determined (density distribution, temperature, velocity). The importance of such investigations is due to the possibility of using laser plasma for thermonuclear fusion (¹), as a source of multiply charged ions for spectroscopic investigations of astrophysical interest (³), for accelerator technology, etc.

Fig. 1. Slit streak record of the emission of a carbon plume, obtained by means of an electro-optical converter, in the light of spectral lines of various ions.
a — CVI, \(\lambda = 5292\) Å; б — CV, \(\lambda = 4946\) Å; в — CIV, \(\lambda = 5801.51\) Å; the emission in the light of the indicated wavelength is localized near the target because of the short lifetime \(\tau = 2\) nsec of the upper level of the transition under consideration \(3s\,{}^{2}S — 3p\,{}^{2}P^{o}\); г — CIII, \(\lambda = 4662.7\) Å; д — continuous background, \(\lambda = 5000\) Å; е — CVI, \(\lambda = 5292\) Å; ж — CV, \(\lambda = 4946\) Å

However, in previous experiments the emission spectra and gas-dynamic parameters of the plasma were investigated over large time intervals exceeding the duration of the laser pulse. The purpose of the present work is an experimental study of the dynamics of motion and the kinetics of ionization processes in laser plasma with high time resolution. Investigation of the expansion of plasma during the action of the laser pulse and at distances \(r\) from the target surface comparable with the diameter \(d\) of the focal spot of the laser radiation made it possible to trace the different phases (including the initial heating stage) of the motion of matter and the “quenching” of the ionization state of the plasma.

2. The radiation of a neodymium laser (energy 10 J, full width at half maximum 15 nsec) was focused by a lens with \(f = 5\) cm onto the surface of a carbon target in a vacuum of \(10^{-6}\) torr. The structure of the expanding plasma was investigated from its emission. A slit image of the plume in the light of the lines of multiply charged carbon ions was swept in time on the screen of an electro-optical photochronograph (⁴). The spatial resolution was 20 lines/mm, the time resolution 0.5 nsec. To measure the electron density, an interferometric method and Stark broadening of lines were used.

Figure 1 shows characteristic streak-camera records of the plume in the light of the corresponding lines of carbon ions. The expansion of each type of ion was investigated by oscillographic \((^{5})\) and electro-optical methods. As a result of processing the data, space–time diagrams of the ion expansion were obtained (Fig. 2). At \(r \leqslant 1\) mm, in the visible region the plasma emits a continuous spectrum (see Figs. \(1a\)–\(d\)). Lines are observed only at distances \(r \geqslant 1\) mm. With an increase to 10 mm, one observes

Fig. 2. \(r\)–\(t\) diagrams of the expansion of carbon ions of different multiplicity. Oscillograms of the emission of the line of these ions at a distance \(r = 4\) mm are shown. The shaded region is where the continuum intensity exceeds the line intensity.

a detachment of the glow of CVI and CV ions from the target (see Figs. \(1e, zh\)). In this case the regions occupied by ions of different charges partially overlap, although there are no jumps in the density of matter in the plasma.

1. From an analysis of the experimental data one can construct the following model of the gasdynamic motion of the heated material. From the heating region \((r < d)\), where, according to measurements of recombination x-ray radiation \((^{5})\), the electron temperature is \(T_e \sim 120\) eV, the plasma moves into vacuum perpendicular to the surface with a velocity \(u \sim 6 \cdot 10^6\) cm/sec. In this region the velocity of plasma motion is close to the speed of sound, and the ion temperature corresponding to this velocity is \(\sim 125\) eV. A substantial acceleration of the plasma is observed at distances \(r \leqslant 1\) mm. The velocity of motion of the material in this case exceeds the initial velocity by several times. Thus, for the outer boundary of ions with \(C_{z=6}\), the asymptotic velocity 10 nsec after their departure from the heating region, determined from the glow of CVI ions, is \(\sim 3 \cdot 10^7\) cm/sec, which corresponds to an energy of directed motion of \(\sim 5\) keV.

Determining experimentally the values of the velocity and density of the plasma along the ion trajectory and assuming the plasma expansion at a distance \(r > d\) to be adiabatic, one can calculate the rate of recombination and ionization. The “freezing” effect arises because, along the ion trajectory, the density falls as \(u^{-1} r^{-2}\), and the recombination time becomes much longer than the characteristic expansion time. Thus, for example, for the process \(C_{z=6} \rightleftarrows \mathrm{CVI}\) \((^{6})\), “freezing” of the maximally attained degree of ionization occurs

several nanoseconds after the onset of motion of the “elementary volume” of plasma. The same freezing process also occurs for the other ions. Hence the layered structure of the plume, which is observed in the photographs in Figs. 1e, ж and in the expansion diagrams in Fig. 2, becomes understandable. It is seen that ions of a given kind fly in the form of a spherical layer. An analogous phenomenon may occur during the expansion of a heated gas cloud in vacuum. (⁷).

1. From the absolute values of the density and velocity, the total mass of bare carbon nuclei and C VI ions was determined; it is equal to \(10^{-7}\) g. The kinetic energy of motion of this mass is \(\sim 3\)—4 J.

Estimates of the energy lost by the plasma enclosed in the region \(r < d\) to radiation in the range 20—100 Å over a time of 40 nsec give a value of about 0.5 J. The tables (⁸) were used for the estimates.

The authors thank I. L. Beigman and A. V. Vinogradov for useful discussions, and V. A. Kovalenko for assistance in the work.

Physical Institute named after P. N. Lebedev
Academy of Sciences of the USSR
Moscow

Received
8 III 1970

CITED LITERATURE

¹ Н. Г. Басов, О. Н. Крохин, ЖЭТФ, 46, 171 (1964). ² N. G. Basov, O. N. Krokhin, G. V. Skilizkov, IEEE J. Quantum Electronics, QE-4, № 12, 988 (1968). ³ Н. Г. Басов, В. А. Бойко и др., Rev. Roumaniade Phys., 13, 97 (1968). ⁴ N. G. Basov, Yu. A. Drozhbin et al., High-Speed Photography, Stockholm, 1968. ⁵ Л. И. Андреева, Н. Г. Басов и др., Препринт Физ. инст. им. П. Н. Лебедева АН СССР, № 157, М., 1968; В. С. Boland, F. E. Irons, R. M. P. McWhirter, Proc. Phys. Soc., 1, 1180 (1968). ⁶ N. G. Basov, V. A. Boiko et al., IX Intern. Conf. Phenomena in Ionized Gases, Bucharest, Romania, 1969, p. 333. ⁷ Ю. П. Райзер, ЖЭТФ, 37, № 2, 580 (1959). ⁸ R. W. P. McWhirter, A. G. Hearn, Proc. Phys. Soc., 82, 641 (1963).