SUBNANOSECOND SCHLIEREN PHOTOGRAPHY OF LASERINDUCED GASBREAKDOWNA. J. Alcock, C. DeMichelis, and K. Hamal Citation: Applied Physics Letters 12, 148 (1968); doi: 10.1063/1.1651931 View online: http://dx.doi.org/10.1063/1.1651931 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/12/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Evolution of laser-induced carbon particle breakdown in gas Appl. Phys. Lett. 95, 181502 (2009); 10.1063/1.3258493 Schlieren Visualization Technique Applied to the Study of LaserInduced Breakdown in Low DensityHypersonic Flow AIP Conf. Proc. 830, 504 (2006); 10.1063/1.2203293 Laserinduced gas breakdown in the presence of preionization Appl. Phys. Lett. 22, 245 (1973); 10.1063/1.1654626 Highspeed photography of laserinduced breakdown in liquids Appl. Phys. Lett. 21, 27 (1972); 10.1063/1.1654204 WAVELENGTH DEPENDENCE OF LASERINDUCED GAS BREAKDOWN USING DYE LASERS Appl. Phys. Lett. 15, 72 (1969); 10.1063/1.1652907

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Volume 12, Number 4 APPLIED PHYSICS LETTERS 15 February 1968

Film thic:kn ...

Q- 0 '!I.Co .,-ZO'!l.Co • - ao '!I.co .-40'!I.Oo

Fig. 2. Coercivity versus film thickness.

9 10

thickness for various cobalt melt percentages. Ap­parently, the rapid increase in coerClvny associated with the onset of stripe domains is suppressed with increasing cobalt percentage.

Figure 3 shows the dependence of the exponent x on cobalt percentage. Clearly, the coerClvity be­comes more independent of film thickness as the amount of cobalt is increased up to at least 40%.

It is not known whether the postulated mecha­nism governing stripe domain formation also governs the coercivity thickness dependence.

(/)

Q)

c o (/)

c

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o

Fig. 3. Exponent x versus weight percent cobalt.

1 Y. Sugita, H. Fujiwara, and T. Sato, Appl. Phys. Letters 10, 229 'l967).

2 M. M. Hanson, D. I. Norman, and D. S. Lo, Appl. Phys. Letters 9, 99 (1966).

3D. S. Lo and M. M. Hanson,I Appl. Phys. 38, 1342 (1967). 4]. M. Gones, M. M. Hanson, and D. S. Lo, presented at the

International Congress on Magnetism, Sept., 1967, in Boston, Mass. (scheduled for publication, I Appl. Phys. 39, (February 1968).

5N. Saito, H. Fujiwara, and Y. Sugita, I Phys. Soc. japan 19, 1116 (1964).

"T. Iwata, R.]. Prosen, and B. E. Gran,I Appl. Phys. 37, 1285 (1966).

SUB NANOSECOND SCHLIEREN PHOTOGRAPHY OF LASER-INDUCED GAS BREAKDOWN

A. J. Alcock, C. DeMichelis, and K. Hamal Division of Pure Physics

National Research Council of Canada Ottawa 2, Canada

. (Received 29 December 1967)

Second harmonic radiation from a mode-locked neodymium:glass laser, prOViding a 400-nsec-long train of pico­second pulses, has been used as a light source for Schlieren photography of a laser-induced spark. The beam trom a ruby laser, Q-switched by means of a Pockels cell, was focused in air to produce breakdown and synchronization of the two lasers was achieved by switching the Pockels cell with a spark gap illuminated by the mode-locked pulse train.

Shortly after the first observation of a laser­induced spark in air it was found that the expansion of the ionized region occurred asymmetrically with the most rapid growth being directed into the focused beam towards the laser. I Strf"ak photo­graphs of the plasma revealed a well defined front moving rapidly towards the laser with an initial velocity of approximately 107 em/sec. This rather

148

unexpected motion of the plasma was interpreted in terms of a new mechanism, namely a radiation­supported shock wave. By using a model analogous to the theory of detonation waves driven by the release of chemical energy it was possible to account for the initial velocity of the plasma and its sub­sequent motion in the direction of the laser.2

Although numerous studies of laser-produced

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Volume 12, Number 4 APPLIED PHYSICS LETTERS 15 February 1968

plasmas have been made since these initial observa­tions they have provided no additional information on the dynamics of the plasma during the initial phase when it is receiving energy from the focused laser beam. The main reason for this has been the lack of suitable diagnostic techniques to study this period.

The technique which we have employed to over­come the limitations imposed by conventional methods involves the use of a mode-locked laser as a light source, thus permitting Schlieren photog­raphy with framing rates in the gigacycle range and exposure times as short as a few picoseconds.

A schematic diagram of the experimental ar­rangement is shown in Fig. I. A neodymium:glass laser Q- spoiled by means of Kodak 9740 saturable dye provided a 400-nsec-long train of pulses, con­taining a total energy of 0.2 J. The individual pulses were separated by 5.5 nsec and had a duration of 5 psec, as measured by the two-photon fluorescence technique.:l Using dielectric coated mirrors having a reflectivity of 75% an output was obtained from each end of the laser cavity. One beam was focused onto the negative electrode of a spark gap while the other passed through an index-matched ADP crystal, in order to illuminate the Schlieren system with the visible second harmonic radiation. Ap­proximately 1 % of the incident radiation was con­verted to the second harmonic which then passed through a cell containing a copper sulphate solution to absorb the fundamental.

The spark was produced by a Pockels cell Q­spoiled ruby laser with an output consisting of a single pulse having a halfwidth of 20 nsec and an energy of approximately 2 J. In order to synchro-

SRL

I I

\t7 I L.

j--oK

<±> L.

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I I I i ~ P02fj"

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--- -ffi-&-H-8~~--E}-8--,Y--~---AOP C as SM

MLL

Fig. 1. Experimental setup. MLL, mode locked laser; SRL, ruby laser; PC, Pockels cell; SG, spark gap; ADP, second har­monic generator; C, copper sulphate cell; BS, beam splitter; SM, silver mirror; PD, photodetector; L, lens; K, knife edge.

nize the two lasers, the spark gap, triggered by the mode-locked laser, was used to switch the Pockels cell. Using this arrangement the ruby laser pulse occurred approximately 200 nsec after the start of the mode-locked pulse, the jitter being less than 20 nsee.

The radiation from the ruby laser was focused in air at atmospheric pressure by a :~.5-cm-focal-length lens. The beam from the mode-locked laser also passed through the focal region in a direction per­pendicular to the ruby laser axis and was then focused by the Schlieren lens onto the knife edge. An additional lens was used to image the spark on the film plane, the overall optical system having a magnification of 10. Corning glass filters plus neutral density filters were used to eliminate the light from the spark. The use of a round obstacle, 1 mm in diam, rather than the knife edge, per­mitted the observation of the complete shock front and an example of the Schlieren photographs ob­tained with this arrangement is shown in Fig. 2.

Previous studies of laser-produced plasmas~ have demonstrated that extremely high electron densities exist during the initial phase and the central region of the photograph shows clearly the edge of the highly ionized plasma at successive stages in its development. The fact that the plasma is not sym-

o 2 3 4 mm Fig. 2. Schlieren photograph of laser-induced breakdown

in air. The ruby laser beam is incident from the left.

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Volume 12, Number 4 APPLIED PHYSICS LETTERS 15 February 1968

metrical about the axis of the laser beam is not par­ticularly surprising since a nonuniform distribution of laser radiation across the beam will result in different parts of the radiation-supported shock wave having different velocities. This was confirmed when other ruby rods, giving rise to different in­tensity distributions in the focal region, were used to create the spark, the Schlieren photographs re­vealing a characteristic shape for the shock front associated with each ruby. However, in each case the shock front expands into the cone defined by the focused laser beam. The photographs allow an accurate determination of the shock front velocity both in the longitudinal and transverse directions. In Fig. 3 the longitudinal velocity along the ruby laser axis and the maximum transverse velocity have been plotted. As expected, the value for the longitudinal velocity during the phase in which the shock wave is strongly driven is much higher than that for the transverse velocity, the ratio of the two velocities being given by the geometry of the light cone. Towards the end of the laser pulse the two velocities approach the same value and the expansion then continues at a greatly reduced rate in what has been described in terms of a perturbed blast wave.5 However, the closely spaced pulses used in this experiment are not suitable for the study of this much slower phase. Microdensitometer traces of the photographs provide an upper limit for the thickness of the shock front during the initial stage, the value of 0.02 mm obtained corre­sponding approximately to the limit of resolution of the optical system.

In conclusion, the technique described above appears to be an extremely powerful tool for the investigation of laser-produced plasmas during the initial phase. Further more detailed studies are being carried out and will be reported at a later date.

150

<.) Q) VI

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>-~

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o Longitudinal motion o Transverse motion

, , '0 0,

........ 0__ ..... 0 ....... 0_ .... 0 -°--0_ ............. 0

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o 10 20 30 40

TIME (nanoseconds)

Fig. 3. Plot of the longitudinal and transverse shock velocity as derived from the Schlieren photographs.

The authors are especially indebted to Dr. S. A. Ramsden for his advice and encouragement throughout the course of this work. They also thank W . .J. Orr for his assistance in the design and con­struction of the laser-triggered spark gap.

1 S. A. Ramsden and W. E. R. Davies, Ph.vs. Rev. Letters 13, '1.'1.7 (I Y64).

'S. A. Ramsden and P. Savic, Nature 203, 1'1.17 (I Y64). ".J. A. Giordmaine, P. M. Rentzepis, S. L. Shapiro, and K. W.

Wecht, Appl. Phvs. Letters Il, '!.16·(IY67). 'A . .J. Alcock and S. A. Ramsden, Appl. Phvs. Letters 8, 187

(1%6). .; E. Panarella and P. Savic, Can. I Phys., to be published.

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