Analysis of a Direct-Drive Ignition Capsule Design for the National Ignition Facility

Transcription

1 Analysis of a Direct-Drive Ignition Capsule Design for the National Ignition Facility R (mm) End of acceleration phase r(g/cc) Gain s (mm) Z (mm) P. W. McKenty et al. University of Rochester Laboratory for Laser Energetics s 2 =.6 s s 2 >1 42nd Annual Meeting of the American Physical Society Division of Plasma Physics Québec City, Canada October 2

2 Analysis of a Direct-Drive Ignition Capsule Designed for the NIF P. W. McKenty Laboratory for Laser Energetics, U. of Rochester The current direct-drive ignition capsule design planned by the University of Rochester s Laboratory for Laser Energetics to be fielded on the National Ignition Facility (NIF) will be reviewed in this paper. The direct-drive requirements to establish a propagating thermonuclear burn on the NIF will be discussed in terms of the constraints on laser-irradiation uniformity and target surface roughness. The ignition design1,2 consists of a cryogenic DT shell (~35 µm thick and ~3 mm in diameter) contained within a very thin(~2-µm) CH shell. To maintain stability during the implosion, the target is placed on an isentrope approximately three times that of Fermi-degenerate DT (α = 3). One-dimensional hydrodynamic studies using LILAC show that the ignition design is robust to uncertainties in laser power history and fuel composition. The two-dimensional hydrodynamics code ORCHID is used to examine the target performance under the influence of the main sources of nonuniformity: laser imprint, power imbalance, and inner- and outer-target-surface roughness. Results from these studies indicate that the reduction in target gain from all sources of nonuniformity can be described in terms of a single parameter related to the resultant inner-surface deformation at the end of the acceleration stage of the implosion. This parameter is constructed from a spectral decomposition of the surface deformation by giving different weights to the long and short wavelengths of nonuniformity. The physical reason for the difference in weighting is discussed in terms of the mechanisms for ignition failure. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC3-92SF C. P. Verdon, Bull. Am. Phys. Soc. 38, 21 (1993). 2. S. E. Bodner et al., Phys.Plasmas5, 191 (1998).

3 Collaborators V. N. Goncharov R. P. J. Town S. Skupsky R. Betti R. L. McCrory J. A. Delettrez D. L. Harding M. Wittman R. L. Keck

4 Summary Scaling target gain with s provides the basis for developing a global nonuniformity budget for the NIF direct-drive point design Results from the nonuniformity budget, accounting for all four sources of nonuniformities, indicate that direct-drive targets can achieve gains in excess of 3 using current NIF specifications and the deployment of SSD with two color cycles. Outer surface roughness does not make a significant contribution to the nonuniformity budget. Distortions at stagnation are dominated by low order modes, however, high order modes cannot be neglected. Gain reduction is caused by target nonuniformities delaying the onset of ignition thereby wasting margin of the stagnating fuel layer. TC5589

5 Outline Point design Numerical modeling Sources of implosion nonuniformities power balance ice/vapor surface roughness outer surface roughness laser imprint Failure analysis Nonuniformity budget Summary TC5484

6 Point design The all-dt direct-drive target is a thick DT-ice layer enclosed by a thin CH shell CH DT ice DT gas 1.69 mm 1.35 mm 1.5 MJ, a = 3 Gain: 45 Yield: rr peak : 13 mg/cm 2 <T i > n : 3 kev Hot spot CR: 29 Peak IFAR: 6 Power (TW) Gain Time (ns) TC5576 R. Town, LLE Review, 79, 121 (1999).

7 Numerical modeling Simulations show that low-order modes can reduce high-gain capsule performance if the perturbation amplitudes are large TC282a Yield (perturbation) /yield (uniform) l = 16 l = 8 l = 4 l = Illumination perturbation amplitude Peak-to-valley (%)

8 To relate the gain reduction to the mode spectrum, a series of 2-D ORCHID simulations with perturbed inner DT-ice interface has been performed s l = s l b 1 1 b = 1.5 Amplitude (µm) b = Initial-mode spectrum Mode number Mode number End of acceleration TC5137 s 2 =.6s s 2 >1 Gain s (µm)

9 Sources of implosion nonuniformities There are four sources of perturbations a directdrive capsule must tolerate to ignite and burn Target fabrication issues Outside capsule finish Laser irradiation issues Laser imprinting Drive symmetry Inner DT ice roughness TC5316a

10 Heuristically, there are four sources of perturbations a direct-drive capsule must tolerate to ignite and burn s rmsdrive symmetry Max allowed value drive symmetry s 1 rmslaser 2 2 imprinting + Max allowed valuelaser imprinting s 2 rmsdt ice srms 1 2 outside capsule finish + + < 1 Max allowed Max allowed value DT ice value outside capsule finish Laser-irradiation-related issues Target-fabrication-related issues TC4113f

11 Power balance NIF temporal power histories are mapped to target and spherically decomposed for input into ORCHID Laser power (arbitrary units) NIF Beamlet pulses 5 1 Time (ns) Rms (%) 8% rms beam to beam 1.5 2% rms on target. 3 Mode number Power imbalance (% rms) Perfect beam timing 4-ps rms mistiming Laser energy (quad to quad) Energy on target Laser pulse Time (ns) TC5495 NIF laser power histories provided by Ogden Jones (LLNL)

12 Results of ORCHID calculations have validated the direct-drive base-line power imbalance specifications 45 Perfect beam timing 3-ps mistiming between beams Gain 3 15 NIF Indirect-drive baseline Initial on target perturbation (% rms) s (mm) TC5563

13 Ice/vapor surface roughness Perturbations located initially on the inside DT-ice surface affects the capsule implosion during both the acceleration and deceleration phases Initial capsule Feedout of perturbation to the ablation surface Ablation surface growth and feedthrough to the inner surface during the acceleration phase Growth of the hot spot main fuel layer interface during the deceleration phase Inside DT-ice surface Time TC5317a

14 ORCHID simulations indicate that target gain depends strongly on the development of the low-order modes R (mm) Gain = 1.75 r (g/cc) Perturbation amplitude (µm) Ice/vapor interface spectra 1. Final spectrum 1. Initial spectrum.1 s c = 95% s tot Z (mm) Fourier mode number TC5211

15 Scaling target gain with s correctly balances the individual contributions of the low and high order modes during the implosion. 45 a = a /l b, b = ( ),.75( ), and 1.5( ) 3 Gain 15 TC s (mm) (modes 2 1) 4 1 s (mm) 2 3

16 Outer surface roughness The smoothness and concentricity of thin-wall polyimide targets (1.5 to 2. mm) have been improved nm NIF indirect-drive standard Filled to ~9 psi Early shell (9/99) Empty Mode number TC5565 High-frequency roughness is a consequence of the coating process; the rms value l > 1 is < 4 nm. Low-frequency roughness is caused by weak shells deforming to accommodate the bending moments that develop during processing. Inflating the shell reduces the low-frequecy roughness.

17 Twice the NIF standard specification (~165 nm) for outer surface roughness does not result in any significant disruption of the ice/vapor interface R (mm) s =.15 mm Z (mm) r(g/cc) t = 8.8 ns Power spectrum (mm 2 ) Surface spectra Ablation (. ns) Ablation (8.8 ns) Ice/vapor (8.8 ns) 1 Legendre mode number 1 TC5455

18 Laser imprint SSD reduces time-averaged laser nonuniformity DPP spectrum Beam Overlay PS Target plane Phase plate Focus lens TC522a s rms ~ t c t s rms rms nonuniformity (%) *S. Skupsky, Phys. Plasmas 6, 2157 (1999) [ ] -1 t * c = Dnsin( kd 2) Dn: bandwidth d: speckle size <t>

19 MCP D TDTpo nd Su SSD gn NF TW p m p o pow m b n p p d Application of SSD bandwidth is necessary for shell integrity during the entire implosion 1 8 No bandwidth r (g/cc) Bandwidth = 1 THz r (g/cc) R (mm) Z (mm) Z (mm) TC5587

20 Scaling gain with s, taken from ORCHID calculations, indicates that NIF must deploy at least 1-THz bandwidth Projected 2-D ORCHID results color cycles Gain 15 NIF Direct-drive specification SSD bandwidth (THz, 1 color cycle) s (mm) TC5588

21 Failure Analysis To achieve high gain for NIF capsules ignition must occur while the cold fuel still retains a significant fraction (margin) of its peak kinetic energy Ignited hot spot Plasma blowoff Fuel burn-up fraction Point design G = 45 f b =.13 margin = ~ 4% Gain threshold G = 1 f b =.3 margin = ~ 1% 1 3 v = cm/s v = cm/s Imploding cold fuel (DT ice) 1 1 (Pusher kinetic energy/1 kj) (b/2) 1.7 (v imp /3 1 7 cm/s) 5.5 TC5545 Graph taken from Levedahl and Lindl, Nucl. Fusion 37 (2), 17 (1997).

22 Shell stagnation determines the margin trajectory that defines the window for high gain 1 75 High gain Margin (%) 5 Non-burning capsule G = G = 1 2 Ignition capsule Ignition timing (ps) 2 TC558

23 ORCHID simulations indicate that as ice/vapor interface perturbations increase, ignition is delayed and gain is reduced 2-D ORCHID results Ignition delay (ps) Margin (%) Gain s (mm) Margin (%) TC5544

24 Nonuniformity budget Scaling gain with s allows forming a global nonuniformity budget for the direct-drive point design 2 Current NIF specifications 4 Sum-in-quadrature s (mm) 1 Two color cycles Multiplier s (mm) Applied SSD bandwidth ( 1THz) On-target power imbalance ( 2% rms) Inner-surface roughness ( 1-mmrms) Outer-surface roughness ( 8 nm) TC5577

25 Summary/Conclusion Scaling target gain with s provides the basis for developing a global nonuniformity budget for the NIF direct-drive point design Results from the nonuniformity budget, accounting for all four sources of nonuniformities, indicate that direct-drive targets can achieve gains in excess of 3 using current NIF specifications and the deployment of SSD with two color cycles. Outer surface roughness does not make a significant contribution to the nonuniformity budget. Distortions at stagnation are dominated by low order modes, however, high order modes cannot be neglected. Gain reduction is caused by target nonuniformities delaying the onset of ignition thereby wasting margin of the stagnating fuel layer. TC5589