IEEE Transactions on Electrical Insulation Vol. 26 No. 6, December 1991 1195

Initial Stage of Extended Laser Breakdown in Liquids

V. S. Teslenko Lavrentyev Institute of Hydrodynamics Novosibirsk,

USSR

ABSTRACT The kinetics of a laser breakdown in liquids has been stud- ied both theoretically and experimentally, taking into account stimulated scattering. It has been shown that the directions of optical breakdowns coincide with those of the stimulated Mandel’shtam-Brillouin and stimulated Raman scattering.

1. EXPERIMENTAL TECHNIQUE

HE kinetics of the initial stage of a nonlinear interac- T tion between a single-pulse ruby laser radiation and a liquid as well as final breakdowns were studied by spec- tral, schlieren and interferometric methods. The pressure fields were measured by piezoelectric pickups.

An experimental arrangement is shown in Figure 1. The radiation of the ruby laser (1) with a single-pulse duration of 10 to 30 ns a t half-height and a 15” diver- gence was focused on the cell (2) by the lens (3) , the fo- cal distance being F = 2.5 cm. An appropriate radiation distribution (Figure 1) was formed by the diaphragm (4) (D = 1.7 cm) and the circular or square mask (5) posi- tioned in its center. In the shadow of the square mask (5) in front of the lens there was the prism (6) which deflected scattered radiation and directed it to either the spectro- graph ( F , = 27 cm), the Fabry-Perot interferometer, or the calorimeter (7).

Similar measurements were made behind the cell using the same devices. The laser radiation parameters were controlled through the dividing plate (8) of the photore- ceiver and through the calorimeters (9) . The schlieren and interferometric photographs were realized with the use of laser illumination and both light and electrical de- lays [ I , 21.

2. EXPERIMENTAL RESULTS

T has been established that there exists a powerful I axial radiation from the focal region for water, ben- zene, nitrobenzene and acetone. The spectral composi- tion of radiation corresponds to the lines of stimulated Mandel’shtam-Brillouin and Raman scattering. Figures lb , c, d illustrate the distributions of the Mandel’shtam- Brillouin stimulated and Raman stimulated combination scattering from the focal region, photographed along the axis directly in the cell, in the sites shown in Figure 1. Pattern Id was taken a t a distance of 12 cm from the cell and pattern l e a t a distance of 0.5 m from the cell. Diver- gence of the radiation emerging from the cell through the lens varied over 8 ~ 1 0 - ~ to 5 ~ 1 0 - ~ rad. Positive outputs of stimulated Mandel’shtam-Brillouin and stimulated com- bination Raman scattering, k, = Q,/Qe (Q, is the ener- gy of back axial stimulated Mandel’shtam-Brillouin and stimulated combination Raman scattering from the focal region, Qe is the laser radiation energy), before satura- tion were k, = 0.02 for benzene, k, = 0.035 for acetone and k, = 0.03 to 0.06 for water. The measurement re- sults for k, for benzene, acetone and water for different single-pulse durations are presented in Figure 2.

Figure 3 represents t h e interferograms and schlieren photographs of the breakdown zone with delays with re- spect to a maximum of a laser single pulse. a, b, d: s, c: 1 6 ~ 1 0 - ~ s , a, b, c: benzene, d: water.

0018-9367 $1.00 @ 191 IEEE

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Teslenko: Initial Stage of Extended Laser Breakdown in Liquids

Figure 1. Experimental setup and patterns of radiation distribution. a: laser radiation behind the diaphragm 4 and mask 5, b, c: radiation of Mandel'shtam-Brillouin stimulated scattering and Raman stimulated combination back scattering on the focusing cone axis, f: laser radiation behind the focus with axial Mandel'shtam-Brillouin stimulated scattering and Raman stimulated forward scattering at a distance of 12 cm from the cell, e: the same at a distance of 0.5 m from the cell.

Figure 2. The measurement results on energy of Mandel'shtam-Brillouin stimulated axial scattering and Raman stimulated combination 'back' scattering (Q.) YS. laser radiation energy ( Q e ) .

Illustrated in Figures 3a and b are the breakdowns in benzene at a radiation intensity of 20 MW after 20 and 40 shots, respectively, inside the 100 ml cell. A time interval between the shots was more than 2 min. Axial stimulat- ed scattering from the cell, filled with benzene, was not recorded after 60 t o 100 shots for several hours. The spec-

trograms of axial stimulated back scattering for benzene without a breakdown are presented in Figure 4a. The distance between the cell and the spectrograph slit is 125 cm. Additional lenses were not placed in front of the slit. The spectrum was registered for one shot. Figure 3b illus- trates the spectrogram of axial back stimulated scattering

IEEE Transactions on Electrical Insulation Vol. 26 No. 6 , December 1991 1197

l a se r

Figure 3. Interpherograms and schlieren records taken with delays. a, b, d: lo-‘ s, c: 16x10-’ s, a, b, 6: benzene, d: water.

s 3 s2 6 9 4 , 3 nm 6 3 2 , 8 nm

Figure 4. The spectrum of axial radiation of back stimulat- ed scattering for benzene. a: without breakdown, b: with breakdown.

Figure 5.

with breakdown in benzene, obtained through 30 shots. The da ta provide a space-angular distribution of axial stimulated scattering from a focus along Stokes and anti- Stokes components. I t has been established that each shot ‘eats away’ part of the axial region of the spectrum (aS3, Figure 4b) in the direction from far components (as,) towards the nearest ones (aS,-I)’ with subsequent

The scheme of dynamic distributed feedback in- ducing high-power stimulated scattering genera- tion and a breakdown in the directions I , w + Ro for spherical apd conical lenses, I ’ , w + 0; for a cylindrical lens.

disappearance of the whole axial stimulated scattering. The anti-Stokes components a.re of a.nnular structure on- ly at powers higher tha.n the threshold breakdown. This indicates that the axial region of scattering is spent by

1188 Teslenko: Initial Stage of Extended Laser Breakdown

the interaction with the medium and then is covered by breakdowns.

The axial stimulated scattering generation threshold is close to the reversal threshold of the stimulated Mandel’shtam-Brillouin and stimulated Raman scatter- ing wave fronts and is estimated from the least values of laser pumping power W = Qe/0.57 (see Figure 2) necessary to register stimulated scattering. The liquid breakdown threshold is higher than the stimulated scattering generation threshold by a factor of 1.5 to 2 and coincides with the axial stimulated scattering generation saturation (kinks of the curves in Figure 2). With no stimulated Mandel’shtam-Brillouin or Raman scattering, the filaments of the structural damages in the liquid are not available (Figure 3c), i.e. the filaments along the laser radiation axis are observed only if the axial stimulated backward or forward scattering takes place, Consequently, the structural damages and breakdowns in the liquid along the focusing axis, observed in a sha.dow of laser beam, are the result of the stimulated Mandel’shtam-Brillouin and combination Raman scattering from the focal region. The structural damages and breakdowns along the axis occur within the section, where the stimulated scattering losses take place, up to the threshold value of the nonlinear interaction between the stimulated scattering and the medium. With high power (W > 20 MW) of laser radiation, the axial thresholds reach the window of the lens and destroy the glass.

On focusing radiation by a cylindrical lens, the stimu- lated scattering generation and breakdowns develop along the focal axis of the cylindrical lens.

In the experiments, the breakdown thresholds in water in the laser radiation field account for 4 ~ 1 0 ~ ’ W/cm2. A specific power of the axial stimulated scattering, taking into account the errors, ranges over 10’ to 1015 W/cm2. The distribution close to Gaussian one is typical for a spatial distribution of the stimulated scattering radiation from the focal region in slightly radiated liquids. To es- timate specific radiation power, the beam cross section was determined from the half-height of microphotome- tering photo-probe beams of radiation and filaments of schlieren photographs [l]. Additionally, it should be not- ed that when a metallic target is placed not far from the cell prior to axial radiation of stimulated back scattering (behind prism 6, Figure l ) , spark breakdown is observed on the target. An analogous picture is observed for a cylindrical lens. The breakdown is directed along the fo- cal axis, i.e. outside the laser radiation field [9].

3. DISCUSSION 3.1 KINETIC MODEL OF A

BREAKDOWN

in Liquids

OLLOWING from the experimental results presented F herein and the data available in [2-51, a kinetic model of the extended optical breakdown has been developed. It may be described as follows. In rather pure trans- parent media, the development of stimulated scattering generation precedes the ionization process (breakdown) on focusing laser radiation by a spherical or cylindrical lens. The stimulated scattering generation saturation co- incides, as a rule, with the beginning of breakdown. Thus, it may be that the stimulated scattering is one of the main factors responsible for the kinetics of any kind of break- down, since it may be considered as a further develop- ment of an absolute instability of the stimulated scatter- ing generation in the system with a point or distributed feedback.

Figure 5 displays the scheme of the distributed feed- back development. A hypersonic grating with a shift X/2, A is formed in an ideally pure medium due to incident J and scattered I radiation. Such a grating provides either an external feedback along the directions J1 - 11, Jz - 12 that corresponds to the wave front reversal or the internal feedback with the stimulated scattering radiation output along the directions I , 1’.

In the medium with no extraneous inclusions and de- fects, the breakdown starts to develop in interferential maxima in the form of microbreakdowns by the model of two-photon or four-photon interaction. The microbreak- downs are of random character dependent on a spatial distribution of laser radiation. The appearance of the first random microbreakdowns leads to a sharp switch-on of the internal feedback between the sites of the break- downs, stimulated scattering energy capture, absorption on the interval between the sites and in the sites them- selves. Thus, a visible breakdown in the form of a filament develops. Its volume increases as the laser power increases due to a series switch on of the generation lines and break- downs, for example, in the direction I(O,1, 2). For slightly contaminated media, this kinetic model of a threshold is valid. Due to extraneous inclusions or defects, the point feedback may develop there in addition to the distributed feedback. The point feedback favors the development of the internal feedback, as well as breakdown a t the points themselves and between them. Breakdowns on defects may occur within the framework of the model of thermal explosion [6,7]. The development of the first random mi- crpbreakdown or the presence of medium defects result in a sharp switch-on of the internal feedback between the

IEEE Transactions on Electrical Insulation Vol. 26 No. 6 , December 1991 1199

breakdown points, in a capture of the stimulated scatter- ing energy and in absorption on the section between the points and a t the points themselves. The conditions of development of absolute instability, stimulated scattering generation and breakdowns may be represented as

R2exp(gJI ) 2 1 (1)

where g is the stimulated scattering amplification coef- ficient for a given medium, J is the power of pumping per unit square, I is the length of the initial distributed feedback, and R is the feedback parameter.

The stimulated scattering generation and the extended breakdown develop along the highest values of amplifica- tion increment M = gJI and of the feedback parameters R < 1. With the same values of g and J , the instability develops, as a rule, along the maximum value of I , that corresponds to axial directions of radiation focusing.

In our experiments, the stimulated scattering genera- tion develops a t M = 2 to 5, and the breakdowns occur at M = 7 to 10. Further increases in power ( J > J o ) leads to a self-intergrowth of the distributed feedback ( a , > IO) and to the increase in the breakdown length l b .

In a rough approximation without taking into account spacetime distribution, the distributed feedback length (& ) and the breakdown length (I,) may be estimated from the condition that a specific power is in excess of the stimulated scattering generation threshold ( J o ) and of the breakdown threshold Jb:

As follows from the experimental results, when the specif- ic power of radiation is higher by an order of magnitude than the breakdown threshold a t IO = 0.07 to 0.1 cm, the breakdown length is 1.2 cm. As power further increases, the shell window is destroyed.

From the experimental results and the above consid- erations it follows that the absorption wave in the form of ‘light detonation’ is not the case here, since the laser radiation falls within the lens caustics region ( l a ) . As far as our situation is concerned, the breakdowns in the laser beam shadow are observed with the use of a spherical lens (direction I , Figure 4), and those beyond the laser radi- ation field are observed with the use of a cylindrical lens (direction 1’, Figure 5). Thereby, the point breakdowns, as if ‘instantaneous’ ones, occur throughout the interval 1, 6 F (at least during 16x10-’ s, which become a con- tinuous breakdown with increasing pumping power. The

calculation by the model of light detonation for J = lo1’ W/cm’, p = 1 g/cm3, y = 7 shows tha t the breakdown length may increase only up to Id = 0.2 cm.

The light detonation model in the stimulated sca.tter- ing field is also unacceptable, since continuity and growth of / b the direction towards radiation are inherent in light detonation. In the case under consideration, the exten- sion of the continuous stage of the breakdown increases as the distance from the stimulated scattering source in- creases and often is of point character, as in Figure 3d.

Thus, it may be concluded that the description of the kinetics of extended optical breakdown is to be based on the assumption that the extended laser breakdown in pure media is a ‘catastrophe’ in the process of develop- ment of absolute or convective instability. Initially this corresponds to the processes of cascade-stimulated scat- tering generation, changing into an absorption band of the liquid, and then transforms into a thermal explosion, providing ionization.

Taking into account that the distribution of axial stim- ulated scattering radiation without breakdowns is close to Gaussian, the axial stimulated scattering radiation prop- agation over a medium is likely to be accompanied by self-focusing effects. This corresponds to a further devel- opment of instability of stimulated scattering radiation in the medium. The latter may result in local breakdowns in the field of stimulated scattering radiation.

In real media, the breakdown mechanisms are difficult t o be assigned to one of the models, model of thermal explosion or that of self-focusing. It is to be considered as a development of absolute instability.

4. CONCLUSION

N conclusion, it should be noted that the results de- I scribed herein enable us to develop and improve the ide- ology and technique of a new generation of high-velocity cumulation devices, governed by laser sources, in which the electrodes are placed along the directions of preferen- tial generation of stimulated laser radiation scattering.

REFERENCES

[l] V. S. Teslenko, “Schlieren and Interferometric Meth- ods to Study Laser Breakdown in Liquids”, Kvanto- vaia Elektronika, Vol. 2, No. 6 , pp. 1248-1252, 1975.

1200 Teslenko: Initial Stage of Extended Laser Breakdown in Liquids

[2] V. S. Teslenko, “Effect of Stimulated Scattering on Spatial Structure of an Optical Breakdown in Liq- uids”, Pis’ma V ZhTF, Vol. 8, pp. 77-81, No. 2, 1982.

[6] F. V. Bunkin, M. I. Tribel’sky, “Nonresonant Inter- action Between Optical Radiation and a Liquid”, Us- pekhi fiaicheskikh nauk, Vol. 130, pp. 193-239,1980.

[7] J . Ready, Action of Powerful Laser Radiation, Mir Publishers, MOSCOW, p. 468, 1974.

[3] H. Kogel’nik and C. V. Shank, “Coupled-wave The- ory of Distributed Feedback Lasers”, J . Appl. Phys., [81 yu* p. Raiser, Laser ’park and Discharge Propaga-

Vol. 43, NO. 5, pp. 2327-2335, 1972. tion, Navka Publishers, MOSCOW, p. 308, 1974.

[9] D. V. Vlasov, Yu. S. Kasyanov, V. V. Korobkin, I. L. Fabelinsky, “Fracture of Transparent Glass by Stimulated Mandel’shtam-Brillouin Scattering Radi- ation”, Fiaika tverdogo tela, Vol. 17, pp. 3574-3578.

[4] S. A. Akhmanov and G. A. Lyakhov, “Optical Pumping Inhomogeneity Effects in Lasers and in Stimulated Scattering. Self-excitation due to Dis- tributed Feedback”, ZhTF, Vol. 66, No. 1, pp. 1975. 96-107, 1974. This paper is based on a presentation given at the 10th In-

ternational Conference on Conduction and Breakdown in Di- electric Liquids, Grenoble, France, September 1990.

Manuscript was received on 2 January 1991, in revised form [5] V. S. Teslenko, “Investigation of Nonresonant Gener-

Proceedings of the 1V All-Union Conference, Novosi- birsk, pp. 182-185, 1984.

ation in Liquids. Frequency-rearrangement Lasers” , 12 June 1991.