Stimulated Raman Scattering in Direct-Drive Inertial Confinement Fusion

Transcription

1 Stimulated Raman Scattering in Direct-Drive Inertial Confinement Fusion View ports 5 FABS Experiments carried out at the National Ignition Facility FABS power (arbitrary units) Plasma-producing beams ( outer ) Interaction beams ( inner ) W. Seka University of Rochester Laboratory for Energetics 5th Annual Meeting of the American Physical Society Division of Plasma Physics San Jose, CA October November

2 Summary Absorption and refraction significantly affect stimulated Raman scattering (SRS) in National Ignition Facility (NIF) planar-target experiments Planar NIF experiments are SRS dominated SRS spectra are strongly affected by absorption and refraction are predominantly caused by sidescattering coronal T e predictions match measurements using spectroscopy Estimates of total SRS levels ~5% of incident are based on simulations, ray-trace calculations, a few measurements, and large extrapolations E579

3 Collaborators M. J. Rosenberg, J. F. Myatt, A. A. Solodov, D. H. Edgell, R. W. Short, S. P. Regan, and A. V. Maximov University of Rochester Laboratory for Energetics P. Michel, C. S. Goyon, and J. D. Moody Lawrence Livermore National Laboratory

4 ~/ light (7 nm) from absolute SRS can escape at Kº from dn although with rapidly decreasing efficiency Absorption of ~/ light is high (>97%) Refraction severely limits the angular emission of ~/ light Only a tilt of the target allows for ~/ light to be observed at NIF ports nm 5.5 SRS -. 7 targ et 7 NIF quads at to 55 to target normal Q SRS diagnostic log Ch E5 CH.5 Te (kev) nc/ M=

5 Absorption of ~/ light is high (>97%) Refraction severely limits the angular emission of ~/ light Only a tilt of the target allows for ~/ light to be observed at NIF ports. E SRS - ve i t la y u m erg u C en I(i) = T* exp [ (i/.5)] 5 5 i ( ).5 7-nm angular emission Multiplier ~ for tilted target. 7. log Ch 57 nm 5 T = % f/ The total absolute SRS emission can be estimated from ray trace and simulated plasma conditions.5 Te (kev) nc/ M= 5

6 ~/ spectra in OMEGA implosions are a signature of two-plasmon decay (TPD), while on the NIF they represent the absolute SRS instability OMEGA implosion ~/ spectrum log 775 (TIM-) Absolute SRS 7 m.5 5 Convective SRS.5. log Ch SRS - Sharp feature: absolute TPD NIF planar-target experiments.. The sharp, red-shifted spectral signature is a useful coronal Te diagnostic Measured and DRACO-predicted electron temperature agree very well E5 TPD-related photons require an inefficient generation mechanism Every SRS decay results in a scattered photon (% efficiency) Absolute SRS may effectively suppress TPD

7 Theory supports that NIF planar experiments are SRSdominated, while the OMEGA experiments are TPD-dominated OMEGA 775 (TIM-) 7 m... NIF SRS 7 OMEGA-EP log Ch SRS - TPDOMEGA Ith,TPD = Ith,SRS.5 5 Te (kev) log Ln (nm) NIF planar-target experiments TPD threshold depends on Te; SRS threshhold does not*.5 7-nm escape: ~%~% 7-nmphoton photon escape: -nm escape: ~5% -nmphoton photon escape: ~5% E5 *R.W. Short, based on single-beam theory (private communication). 7

8 SRS spectra observed at,, and 5 indicate that SRS sidescattering dominates over backscattering 5 QB 5 Ch Q - Ch Q - South pole 5 Sidescatter only 7 log 7 nd QB 5 QB South pole Ch Q - a 5 Mostly sidescatter 7 QB 5 and ~5% of the incident laser radiation reaches SRS densities with minimal refraction SRS light is significantly affected by refraction, particularly at m > 5 nm For incident laser at, local SRS sidescattering at > is required E5

9 The total SRS emission can be estimated from measurements and simulations Estimates of total SRS energy of ~5% of incident (CH target shot ) and measured SRS energies (fast diodes) measured spectra assuming sidescattering as deduced from ray trace using DRACO plasma parameters Because SRS energy measurements are restricted on the NIF to two (non-optimal) locations, the extrapolations are problematic but still useful. T = 5% -nm emission Q Multiplier ~ f/ f/ Cumulative energy i( ) I(i) = T * exp{ (i/5) } (primarily caused by refraction) 5 7 Transmission measured only to n SRS /n c. E55 9

10 Summary/Conclusions Absorption and refraction significantly affect stimulated Raman scattering (SRS) in National Ignition Facility (NIF) planar-target experiments Planar NIF experiments are SRS dominated SRS spectra are strongly affected by absorption and refraction are predominantly caused by sidescattering coronal T e predictions match measurements using spectroscopy Estimates of total SRS levels ~5% of incident are based on simulations, ray-trace calculations, a few measurements, and large extrapolations E579

11 ~/ spectral shifts can be used for coronal T e measurements D 5 5 OMEGA implosion ~/ spectrum CH target NIF planar-target experiments (~/ spectra) # counts D 5 DRACO.5 kev 5 T. kev e 7 nm 7 nm SRS QB (TIM-) T e (kev) Ch CH target Stationary plasma Plasma flow 5 T e (kev) # counts Diverging plasma CH... Dm nm =.9*T e,kev dm Doppler dm Dewandre E595