Laser-plasma instability is a serious concern for indirect-drive inertial confinement fusion (ICF), where laser beams illuminate the interior of a cavity (called a hohlraum) to produce X-rays to drive the implosion of a fusion capsule. Stimulated Raman and Brillouin backscattering (SRS and SBS) could result in unacceptably high laser reflectivities. Unfortunately, it is impossible at present to fully simulate these processes realistically. The authors experimental program aims to understand these instabilities by pursuing a dual strategy. (1) They use a gas-filled hohlraum design, which best approaches ignition-hohlraum conditions, on the Nova laser to identify important non linear trends. (2) They are shifting towards more fundamental experiments with a nearly diffraction-limited interaction laser beam illuminating extremely well characterized plasmas on the Trident laser facility at Los Alamos to probe the relevant fundamental processes

Cobble, J. A.

Fernandez, J. C.

Montgomery, D. S.

Wilke, M. D.

UNT Digital Library

Title.Author(s)Submitted toLos AmmosNATIONAL LABORATORYExperimental Program to Elucidate And Control Stimulated Brillouin4nd Raman Back Scattering in Long-Scale PlasmasJuan C. FernandezJames A. CobbleDavid S. MontgomeryMark D. WilkeInternational Atomic Energy AuthorityFusion Energy ConferenceYokoham% Japan10/19/98-10/23/98_ –———.. .—...—r————=—.-——.—_--.-_—.—-—.—. —-—_——..—_——--——.. . . .. --. .——- -==-:Los Atamos Nat lord Laboratory, an aff irmative actioti equal opportunisty employer, is operated by the Universit y of California for the US Oepartment of E_mrgyundar cant ract W-74 05 -R4G36. W accetIt ante of this article, the publisher recognizes the t he US &wernrnant retsins a nonexclusive, royalty-free license to-..—..puplish or reporduce the published iorm of t hk cent ribution, or to “allowothers to do so, for U.S &vernment purpos=. The Los Alamos National Laboratoryrequaata that the publisher identify this article as work performed under the auspices of the US Department of energy.Form No. 636 f%S72629 10/91DISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document r NYyhe illegiblein electronic image products. Images areproduced from the best ava;lable originaldocument.. ..f:EXPERIMENTAL PROGRAM TO ELUCIDATE AND CONTROLSTIMULATED BRILLOUIN AND RAMAN BACKSCATTERING INLONGSCALE PLASMAS*Juan C. Fermindez, James A. Cobble, David S. Montgomery and Mark D. WilkeLos Alamos National Laboratory, Los Alamos, NM, 87544 USALaser-plasma instability is a serious concern for indirect-drive inertial confinement fusion(ICF), where laser beams illuminate the interior of a cavity (called a hohlraum) to produce X-rays to drive the implosion of a fusion capsule. Stimulated Raman and Brillouin backscattering(SRS and SBS) could result in unacceptably high laser reflectivities. Unfortunately, it is impos-sible at present to fully simulate these processes realistically. Our experimental program aimsto understand these instabilities by pursuing a dual strateg- (1) We use a gas-filled hohlraumdesign, which best approaches iguition-hohlraum conditions, on the Nova laser to identify impor-tant non linear trends. (2) We are shifting towards more fundamental experiments with a nearlydiffraction-limited interaction laser beam illuminating extremely well characterized plasmas onthe Trident laser facility at Los Alamos to probe the relevant fundamental processes.1. PLASMA CONDITIONSHohlraum designs planned for the National Ignition Facility (WI?) use plasma pressurefrom a He-H gas fill to tamp the gold-wall plasma[l]. The underdense HeH plasma allows linerpropagation due to its low ionization state Z. High spatial-growth rates for stimulated Ramanscattering[2] (SRS) and Brillouin scattering[3] (SBS) are predicted in the long-scale (size N fewmm) He-H plasma by linear convective theory[4], particularly backscattering. SRS and SBS arethe growth of electrostatic waves, i.e., electron-plasma waves (EPWS) and ion-acoustic waves(IAWS) respectively, which scatter laser light that in turn reinforces the electrostatic waves.Calculated linear gains are enormous and nonlinear saturation is expected.We discuss two targets, designed using radiation-hydrodynamic simulations with the LAS-NEX code[5]. (1) A toroidally-shaped hohlraum ~ed with a low-Z gas and illuminated withthe Nova laser (wavelength A = 351 nm) best approaches NIF conditions and spatial scales[6,7].Nine of the ten j / 4.3 beams (heater beams) are on for 1.4 ns. An interaction beam (IB) isturned on once the plasma equilibrates at 0.4 ns and is kept at constant power for 1 ns. Variousfills have been used at pressures up to 2.2 atm which fully ionize up to n,/nc = 0.20, wheren~ is the electron densitymid- I window 5planex frame ,stalk -4/and nc is the density above which 351 nm light becomes evanescent.z , 1\, ~i o -2 -1 2I2 mm” I location alongl path (mm*) positi& (mA)Figure 1: Left: The gas-jilled hohh-aum and beam footprints me illustmled. Middle: LA.5YE.Ypmjiles within the hohlraum along a beam path at time I ns are ploftd. Righ!: Hohlmurn z-myimage (O.08 ns framing) taken at time 0.12 ns is shown. By using thin (.2 pm) gold u~allswi!hno gas fill, .wflcient L-shell photons reach our pinhole camem to Econ-1an image.1.-------- ----- ‘. -v.-., , . - ,.. f,x~,-. >..?.. . .... . . ...m=.fl..~ . . .. . ,- 7-7 <.y;.-rci-i-. .~.~ ~---- .- ?-.....,. ,,,~z. \ . $,,.,>.“,’-4(v%%%pl%%m),11(Y1“’””,’””’1”’’”1’8 IOT6 lC?F4 10+~2107’”o.ol~. . ..l . . .. ’.. .t . . ..t . . ..lL o Id100200300400600z (pnl)Figure 2: Left: The geometry for the Thomson scattering measurement is illustmted. Scatteringjmm EP Ws yields the n, profile, while scattering by 1AWs yields the T,, Ti and j?ow velocity v.pmjiles. Right: Measured versus predicted spatial prvjiles normal to the target (.) at 1.0 ns.(2) Plasmas that are quasi homogeneous along the direction of beam propagation (~o) are pre-formed by line-focusing a beam of the Trident laser[8] (A = 526.6 nm) for 1.2 ns onto a CH disk6.7 pm in thickness and 1 mm in diameter[9]. A second Trident beam for SRS and SBS inter-actions, either a regular beam or a well characterized j/6.9 nearly diffraction-limited beam[lO],travels parallel to the disk and along the line focus plane. The pl~ma expansion is mostly,.normal to ko, so the IB beam samples a x 1 mm-long plasma with constant n,, depending onthe chosen IB-disk separation. Trident and Nova plasmas are similar in important areas suchas the onset and saturation of SBS[9,1 1]. Moreover, Trident plasmas have been characterizedwith unprecedented detail with a dia=~ostic that images light from collective Thomson scatter-ing along the direction of plasma expansion with a framing time of 0.12 ns[lO]. The diagnosticgeometry and the plasma profiles, predicted and measured, are shown in Fig. 2. The profilemeasurements are necessary for truly detailed comparisons between theory and experiment.2. EXPERIMENTAL RESULTS AND FUTURE DIRECTIONSHigh-energy ghs-laser beams used for ICF are not spatially uniform. Mitigation tech-niques include passive optics for beam smoothing. For h-ova and some Trident experiments wehave used a binary random-phase plate (RPP)[12,13]. RPPs produce a “smooth” focal spotenvelope with a superimposed fine-scale speckIe (or “hot spot”) pattern. A significant fractionof the beam energy is in hot spots. RPP hot-spot statistics aKect the character of SBS and SRSonsets profoundly, as seen in models and simulations [14,15]. The result of integrating the SBSgain over the hot-spot distribution, including beam diffraction and pump depletion, is a non lin-ear system which exhibits critical behavior[14]. In the long-scale regime the instability -dephasinglength is much Ionger than the typical hot-spot length (~hs N 7~2J). There, as the average beamintensity 1 is increased the reflectivity R sharply increases from l?,.~~ once 1 exceeds a criticalintensity value l=. ~ =1= when the hear gain R/Rs.ed = exp[G(l., Zh.)] X e], in contrast withthe customary threshold G = 27r. Once 1 >> 1= hot-spots undergo pump-depletion or nonlinear saturation and R levels off. Sharp ousets are also evident on 3-Dim. SBS simulations [15].High-T’, and long density scales also allow SRS study where the same theory applies.SBS 1. magnitudes and dependencies on v; and on j number measured in the Nova toroidalhohlraums(7,16] and in the Trident line plasmas[9] are predicted theorwically reasona’ i well!’ ~].21?Uxf G#s-f2Mn w2mKwus,1u!’ l’;{uIII!10.1 I0.1 1 folntenslty at LEH (1 O’s W/cm’)Figure 3: Left: Time-avemgedH-_Ed%0.05 0.10 0.15 0.20w U5-m.l.m MawAws flu m10 , I *D●Y%Awumc&,5j ~i,,,,,,,;o ,#, , *+0.05 0.10 0.15 0.20.Wdo (did,Rsm and RSBS are plotted versus I. Center 6 Right: RSRS& ‘RsBs are plotted versus initial nejn.. RSRS ualues am co~cted for inverse-B~msstmhlungabsorption (x1.65 at ne/nc = 0.18, x1.25 at ne/nc = 0.11 and x1.05 at n,/nc = 0.07).SRS onsets in toroidal hohlraums have been measured also. For C5H12 (n,/nc = 0.11) theobserved 1= - 10]4 W/cm2[16] is consistent with theory within our ability to ascertain the valueof the Landau-damping frequency of EPWS (v=) within hot spots[17]. Fig. 3-Left shows RSRSand RSBS versus 1 of the RPP-smoothed III from 1 atm C3~8 fills (ne/rzc = 0.07). All thereflected light near and within the beam cone is measured[18] and included in the values shown.The observed 1= - 3 x 1014 W/cm2 for SRS is significantly higher than with n,/nc = 0.11because of the higher v, from Landau damping at lower n,. In contrast, the observed SBS 1.are similar for both ne values because the Landau-damping rates for IAWS (vi) do not change.Fig. 3-Left includes fits to R.sR,s and RsBs from the model in Ref. ([14]).We have studied SBS and SRS saturation extensively. In a fixed-length homogeneousplasma the convective gain exponents are GSBS M (nJ)/~i and GSRS m 1/v,. SRS and SBS arenot simply gain-limited when 1>> lC since R levels off with increasing 1[9,7,16], as in Fig. 3-Left.Only timeintegrated hohlraum RSRS and R.SBS are discussed, as the plasma is approximatelyin steady state while the IB is on. RSRS and RSBS vary in time due to hydrodynamic-evolutiondetails[19], but this is outside the scope of this paper. RSRS is remarkably constant over awide range of ne, as shown in Fig. 3-Center. According to LASNEX, changing n= changes T.and Ti little, allowing fairly clean comparisons. RSBS would increase exponentially with n. iflinear convective theory applied. Fig. 3-Right shows a vastly different result. In plasmas withmultiple ion species, v;/u; depends on the relative ion masses[20,21]. we exploit this by usingthe hohlraum gas till to vary vi/wi while keeping T;/T. fixed. The measured Rsx from toroidalhohlraums depends approximately linearly on ~~/~;[22,16], even though SRS does not involveIAWS. This is attributed to the coupling of SRS to the Langmuir Decay Instability (LDI),where the SRS EPW further decays into another EPW and an IAW[23]. We have imaged theSRS light at the target plane with 25 pm resolution and 0.15 ns framing[24]. We deduce fromthese images that significant SRS occurs throughout the beam path, contrary to expectationsfrom convective theory, i.e., highest scattering at the end of the SRS gain region. UbiquitousSRS is more consistent with saturation mediated by LDI, i.e., SRS clamped by secondary decaythroughout the plasma except for a short growth region. In spite of modeling successes inSRS-LDI coupling~25,26], much more work will be required to explain our data thoroughly ~27].f?SBS from toroidal hohlraums is influenced also by ~i/~i[16]. Since SBS is not simply gain-limited, any vi dependence is difficult to explain. Competition between SRS and SBS might beinvolved, but it is strange that the SBS reflectivity is constant for a wide range of vi/~i whilethe SRS reflectivity is changing significantly. SBS saturation is apparently sensitive to boundarvconditions. In Trident line plasmas when Z >> l=, we have raised the intensity-saturated RSE;31.51.6 Time (ns)1.7 1.8Figure 4: The instantaneous RsBs and RsM ~flectim”ty of the Tn”dent difl’ction-limited beamfmm a CH line plasma is plotted versus time.by seeding SBS with an external light source[ll].Much work remains to understand SRS and SBS saturation. Underlying observed trendsin complicated NIF-relewmt plasmas, there are undoubtedly coupled physical processes whichwe hope to uncover by detailed comparisons of theory to the single-hot-spot experiments. Forexample, Fig. 4-Right shows the evolution of R.sm and RsBs of the diffraction-limited Tridentbeam. RsR.s and RSBS are anticorrelated and oscillate in time with a period similar to acoustictransit times across the IB. Promising modeling efforts for these experiments are underway.*This work was supported by the US DOE.References[1] HAAN, S. W. et al., Phys. Plasmas, 2 (1995) 2480.[2] GOLDMAN, M. V. et al., Phys. Fluids, 8 (1965) 1404.[3] GORBUNOV, L. M., Soviet Physics JETP, 28 (1969) 1220.[4] ROSENBLUTH, M. N., Phys. Rev. Le.tt., 29 (1972) 565.[5] ZIMMERMAN, G. et al, Comments Plasma Phys. ContruUed Fusion, 2 (1975) 51.[6] WILDE, B. H. et al., in Proc. 12th International ConJ on Laser Interaction and RelatedPlasma Phenomena, Osaka, 1995, G. H. MILEY Ed., page 255, AD? Press, NY, 1996, Conf.Proc. No. 369, Part 1.[7] FERNANDEZ, J. C. et al., Phys. Rev. E, 53 (1996) 2747.[8] MONCUR, N. K. et al., AppL Optics, 34 (1995) 4274.[9] WATT, R. G. et al., Phys. Plasmas, 3 (1996) 1091.[10] MONTGOMERY, D. S. et aL, accepted to Lasers and Particle Beams, 1998.[11] FERNANDEZ, J. C. et al., Phys. Rev. Lett., 81 (1998) 2252.[12] KATO, Y. et al., Phys. Rev. LetL, 53 (1984) 1057.[13] DIXIT, S. M. et al., AppL Optics, 32 (1993) 2543.[14] ROSE, H. A. et al., Phys. Rev. Leit., 72 (1994) 2883.[15] BERGER, R. L. et al., Phys. Rem LetL, 75 (1995) 1078.[16] FERNANDEZ, J. C. et al., Phys. Plasmas, 4 (1997) 1849.[17] AFEYAN, B. B. et al., New regimes of parametric instabilities in plasmas with flat-toppedand depleted tail velocity distribution functions, submitted to Phys. Rev. Lett., 1997.[18] KIRKWOOD, R. K. et al., Rev. Sci. Insirum., 68 (1997) 636.[19] MONTGOMERY, D. S. et al., Phys. Plasmas, 5 (1998) 1973.[20] VU, H. X. et al., Phys. Plasmas, 1 (1994) 3542.[21] WILLLAMS, E. A. et al., Phys. Plasmas, 2 (1995) 129.~22] FERNANDEZ, J. C. et al., Phys. Rev. Leit., 77 (1996) 2702.[23] DUBOIS, D. F. et al., Phys. Rev. Lett., 14 (1965) 544.~24] WILKII, M. D. et al., Rev. Sci- Irzshwm., 68 (1997) 672.[%] DRAKE, R. P. et al., Phys. l%ids B, 3 ( 1991) 2936.[26] BEZZERIDES, B. et al., ~hys- Rev. Lett-, 70 (1993) 2569.[27] RUSSELL, D.A. et al., submitted to Phys. of Plasmas, 1998.4..,.’

Experimental Program to Elucidate and Control Stimulated Brillouin and Raman Backscattering in Long-Scale Plasmas

https://digital.library.unt.edu/ark:/67531/metadc703271/m2/1/high_res_d/759442.pdf

Experimental Program to Elucidate and Control Stimulated Brillouin and Raman Backscattering in Long-Scale Plasmas

Abstract

Similar works

Full text

Available Versions

UNT Digital Library