Stimulated Raman Scattering in a Laser-Produced Plasma Heated by Laser at Wavelength of 0.53 pm

E.A. Bolkhovitinov, V.Y U. Bychenkov, M.O. Koshevoi, M.V. Osipov, A.A. Rupasov, A S . Shikanov, and V.T. Tikhonchuk P N . LeDedeL Physics Institute, Russran Academy of Scrences

Moscow. Russia

A.V. Kilpio, N.G. Kiselev, D.G. Kochiev, P.P. Pashinin, E.V. Shashkov, and Y.A. Suchkov

General Physics Institute, Russiun Acudemy of Sciences Moscow, Russia

ABSTRACT

Stimulated Raman scattering (SRS) has been studied in a plasma produced by a second har- monic Nd-glass laser irradiating a limited mass target, which does not burn through during the interaction time. It is assumed that flat density regions form and move through an expanding plasma, and the SRS results have been analyzed on that basis. The SRS spectrum cut-offs are discussed, as well as features of time-integrated and time-resolved SRS spectra.

I. INTRODUCTION

Prior experiments have investigated SRS in plasmas produced from massive targets, thin foils, and gas jets [ 1-51. Laser interaction studies of intermediate cases, where the thickness of the limited size targets only slightly exceeded the burn-through depth, have been presented [6], and recent studies of stimulated Brillouin scattering (SBS) [7] and SRS from spherical targets [8] have been reported.

Such targets are of interest in controlled inertial confinement fusion (ICF), because they provide the most efficient conversion of the laser energy into the target kinetic energy. In this paper, intermediate-thickness planar targets have been used to model real spherical ICF targets. De- tailed time- and space-resolved measurements of SRS from this type of target are presented.

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11. EXPERIMENTAL SET-UP and METHODOLOGY

The single-channel laser facility KAMERTON has been used to study SRS from a laser-pro- duced plasma [9]. A plasma was produced using a 2.5-ns and 30-80-5 (A, = 527 nm) laser pulse focused onto planar polyethylene foils of varying thicknesses (5-20 p). The focal spot diameter was =lo0 p, and the laser light intensity on the target was 3e10I4 Wlcm’. The diagnostic array, including a multi-channel optical analyzer, two spectrometers, and a spectrometer coupled to a streak camera, is shown in Fig. 1. The observa- tion angles (e), spectral resolution (Ah), time resolution (At), and spatial resolution (Ax) for the various diagnostics are summarized in Table I.

Fig. 1 Diagram of the experimental set-up and the diagnostics of the KAMERTON single-channel laser facility.

Table I. Diagnostics used to Record SRS Spectra in Laserproduced Plasmas

~~~

Observation Spectral Time Spatial Spectral Diagnostic Angle, Resolution, Resolutiion, Resolution, Range,

0 Ah At Ax nm

Multi-channel optical analyzer OVA-284 =:135" =1 nm

Spectrometer ISP-5 1 =90", 180" 4 . 5 nm ... =20 pl 400-900

5 80-900 ... ...

450-900 ... ... Spectrometer STE- 1 =90", 180" ~ 0 . 1 nm ~ ~ _ _ ~~ ~ ~~

Spectrometer coupled to streak camera -135" 1 nm =30 PS Hamamatsu-C979

630-830 ...

All SRS spectra observed at angles 8 = 135" and 180" for 6 - p foils were located in a wavelength range of h,,,,, < h < A,,, where h,, = 610-650 nm, and h,,,, = 690-750 nm.

A typical densitometer trace of a time-integrated SRS spectrum is shown in Fig. 2. In accordance with the SRS dispersion equation, each wave- length h corresponds to the definite plasma density n:

n/n, = (1 -h,,/h)' , (1)

where n, is the electron critical density and h, is the incident laser wavelength. Therefore, the measured values of h,,,,, = 640 nm and &,,& = 725 nm, shown in Fig. 2, correspond to a plasma density of n,,,,, = 0.030 n, and nlllax = 0.075 n,. Two characteristic scales of intensity modulation were observed in SRS spectra: one having a distance between peaks of 0.5-1.0 nm, and the other having a 10-20-nm distance between peaks. When the foil thickness was increased to 20 pl, the SRS intensity decreased, and the SRS spec- trum narrowed and shifted toward the red. No SRS was recorded at 90".

3 1

1 1

750 700 6 SO 660 >,hm

Fig. 2 Typical densitogram of time-integrated SRS spectrum from a 6-pm foil target.

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Time-resolved SRS spectra have a complicated structure. Typical time-resolved spectra from thin targets is shown in Fig. 3 (a). During the rise time of the laser pulse, the long wavelength portion of the SRS spectrum is emitted, and then in the second half of the laser pulse, the SRS spectrum shifts toward shorter wavelengths.

Recalling the relation between the SRS spectral shift and the plasma density, this phenomenon may be interpreted as a movement of the scatter- ing region toward lower densities. The duration of the SRS signal at a given wavelength is rather short (0.8 ns), while the total SRS duration ( ~ 2 . 5 ns) is comparable to the pulse length.

8SO aoo 780 760 680 A m

- - I

BSO 800 710 7bo h.nm 1

Fig. 3 (a) The time-resolved SRS spectra from a thin foil target, and the SRS radiation intensity distributions at 0.5 ns (b), 0.9 ns (c), and 1.9 ns (d), respectively.

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Two types of SRS signal modulation--temporal (-0.1 ns) and spectral ( 4 0 nm)--have been observed. The SRS radiation intensity distribu- tions obtained from Fig. 3(a) are shown in Figs. 3(b), (c), and (d) for three distinct points in time- -at 0.5 ns, 0.9 ns, and 1.9 ns, respectively. For thicker foils, the SRS dynamics changed--scat- tered emission starts simultaneously in a broad spectral range, and there is no significant varia- tion of the radiation wavelength with time.

111. DISCUSSION

The SRS spectra shown earlier suggest that the laser intensity surpasses the threshold level in certain density regions, but is below the thresh- old at both very low and very high plasma densities. Usually the low density cut-off of the SRS spectrum is related to the rise of the SRS threshold as a result of enhanced Landau damp- ing. Because of the exponential growth of the Landau damping with the wavenumber, it is reasonable to consider the inequality

as the necessary condition for SIRS (where k, is the wavenumber of the plasma ?wave and rDe is the electron Debye radius. For backscattered SRS, k, = 2 k, (where 16 = 27&), the electron temperature can be obtained from Eq. (2), thus:

Te = (met 2/36> (qllul\nc ) . ( 3)

From Eqs. (1) and ( 3 ) , for a minimum observed wavelength km = 630 nm, the value of the low density cut-off (nInm = 0.03 n,) and the electron temperature (T, = 350 eV) can be determined. This estimate may be considered to be the over- all plasma corona temperature, which is also valid for the higher densities n_<ri,.

The high density cut-off nlllax, which is equal to 0.075 nc for Amax = 750 nm, should also be related to the SRS threshold condition. The formula for such a SRS threshold is

where v, is the electron quiver velocity in the laser field, is its threshold magnitude of SRS instability, v,, is the electron-ion collision fre- quency, ancl L,, is the electron density scale length. The dissipative part of the SRS threshold [the first term in Eq. (4)] is below IOl3 W/cm2 for T, = 350 eV. However, the convective part of the threshold [the second term in Eq. (4)] is much larger, so the inhomogeneity of the scale length L,, = 3 cm should be assumed, in order to match the observed threshold intensity of 3.1 014 W/cm'. This is an unrealistically large scale length. At the same time, SRS numerical simu- lations[ 101 of a homogeneous plasma slab model show that the interaction length at 4 0 pm is enought to amplify the backscattering signal to 400 times that of the noise level.

IV. CONCLUSIONS

Thus, it is possible to explain the excitation of SRS for the given laser intensities, if it is as- sumed that there are one or more local flat density regions in an expanding plasma, similar to what was observed in Ref. 11. Such flat regions would be transitory, but a time scale of =lo-20 ps is enough for SRS to become excited and saturatedi. After that time period, the flat interaction region could be destroyed (as a result of the SRS itself or because of the hydrodynamic motion of the plasma), reappearing later in a different area of the plasma. This scenario suggests that instantaneous SRS emission may have a very narrow spectral line, but it should randomly change position during the laser pulse.

This anticipated temporal behavior may be associated with the peak structure of the ob- served SRS spectra. The downward motion of flat regions along the density profile will result in a blue shuft of the SRS radiation, which agreed well with the spectra shown in Figs. 3 (b)-(d).

The high density SRS cut-off can be attributed to the steeper gradients in the region d n , > 0.1, where a laser beam cannot produce extensive enough flat density regions. Also, the plasma produced by thicker foil targets should have a steeper density profile, and therefore the flat density regions should be shorter and generate less SRS. That also agrees with experimental observations.

ACKNOWLEDGMENTS

This work was partially supported by the Interna- tional Science Foundation, grant MM-1000, and the Russian Foundation of Fundamental Re- search, grant 94-02-03864-a. Presentation of thls report at the 16th Symposium on Fusiorf E@- neerirzg was made possible by the International Science Foundation Travel Grant program.

REFERENCES

1. C.J. Walsh, et al., Phys. Rev. Lett., 53, 1445 (1984).

2. E.D. Bulatov, et al., Proc. 12th ECLIM, 13 (1979).

3. R.P. Drake, Lnser and Particle Beams, -, 10 599 (1992).

4. H.A. Balds, et al., "Parametric instabili- ties in picosecond time scales," Lmer Iritercrction with Atoms, Solids, m d Plasmas, ed. R. More, New York, Ple- num Press (1994).

5 . S.H. Batha, et al., Phys. Fluids B, 5, 2596 (1993); Plzys. Plasmas, _1, 1985 (1994).

6. A.V. Kdpio, et al., Sov. 1. Quantum Electronics, 20, 536 (1990).

7. B.S. Bauer, R.P. Drake, K.G. Estabrook, et al., "Narrowband SBS in a Laser- Produced Plasma with a Controlled Row Profile," 25th Anomalous Absorption Coifererice Abstracts, Lawrence Liver- more National Laboratory, Livermore, CA, AP-3 (1995).

8. Tsukamoto et al., Phys. Plasmas, 2, 486 (1995).

9. A.V. Klpio, et al., "Interaction of Nd- glass laser second harmonic radiation with plane targets on Kamerton installa- tion," Proc. 2Ist ECLIM, 95 (1991).

10. T. Kolber, W. Rozmus, and V.T. Tik- honchuk, Phys. Plcismus, 2, 138 (1995).

11. Yu.A. Zakharenkov, et al., Sov. Phys. JETP Lett., 2, 557 (1975).

109

Heavy Ion Fusion: Prospects arid Status*

W. B. Herrmannsfeldt Stanford Linear Accelerator Center

Stanford University, Stanford, California1 94309

AB S TRACT

The occasion of this Symposium on Fusion Engineering marks almost exactly two decades since the late AI Maschke (BNL) and Ron Martin (ANL,) began working on the prom- ising synthesis of high-energy accelerators and inertially confined fusion. Although a start on a large-scale fusion driver is still far in the future, it is possible to mark this occasion by noting progress on several fronts. Key events in the Heavy Ion Fusion (HIF) field are usually marked by the dates for the Intemational Symposium series which began in 1976 at the Claremont Hotel (Berkeley /Oakland), and most recently in the eleventh meeting in the series at the Princeton Plasma Physics Laboratory in Sep- tember 1995. The main purpose of this talk will be to re- view the status of HIF as it was presented at Princeton, and also to try to deduce something about the prospects for HIF in particular, and fusion in general, from the world and U.S. political scene. The status of the field is largely, though not entirely, ex- pressed through presentations from the two leading HIF efforts:

The U.S. program, centered at LBNL and LLNL, is p i - marily concemed with applying induction linac technol- ogy for HIF drivers. (Details of the LBlVL pro-gram will be presented by Roger Bangerter in a paper later in this session.) The European program, centered at GSI, Darmstadt, but including several other laboratories, is primarily directed towards the rf linac approach using storage rings for en- ergy compression. However, in contrast to the U.S. pro- gram in which target development is in the separate Inertial Confinement Fusion (ICF) program in the DOE, the European program includes target study groups in Spain, Italy, Germany, and Russia. These groups are col- laborating with the experimental target groups at the In- stitute of Laser Engineering, Osaka. Supporting technologies, such as systems studies,, reactor chamber studies, etc., are also included in the Ehropean HIF pro- gram, while in the U.S. these efforts are found both in Inertial Fusion Energy (IFE) and in ICF.

Several developments in the field of HIF should be noted:

1. Progress towards construction of the National Ignition Facility (NIF), which was reported to this symposium on Monday, gives strength to the whole rational for & veloping a driver for Inertial Fusion Energy.

2. The field of accelerator science has matured far beyond the status that it had in 1976. Although the field was bloodied in the politics of the Superconducting Super Collider, technically accelerator projects for basic re- search, industrial processing, and the production of trit- ium have all pushed on the frontiers of efficiency, reliability, and high intensity.

3. Heavy Ion F:usion has passed some more reviews, includ- ing one by the Fusion Energy Advisory Commit-tee (FEAC), and has received the usual good marks.

4. Sadly, HIF has lost one of its founding fathers; AI Maschke passed away in the Spring of 1995. The HIF Symposium in September was dedicated to Al's memory. Ron Martin gave a fascinating talk relating the history of HI[F to his own and AI Mascke's parallel efforts.

5. A!; the budgets for Magnetic Fusion have fallen, the pressures 011 the Office of Fusion Energy (OFE) have in- tensified, arid a move is underway to shift the HIF pro- gram out of the IFE program and back into the ICF program in the Defense Programs (DP) side of the DOE. The outcome of this strategy is not known at the time when these lines are being written. In any case, it is im- portant to note that inter-national collaborations in IFE/HIF would still be with the civilian energy program in OFE, not with the ICF/DP program. We hope the above acron.ym-loaded paragraph is understandable to our foreign colleagues for whom this is an important politi- cal issue.

INTRODUCTION

The Heavy I011 Fusion (HIF) program is the only program addressing inertial confinement fusion in the Office of En- ergy Research (OER) of the DOE. The purpose of the HIF program is to evaluate the technology of heavy-ion accelera- tors for prospects as drivers for commercial power produc- tion from ICF. Early in the study of HIF, two types of accelerators were identified as suitable driver candidates:

*Work supported by Department of Energy contract DE-AC03-SF00515

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