EXPERIMENTS ON INTENSE LASER IRRADIATION OF PLASMAS

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EXPERIMENTS ON INTENSE LASER

IRRADIATION OF PLASMAS

R. Sigel

To cite this version:

R. Sigel. EXPERIMENTS ON INTENSE LASER IRRADIATION OF PLASMAS. Journal de

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 38, Dkcembre 1977, page C6-35

EXPERIMENTS ON INTENSE LASER IRRADIATION

OF

PLASMAS

R. SIGEL

Projektgruppe fur Laserforschung der Max-Planck-Gesellschaft, D-8046 Garching, F R G

Rhumb.

-

Avec son application possible a la fusion par laser, la production de plasmas denses par irradiation de matihre solide avec une lumiBre laser intense suscite un interkt considerable. Ceci requihre une comprehension des mhnismes d'interaction lineaires et non linkaires, entre le rayonnement laser et le plasma. Dans cet article, nous discuterons d'experiences consacrks au probleme de I'absorption et de la diffusion du rayonnement laser.

Abstract.

-

With its possible application for laser fusion the generation of dense plasmas by irradiation of solid matter with intense laser light has found considerable interest. This calls for an understanding of the linear and nonlinear interaction mechanisms between laser radiation and plasma. In the paper we will discuss experiments devoted to the problem of absorption and scattering of laser radiation.

1 . Introduction.

-

The interaction of intense laser radiation with dense plasmas has recently found considerable attention in the context of laser fusion. As a prerequisit for economic power production, absorption in laser irradiated solid targets is a problem of particular importance. We will discuss the experimental status of the problem in this paper.

Unfortunately it is true that the experimental observations on light absorption and scattering reported in the literature till today form a patchwork of results where common features are difficult to isolate. Apparently identical conditions have lead sometimes to quite contradictionary results. This indicates that not all the problems in setting up such experiments have till now been fully mastered.

Some of the difficulties have, however, recently been identified and resolved. For example, as we shall see below, meaningful reflection measurements require detectors which collect the scattered radiation over the full solid angle. Meticulous care is required with respect to focus localization, target adjustment, cali- bration of the detectors and target surface quality before reproducible results are obtained. Perhaps the most significant progress has been made with respect t o beam quality and reproducibility of lasers. With the Nd-glass laser self-focusing in the optical components of the laser which makes the beam unfo- cusable above a certain power limit is now under control. The importance of prepulses though still a problem in high gain gas lasers has been recog- nized and is taken care of. It seems in fact that with the much improved performance of modern high- power lasers thoroughly performed and well documented experiments will soon come out from many laboratories.

Nevertheless it is still too early to try a compre- hensive review of experimental observations. Here

we shall describe a series of experiments which have been performed in our laboratory using Nd-glass lasers. Though with this restriction certainly a number of interesting observations are not adequately cove- red, it enables us to sketch a picture of light absorption and scattering under conditions which are sufficiently well-known to us in order to make a comparison between various results - actually over a broad range of intensities from picosecond to nanosecond pulses.

2. Mechanisms of Light Absorption and Scattering.

- At the intensities of interest for laser fusion the plasma produced by irradiation of a solid target should reach such a high temperature that electron-ion collisions are no longer efficient for light absorption. As a result a laser light wave, normally incident on an overdense plasma, may be completely reflected in much the same manner as from a metal. Thus efficient light absorption in these plasmas appears problematic.

Actually the situation is more favourable. For oblique incidence linear optics generally predicts enhanced absorption of p-polarized radiation depend- ing on the density profile [ I , 2, 31. In a linear density profile resonant coupling to plasma waves (resonance

absorption) typically leads to 50

%

absorption [2]. Since the coupling is linear, absorption does not depend on intensity. The standing field pattern as calculated from the linear wave equation for the two polarizations is shown in figure 1. Absorption disappears at normal and tangential incidence. It is strongest at an optimum angle given by (k, L)213 sin2 Om 1.0.6, where k, = 2 n/lo is the laser light

wave vector and L the characteristic length of plasma inhon~ogeneity.

It is interesting to note that the specularly reflected light beam should be accompanied by a beam of

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R. SIGEL

on the theory of these processes may be found through the recent references [ 5 , 6 ] .

3. Experimental Techniques. - A technique which

we have used [7] to measure absorption in the target as well as the angular distribution of scattered radia- tion is shown in figure 3. A 2 n ellipsoidal mirror,

DIFFUSE SCATTERING EXPERIMENT

,

Quontel LASER 106p 300mJ/30ps

FIG. 1 . - Standing field pattern produced on oblique reflec-

tion of an either s- or p-polarized electromagnetic wave from an FIG. 3. - Mirror arrangement for measurement of diffuse

inhomogeneous plasma layer. After reference [I]. scattering losses from a laser-irradiated target. From refe-

rence [7].

frequency doubled laser light which is generated as a result of non-linear electron motion in the resonance layer [4].

At the intensities applied in present experiments one expects that nonlinear, parametric interactions of the light wave with the plasma become important leading to the decay of the light wave into new waves. A few

of the processes are shown in figure 2. Enhanced

FIG. 2. - Some of the instabilities which can be excited by an

intense laser light wave in a laser-produced plasma.

absorption may be expected if the light wave excites plasma waves which deposit their energy in the plasma without leaving it. Among these processes are the parametric decay instability t + I

+

s(t = transverse photon, 1 = longitudinal electron plasma wave, s = sound wave) and the 2 o,, - decay t -t I

+

1.

If, however, one of the excited waves is again an electromagnetic wave as in the case of stimulated Brillouin (t + t

+

s) and Raman (t + t

+

I) scattering, this wave may leave the plasma and enhanced losses may arise. Particularly dangerous in this respect it stimulated Brillouin scattering where the excited electromagnetic wave has nearly the same frequency as the laser light wave and hencz, according to the Manley-Rowe relations may carry off nearly all the incident flux. Access to the very extended literature

with target and detector in its two foci, collects all the scattered radiation onto the detector R,,,,. The amount of radiation backscattered through the lens (R,) as well as the radiation transmitted through the target (T) is measured separately. The angular distribution of scattered radiation can be detected by replac- ing the detector R,,,, by a camera which focuses onto the inner surface of the ellipsoidal mirror.

If we wish to study quantitatively oblique reflection by tilting the target, the mirror is not very well suited. For such measurements we use an Ulbricht sphere, the interior of which is painted with a white, diffusing paint (Fig. 4). Two detectors are used to average out asymmetries arising for example from the beam access hole. The recent measurements made with the sphere will be discussed in more detail elsewhere [8].

ULBRlCHl SPHERE

palnt F," ' - " -

OUdNlEL LASER IARGET

FIG. 4. - Sphere arrangement for measurement of light

absorption in the general case of oblique incidence. From

reference (81.

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EXPERIMENTS O N INTENSE LASER IRRADIATION O F PLASMAS C6-37 4. Experiments with Picosecond Pulses. - 4.1 NOR- 0.4 to 0.5. The diffuse scattering of laser light into

MAL INCIDENCE. - Several years ago, it was noticed the mirror is clearly seen from the photograph figure 7. that measurements of reflectviity back through the

lens (normal incidence, plane targets) showed a decrease at high intensities (Fig. 5). It was therefore speculated that this might indicate very good absorption due to nonlinear interaction of the laser light with the plasma.

FIG. 5. - A decrease in target reflectivity is found at high intensities if only reflection back through the focussing optics is

measured. From references [9-1 I].

80%

f

-

60.1. .- N .-

-

w 0 s 40.1. C 0 .- X 0

-

&

zOx

We have examined this question in detail. Figure 6

0 Rochester 0 Lirnell - a Garch~ng

-

n

/= a d

-

I

Pl

I

shows mirror measurements with a plane glass target. Intensity is varied by varying the position of the focus relative to the fixed target. The focus is on the target surface at position 14.9 mm. The intensity given in W cm-2 in figure 6 is calculated from the geometry of the experiment. In the range 2.5 x 1012

to1= tou tolL 1015 io16 Intens~tat [Wlcm']

FIG. 6.

-

Reflection of 30 ps-pulses from a glass target at normal incidence versus focus position. For Z < 14.9 mm the focus is in front of, for Z ; 14.9 mm at and for Z > 14.9 mrn inside the target surface. Measurements were made with the

mirror arrangement of figure 3. From reference [7].

to I O l 4 W cm-2 the glass target has become highly reflecting back through the lens (RL = 0.5 to 0.6). In the focus, at the highest intensities, RL decreases sharply to RL = 0.15 as suggested by figure 5. Howe- ver, the decrease in lens reflectivity is largely com- pensated by strong diffuse scattering into the ellipsoidal mirror. Absorption A = 1 - A,,, never exceeds

FIG. 7. - Distribution of laser radiation scattered into the mirror with the target in focus. Concentric grooves come from imperfect machining of the mirror surface. Photographs taken at 14.0 m m and 16.0 mm show no diKuse scattering, therefore only their central part is shown. One may note that a lens reffec- tion from the focussing lens becomes very bright at these posi- tions indicating a high reflectivity back through the focussing lens

as expected from figure 6. From reference [7].

We have also investigated under the same conditions reflection from a copper target. The behaviour in the vicinity of the f o c ~ s is very similar to the glass case with again Am,,

--

0.5 [ 7 ] . It becomes different for low intensities, i. e. far from the focus due to the different dielectric properties of the two materials. From Fresnel's formulas, we expect T = 0.92,

A,

= 0.08 for a two-sided glass sheet and RL z 1

for copper in the limit of low intensity without plasma formation. The transition to these limiting values has in fact been measured at reduced energy, where the corresponding focus position remains in the range of the detectors (the trend is clearly seen in figure 6). Plasma formation on glass with 30 ps pulses begins at 2 x 10''

w

~ m - ~ , o n c o p p e r a t 10l0 W ~ m - ~ .

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infrared beam, accompanied by a green second har- monic (SH) beam if the incident light is p-polarized. Figure 8 shows the specular infrared beam from a glass target as observed again with mirror and camera. The conditions correspond to figure 6 with focus at the position 14.9. The central bore in the mirror and the target holder are marked (compare figure 7). At position 13.0, with a corresponding focal spot size of

-

1 mm diameter, we observe a well defined specular beam with the same angular width as the incoming beam. Upon moving toward the focus the specular beam becomes more and more diffuse as

FIG. 8.

-

Specular infrared bcam reflected into the mirror from a tilted glass target at an angle of incidence of 17". The Position of the central bore and the target are marked (compare

to Fig. 7). From reference (71.

expected on the basis of the normal-incidence measurements.

The frequency doubled SH beam is seen in figure 9 (unfortunately in this series the target was tilted into the opposite direction). As in the case of the funda-

13.0 mm

14.3 mrn

14.6 rnrn

FIG. 9. - A green beam of frequency doubled light is produced together with the specularly reflected laser beam. The target was tilted by 170 in the opposite direction compared to figure 8. Photographs taken with 2. = 0.53 pm J F filter and J R blocking filters on Polaroid film. Similar as the infrared beam, the frequency doubled beam becomes more and more diffuse towards

focus (14.6 mm). From reference 171.

mental the SH beam is well defined for a large focal spot and becomes very diffuse in the focus (at 14.6 in this case). The electric field vector of the linearly polarized laser beam was horizontal, thus SH-generation is observed for p-polarization as expected. The polarization dependence of SH-generation was demonstrated earlier with nanosecond pulses [12], hence it was not investigated further in this series of experiments.

4 . 3 POLAR~ZATION DEPENDENCE OF REFLECTIVITY.

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EXPERIMENTS ON INTENSE LAS ;ER IRRADIATION OF PLASMAS C6-39 Results obtained with a copper target are shown

in figure 10. The focus position was

--

200 pm in front of the target with a nominal intensity of 2.5 x 1014 W c m e 2 on the target surface. For nor- mal incidence we have A = 1

-

R,,,

= 0.3 and for

0 2 600, A 2. 0 for both polarizations. In the inter- mediate range there is a clear distinction between the s- and p-polarization : p-polarized light is always stronger absorbed with a minimum in reflectivity at 0 , = 20° to 300. Very sim~lar results have been obtained with the glass target. With the focus exactly on the target surface the polarization effect was practically absent and this may explain why it was not observed in [13]. Disappearance of the effect at focus is not unreasonable, since we know (see Fig. 6 and Fig. 7) that in the focus very strong diffuse scattering occurs. The underlying apparent roughness of the plasma layer should smooth out the difference between the two polarizations.

If we attribute the enhanced absorption of p-polarized radiation to resonance absorption, we estimate an inhomogeneity length of L

-

1, from the minimum in reflectivity at 8, = 200 to 300. Such angles and the corresponding profile steepening are in fact obtained in 2 D numerical simulations [5,6].

If reflection from the target were purely specular one would expect that for an angle of incidence of the target of greater than 140 (the half-angle of the F/2 lens) only a very small amount of backscatter through the lens would occur

( 5

3

%

for a diffusively scattering target). However, apparently preferentially at the optimum angle for resonance absorption (p-pol., 8 = 20 to 300) we sometimes observe lens reflectivities RL > 0.5. According to our preliminary experience, large lens reflectivities seem to correlate with double pulses from the laser, i. e. they seem to be favoured by the presence of a certain amount of plasma in front of the target. Our investigations of this effect are, however, not yet complete.

The fact that besides specular reflection a component of highly directional backscatter exists in laser-produced plasmas is well known from experiments with nanosecond pulses [ l l , 141 and will therefore be discussed in more detail below.

5 . Experiments with Nanosecond Pulses.

-

In picosecond experiments such as those described above the plasma layer is thin and the hydrodynamic motion of the plasma is nonstationary. With nanosecond pulses one approaches the limiting case of a statio- nary plasma profile. It is therefore interesting to make a comparison between the two cases.

I t turns out that the behaviour of the plasma is very similar. Actually many of the results presented above were anticipated by our previous nanosecond experiments [l 1, 12, 141.

Figure 11 shows results obtained by irradiating a plane solid deuterium target at an intensity of 10" W with a 5 ns laser pulse 1141. Instead of

Rtot 1.0 - 0.9

-

Coppsr hrget 3WmJ130ps 5 1 1977 Target pos. 15.9 rnm Sphere Angle of Incidence ( O f

FIG. 10. - Total reflectivity Rtot as a function of angle of incidence for s- and p-polarization. Maximum absorption

A = 1

-

Rbt occurs at 20° to 30° for p- polarization. Position of focus

-

200 pm in front of the target surface. Copper target.

From reference [8].

detecting the specular beam with the help of a ellipsoidal mirror, observations were made only back through the focusing lens. To simulate oblique incidence, the right half of the lens was shielded from incident laser radiation. The photograph shows the distribution of backscattered radiation in the lens plane at o, (lower) and 2 w, (upper part). A diffuse specular beam is observed as expected a t the right edge of the lens, accompanied by a corresponding SH beam, in a manner which is basically similar to the observations with picosecond pulses. In the left half of the lens we see hot spots of so-called collimated backscatter (not accompanied by S H radiation). Actually the distribution of these spots represents a copy of the incident light distribution, i.e. the wave- fronts backscattered from the plasma are identical with the incident ones except that they travel backward

in time.

The phenomenon of collimated backscatter which seems to be observed in experiments with Nd [ l l , 14-16], ruby [17] and CO, [I81 lasers is generally attributed to stimulated scattering from ion sound waves, either by Brioullin scattering in the extended volume of the underdense plasma [19] or, more loca- lized by the Radiative Decay Instability [20] near the critical density layer. It is the most clearly observed example of parametric interaction of the light wave with the plasma and is most probably of practical importance in many laser fusion experiments.

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C6-40 R. SIGEL

FIG. 1 1 . - Photographs of the focusing lens as illuminated

from behind by the radiation backreflected from the target. The symbol indicates that illumination by the incident laser beam was only through the left half of the lens. The photographs show diffuse specular reflection at U L and 2 (DL and intense

spots of collimated backscatter. 5 ns incident laser pulse. From reference [14].

incidence (Fig. 12) using the Ulbricht-sphere descri-

bed abovc. It is found that in focus a fraction of

0.25 is backscattered through the focusing lens (half

angle 14O) though the specular direction is far outside the lens cone at the applied angle of incidence of 200. This confirms once more the existence of a highly directional backscatter component. Photographic tests with masks similar to those described above gave again the same results though a different laser system, focusing lens and target material were used. Note that in focus we have also with the nanosecond pulses

A

=

0.5.

Figure 12 rcpresents the remarkable case where we were able, by means of defocusing, to absorb practically all the laser radiation in the target. Defocus- ing probably leads to the formation of an extended, cold plasma cloud in which the light is absorbed by electron-ion collisions (inverse bremsstruhluizg). The range of strong absorption seems, however, restricted

GLASS. 20°

+

, p

Y L 2 5 2 - 4 2 8 5

l.2JllOns

,

FIG. 12. - Nanosecond reflectivity of glass measured with the sphere. Angle of incidence 20°, p-polarization. From

reference 181.

to low intensities in the range 10'' to 1012 W cmP2.

Very similar results were obtained with copper. Spectra of the radiation scattered from the target show, besides frequencies o, ans 2 o,, also a (double-) line centered at 312 w,. This line is attributed to

scattering from plasmons with frequency 0,/2 gene-

rated by the 2 ope-instability [21]. Figure 13 shows

that 312 wL emission has a threshold behaviour ; it

occurs at

a

= 1013 W in agreement with

theoretical predictions. Measurements of lens reflec-

INCIDENT ENERGY ( I N JOULES)

FIG. 13.

-

Threshold behaviour of 312 U L radiation, 5 ns laser pulse. From reference [21].

tivity at o, below and above threshold showed, however, no detectable influence on laser light absorption. Thus the 2 ope instability may, via the 312 o,

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EXPERIMENTS ON INTENSE LASER IRRADIATION OF PLASMAS C6-41

6 . Summary.

-

On the basis of the experiments I

described above we may sketch the following picture _{R , ~ ,} of light scattering from a laser produced plasma (Fig.

14) : An obliquely incident light beam produces a

05

i n c i d e n t

0

I o9 loll 1013 1015

- -

_FIG._15._-_{Measured total reflectivities}_Rtot_{from recent}_expe-

riments. Curve l and 2 : from figure 3 and 4 in reference 171,

FIG. 14. - Schematic representation of light scattering from a laser produced plasma.

well defined specular beam as long as the irradiated target area is sufficiently large. The specular beam is accompanied by a SH beam as theoretically predicted. Absorption is enhanced for p-polarized radiation and has a maximum at an optimum angle. It is very satisfactory that this simple behaviour is predicted by linear optics. From the measured angle we deduce a steep density gradient with L 1.

A,.

Such profile

steepening by ponderomotive force effects is expected from simulations.

Besides specular reflection there exists a component of highly directional backscatter, so-called collimated backscatter. Its intensity seems to depend on the extension of the underdense corona, but in general it must be considered as an important loss mechanism. It is attributed to stimulated scattering from ion sound waves in the plasma. Note that specular and SH-beam and collimated backscatter are also the main lobes seen in 2 D-simulations [19].

There is evidence that the 2 o,, instability is excited near ope = 0,/2, i. e. at one quarter of the critical density but its effectiveness in light absorption is doubtful. As concerns excitation of parametric instabilities, simulations show that their efficiency is reduced by profile steepening near the critical density [5, 61. The present experiments give no direct evidence for their excitation ; as concerns their possible role as well as that of other mechanisms we refer to the thorough discussions given in references [ 5 , 6 ] .

Quantitative predictions for light absorption come from models which assume a simple geometry, most of them a plane wave incident on a plane target. The geometry of the experiments is clearly much more complicated ; hence we are faced with the problem of how to compare quantitatively experiment and theory. As we shall see simply calculating the intensity from the diameter of the irradiated target area and taking it as a basis for comparison is completely unsatisfactory.

In figure 15 we have compiled data for R,,, = 1

-

A from this series of experiments ; also includcd are data of recent LLL expcriments (Fig. 7 in Ref. [25]). These are at present the most complete and thoroughly

curve 3 : from figure 7 in reference [25]. curve 4 : from figure-12

this paper.

documented data available. In all of these experiments intensity was varied by moving the focus relative to the target, thus changing the irradiated area at cons- tant energy. Together they span a range of 8 orders of magnitude in intensity.

At very low intensities (< lo9 W cm-2), the targets do noy absorb due to their dielectric nature (copper, glass). In the range 10" to 10" W c m - 2 we have a quite different behaviour : Long pulses are completely absorbed, picosecond pulses are nearly completely reflected. As mentioned above, this behaviour is plausible if we assume collisional absorption of long pulses in an extended, cold corona which cannot form with picosecond pulses.

The experiments in figure 15 are all made at normal incidence, hence we wish to compare them with 2 D-simulations at normal incidence, which predict typically A 1.0.15 [5]. Clearly we approach the limit of a plane wave in the experiments if we make the focal spot and hence the laser energy very large. Thus, at a given intensity, we should compare with the highest available energy data. In fact, it is seen from figure 15 that for a given intensity (picosecond pulses) absorption tends to decrease steadiIy with increasing pulse energy (spot diameter). The data do not yet allow a very accurate extrapolation but seem quitc consistent with A = 0.15 or less for truly

normal incidence. At oblique incidence we would

clearly expect enhanced absorption for p-polarized radiation, but there are not enough data to make any extrapolations.

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C6-42 R. SIGEL Nevertheless it is quite astonishing that the reflection curves in figure 15 end all at nearly the same reflectivity (R,,,

-

0.5) when the target is in the focus of the laser beam. In this position absorption is independent of intensity over nearly five orders of magnitude. It is interesting to note that in this case absorption is strongest, pointing towards a connection between absorption and surface roughness. It is well known that coupling to surface plasmons on rough metal sur- faces may lead to considerable absorption [26]. It is interesting to speculate that a similar linear, i. e. intensity independent coupling mechanism may exist in laser-produced plasmas.

In summary we have shown that laser light propa- gation in laser-produced plasmas follows a pronoun-

ced pattern, partly even anticipated by the predictions of linear optics. As far as reliable data are available they agree in surprising detail with 2D-computer simulations. With forthcoming experiments including different wavelength lasers, especially with CO,

lasers [IS, 27-30] it should be possible to establish a much more detailed picture than presented here within the not too distant future.

Acknowledgement. - The recent results on oblique irradiation of targets with picosecond pulses presented in this paper were obtained in collaboration with R. P. Godwin, on professional research and teaching leave from the Los Alamos Scientific Laboratory, Los Alamos, New Mexico, U. S. A.

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Waves in Plasmas (Pergamon, New York) 1964, p. 213.

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[4] EROKHIS, N. S., ZAKHAROV, V. E. and MoIsEEv, S. S.,

Zh. Eksp. Teor. Fiz. 56 (1969) 179 [Sov. Phys. JETP

29(1969) 1011.

(51 KRUER, W. L., HAAS, R. A., MEAD, W. C., PHILLION, D. W. and RUPPERT, V. C., in Plasma Physics : Nonlinear Theory and Experiments, 36th Nobel Symposium Aspeniisgarden, Sweden, June 10-18, 1976, edited by

H. Wilhelmsson (Plenum Press) 1977.

[6] BEZZERIDES, B., D U BOIS, D. F., FORSLUND, D. W., KIN- DEL, J. M., LEE, K. and LINDMAN, E. L., in Plasma Physics and Controlled Nuclear Fusion 1976, Vol. I (International Atomic Energy Agency, Vienna) 1977, p. 123.

[7] VAN KESSEL, C. G. M., OLSEN, J. N., SACHSENMAIER, P., SIGEL, R., EIDMANN, K., GODWIN, R. P., Max-Planck- Gesellschaft zur Forderung der Wissenschaften e.V., Projektgruppe fiir Laserforschung, D-8046 Garching, Laboratory Report IPP IV/94 (1976), to be published in

Z. Naturf.

[8] GODWIN, R. P., SACHSENMAIER, P. and SIGEL, R., to be

published in Phys. Rev., Lett.

[9] LUBIN, M., GOLDMAN, E., SOURES, J., GOLDMAN, L., FRIEDMAN, W., LETLRING, S., ALBRITTON, J., KOCH, P., YAAKOBI, B., in Plasma Physics and Controlled Nuclear Fusion Research 1974, Vol. I1 (International Atomic Energy Agency, Vienna), 1975, p. 459.

[lo] SALERES, A., FLOUX, F., COGNARD, D. and BOBIN, J. L.,

Phys. Lett. 45 A (1973) 451.

[ l l ] EIDMANN, K. and SIGEL, R., in Laser Interaction and Related Phenomena, edited by H. J. Schwarz and H. Hora (Plenum, New York), 1974, Vol. 3, p. 667. [12] EIDMANN, K. and SIGEL, R., Phys. Rev. Lett. 34 (1975) 799.

[I31 RIPIN, R. H., Appl. Phys. Left. 30 (1977) 134.

[14] SIGEL, R., EIDMANN, K., PANT, H. C. and SACHSENMAIER, P.,

Phys. Rev. Lett. 36 (1976) 1369.

[I51 RIPIN, B. H., MCMAHON, J. M., MCLEAN, E. A., MAN- HEIMER, W. M. and STAMPER, J. A., Phys. Rev. Lett.

33 (1 974) 634.

[I61 LEE, P., GIOVANELLI, D. V., GODWIN, R. P. and MCCALL G. H., Appl. Phys. Len. 24 (1 974) 406.

[I71 JANNITTI, E., MALVEZZI, A. M. and TONDELLO, G., J.

Appl. Phys. 46 (1 975) 3096.

[It(] MITCHELL, K. B., STRATTON, T. F. and WEISS, P. B.,

Appl. Phys. Lett. 27 (1 975) 11.

[19] BISKAMP, D. and WELTER, H., in Plasma Physics and Controlled N~clear Fusion (International Atomic Energy Agency, Vienna, Austria) 1975, Vol. 11, p. 507. 1201 FORSLUND, D. W., KINDEL, J. M., LEE, K. and LINDMAN, E.

L., Phys. Rev. Lett. 34 (1 975) 193.

[21] PANT, H. C., EIDMANN, K., SACHSENMAIER, P. and SIGEL, R.,

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[24] A v ~ o v , A. I., BYCHENKOV, V. Yu., KROKHIN, 0. N., PUSTOVALOV, V. V., RUPASOV, A. A., SILIN, V. P., SKLIZKHOV, G. V., TIKHOUCHUK, V. T., SHIKANOV, A. S., Pisnla ZhETF24 (1976) 293 (in Russian).

[25] HAAS, R. A., MEAD, W. C., KRUER, W. L., PHILLION, D.

W., KORNBLUM, H. N., LINDL, J. D., MACQUIGG, D., RUPERT, V. C. and TIRSELL, K. G., Phys. Fluids 20 (1977) 322.

[26] For a recent review on surface plasmons see : Orro, A.,

in Feskorperprobleme/Advances in Solid State Physics

(PergamonlVieweg) 14,1974, p. 1.

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[29] MARTINEAU, J., RABEAU, M., BOCHER, J. L., ELIE, J. Ph. and PATOU, C., Opt. Commun. 18 (1976) 347.