Direct-Drive, High-Convergence-Ratio Implosion Studies on the OMEGA Laser System

Transcription

1 Direct-Drive, High-Convergence-Ratio Implosion Studies on the OMEGA Laser System F. J. Marshall, J. A. Delettrez, R. Epstein, V. Yu. Glebov, D. D. Meyerhofer, R. D. Petrasso,P.B.Radha,V.A.Smalyuk,J.M.Soures,C.Stoekl,R.P.J.Town, and B. Yaakobi. Laboratory for Laser Energetics, U. of Rochester The effects of beam smoothing, pulse shaping, and target dimensions on the compressed core and shell performance of directly driven plastic capsules are studied on the 30-kJ, 60-beam OMEGA laser system. Experiments are performed on surrogate-cryogenic capsules where the main fuel layer is a polymer shell (either CH or CD + CH) and the hot spot is provided by the fill gas (D 2, DHe 3, DT, or H 2 ). The spatial evolution of the fuel and shell regions is recorded using both broadband and monochromatic time-resolved x-ray imaging techniques. Similar targets with inner Ti-doped layers provide additional spectral diagnostics of the shell and a source of monochromatic emission. Core conditions are diagnosed with measurements of the emergent x-ray, neutron, and particle spectra. For 1-ns-square drive pulses the calculated convergence ratios are in excess of 30, and the primary neutron yields are 20% of clean 1-D with shell areal densities >100 mg/cm 2. Shaped pulse implosions have higher convergence ratios. Compressedtarget conditions measured include those of fuel and shell areal density, fuel ion temperature, shell electron temperature, and primary (DD) and secondary (DT) neutron yield. The effect of pulse shaping and the beneficial effects of beam smoothing on the final core conditions are seen in these measurements. Mixing, resulting from laserirradiation nonuniformities and target imperfections that seed the Rayleigh Taylor instability, are observed in the x-ray and neutron spectra and are seen to depend on both the level of beam smoothing and the pulse shape. Comparison of these results with 1- and 2-D hydrocode simulations (including models of fuel-shell mixing) is ongoing. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460, the University of Rochester, and the New York State Energy Research and Development Authority.

2 Direct-Drive, High-Convergence-Ratio Implosion Studies on the OMEGA Laser System Frederic J. Marshall University of Rochester Laboratory for Laser Energetics 41st Annual Meeting of the American Physical Society Division of Plasma Physics Seattle, WA November 1999

3 Contributors Experimental: Vladimir Glebov David Harding Richard Petrasso Wolf Seka Vladimir Smalyuk Christian Stoeckl David Meyerhofer Barukh Yaakobi John Soures OMEGA Laser Operations: Jack Kelly Sam Morse Greg Pien Laser Operations Staff Theory: Jacques Delettrez Richard Town Pat McKenty Reuben Epstein P. B. Radha MIT: C. K. Li Damian Hicks Fredrick Séguin LLE Staff: Many

4 Summary We are investigating the effects of beam smoothing and pulse shape on target performance with OMEGA The core conditions of high-convergence-ratio targets have been diagnosed using x-ray, neutron, and particle spectroscopy: [ρr fuel, ρr shell, T e, and T i, Y n (DD), Y n (DT)]. Increased SSD bandwidth improves target performance. The improvement is most evident for the least-stable conditions (i.e., longer, shaped pulses). 2-D simulations of single-beam nonuniformity effects can account for the performance of the outer region of the shell. Low l-mode contributions due to beam balance appear to limit the performance of the most-stable, low-convergence-ratio targets. E10147

5 Current OMEGA experiments are preparatory to cryogenic implosions on OMEGA and NIF ignition OMEGA cryogenic implosions are energy scaled from NIF ignition target designs. Current noncryogenic implosions study hydrodynamics and stability issues. The CH shell corresponds to the main fuel layer (DT ice), and the fill gas corresponds to the hot-spot-forming central DT gas. CH (20 to 40 µm) OMEGA noncryogenic D 2, DT, DHe 3 < 50 atm 0.5 mm OMEGA cryogenic DT gas 3 atm CH < 2 µm DT ice (19 K) & 100 µm E10148

6 20-µm-thick plastic shells driven by 1-ns square pulses show similar stability to the OMEGA α = 3 cryo design α = 3 OMEGA cryogenic design 20 µm CH, 1-ns square pulse 10 1 Shell thickness Shell thickness Distance (µm) 10 0 Mix width: 1 THz, DPR s Mix width: 0.2 THz, no DPR s Mix width: 1 THz, DPR s Distance traveled (µm) Distance traveled (µm) TC5267 The mix width is calculated using an instability model with Takabe growth rates and Haan saturation.

7 Outline Direct-drive, high-convergence ratio implosion studies on the OMEGA laser system Review of recent OMEGA results Gas-filled shells Beam-smoothing effects (SSD bandwidth) Pulse shaping Diagnostics Neutron yield and spectroscopy [Y n (DD), Y n (DT), ρr fuel, ρr shell, T i ] Charged-particle spectroscopy [Y p (DHe 3 ), ρr total, T i ] X-ray continuum spectroscopy and imaging of Ti-doped shells (ρr shell, Te) Conclusions SSD smooths the high l-mode nonuniformities. SSD benefits the least-stable implosions (shaped pulses) the most. E10149

8 Fuel performance improves with increasing shell thickness Gas convergence ratios (r gas ) i /(r gas ) f range from 30 to Yield (DD)/calculated (YOC) YOC (%) 10 1-ns square pulse Empty CD/CH shells 3-atm-filled CD/CH and CH shells E10150 Shell thickness (µm)

9 Primary fusion yield (DD) from CH targets is a function of the shell stability Shell convergence ratio (shell CR) is a measure of shell stability. 100 Empty CD/CH (1 ns) YOC (%) ns square pulse PS26 3- to 15-atm-filled CH (1 ns) 3- to 15-atm-filled CH (PS26) SSD off = open symbols (R shell ) Shell CR = i, (R shell ) f Calculated shell convergence ratio (R shell ) f = 3M f 4π<ρR> f The least-stable implosions show the most improvement with SSD. E10151

10 We are using three principal methods to determine ρr in compressed CH shells X-ray absorption: continuum or high-z K-shell [ ρr shell, T e ] CD X rays Secondary (DT) neutron yield from D in gas or shell (CD) [Y n (DD),Y n (DT), ρr fuel, ρr shell, T i ] T* D n DT * Charged-particle energy loss of 14.7 MeV proton (D-He 3 reaction) [Y p (DHe 3 ), ρr total, T i ] p p* E10152

11 ORCHID simulations are used to estimate the effects of high-l-mode nonuniformities on measurements 20-µm-thick, 3-atm-filled CH shell, 29 kj, 1-ns square pulse t = 1.70 ns t = 1.72 ns t = 1.74 ns t = 1.76 ns 100 µm Density profiles Radius E10153 X-ray emissivity 50 µm ρr (mg/cm 2 ) 200 Total ρr Cold-shell ρr D D 50 1-D 2-D Radius (µm) Radius (µm)

12 SSD increases the fuel rr of shaped-pulse implosions 20-µm-thick, 3-atm-D 2 -filled CH targets Normalized secondary spectrum to 0.3 THz rr fuel = 7 mg/cm ns PS to 0.3 THz rr fuel = 8 mg/cm to 0.1 THz rr fuel = 4 mg/cm 2 Energy (MeV) Energy (MeV) Secondary yields measured by MEDUSA E10154

13 The experiments performed with and without CD layers put limits on the amount of mixing at stagnation 27-µm-thick, 3-atm D 2 -filled CH targets, 1-ns square pulse. 5 Normalized secondary spectrum Inner CD layer 1-µm offset embedded CD layer No CD layer Energy (MeV) E10155 Minimal mixing within the inner 2-µm shell region is observed.

14 The OMEGA charged-particle spectrometers (CPS) measure the total rr 20-µm-thick, 3-atm DHe 3 -filled CH targets, 1-ns square pulse Proton spectrum D-He 3 reaction proton spectrum Observed downshift DE Birth energy Energy loss DE = 1.63±0.04 MeV Implied areal density rr total = 70±5 mg/cm 2 Averaged over three shots Energy (MeV) E10156

15 The shell ρr is determined from the secondary neutron yield and from the 14.7-MeV proton energy loss The methods give consistent values for the shell ρr (the yield ratio determined values are lower limits). Shell ρr (mg/cm 2 ) ns square pulse LILAC limit Voided CD/CH 3-atm-filled (D 2, H 2, DHe 3 ) DHe 3 proton energy loss Shell thickness (µm) E10157

16 The 5- to 7-keV x-ray images show a more-compact and integral stagnation region with increased SSD bandwidth 20-µm CH shells, 1-ns square pulse, 25 kj, Ti-doped embedded layers CH Ti-doped CH 0.1 THz 0.2 THz 0.3 THz µm 2 µm 1 µm 3-atm D 2 2 ~ 50 µm Intensity (arbitrary unit) Radius (µm) Radius (µm) Radius (µm) E10158

17 ORCHID is used to simulate the effect of shell nonuniformities on x-ray images 3-atm-filled, Ti-doped, 20-µm CH shells, 1-ns square pulse ORCHID simulated x-ray emissivity 50 µm Intensity (arb. units) Comparison of radial profiles OMEGA shot ORCHID KB image Radius (µm) ~ 50 µm KB microscope x-ray images 2-D ORCHID x-ray emission profile shows a similar size but different width than seen in the KB microscope image. E10159

18 SSD has a dramatic effect on the final stagnation of direct-drive ICF capsules 20 kj, PS26 pulse shape, with DPP s, 0.95-mm diameter 3 atm H 2 -filled, 27-µm-thick shells OMEGA shot 12565, SSD on Gratingdispersed images OMEGA shot 12569, SSD off 1 mm 6 50 µm Hard x-ray images (> 4 kev) (undispersed) E9438

19 X-ray spectral measurements have confirmed the beneficial effects of SSD on direct-drive implosions OMEGA shots and atm-DHe 3 filled, 25-µm-thick CH targets 25-kJ, 1-ns square pulse Spectal fluence (kev/kev) SSD on (0.2 THz) LILAC SSD off E9436 Energy (kev)

20 2-D ORCHID simulations qualitatively agree with experimental measurements of cold shell ρr 100 Cold shell ρr versus shell thickness 3-atm-filled CH shells, 1-ns square pulse Cold shell ρr (mg/cm 2 ) 10 ORCHID (SSD on) 1-D 2-D EXPT Experiments SSD on SSD off E10167 Thickness (µm)

21 Summary/Conclusions We are investigating the effects of beam smoothing and pulse shape on target performance with OMEGA Increased SSD bandwidth improves target performance. The improvement is most evident for the least-stable conditions (i.e., longer, shaped pulses). 2-D simulations of single-beam nonuniformity effects can account for the performance of the outer region of the shell. Low l-mode contributions due to beam balance appear to limit the performance of the most-stable, low-convergence-ratio targets. E10168