Determination of the Flux Limiter in CH Targets from Experiments on the OMEGA Laser

Transcription

1 Determination of the Flux Limiter in CH Targets from Experiments on the OMEGA Laser f = Neutron rate (1/s) Exp. f = f = J. A. Delettrez et al. University of Rochester Laboratory for Laser Energetics st Annual Anomalous Absorption Conference Sedona, AZ 3 8 June 2001

2 Contributors C. Stoeckl S. P. Regan P. W. McKenty D. D. Meyerhofer J. P. Knauer University of Rochester Laboratory for Laser Energetics

3 Summary Simulations of experiments on the OMEGA laser suggest slightly larger values than currently used for the flux limiter The increased precision of the neutron time-of-flight (NTD) diagnostic and the reproducibility of laser shots permit a more precise determination of the flux limiter and central cell zoning. The timing and the level of both the shock yield and the onset of the compression yield are sensitive to the flux limiter. A flux limiter of (sharp cutoff) gives general agreement with experiment for 1-ns square pulses and 20-µm CH shells. Differences for the 400-ps square pulse and the 28-µm CH shell imply that other factors affect target dynamics. TC5650

4 Conditions are right for improving parameters in the simulations of CH-shell targets using the neutron time-of-flight detector (NTD) The OMEGA laser provides reproducible pulse shapes and energy over the span of several months. NTD, along with the laser pulse shape diagnostics, provides the timing of neutron yield to within 50 ps. The timing of the shock yield and the onset of the compression yield are not affected by two-dimensional effects and mix. They can be used to refine simulation parameters, in particular the flux limiter. The previous method of determining the flux limiter from the trajectory of glass shells is not satisfactory because CH shells are visible only during the peak of the pulse, and the shell trajectory is not as sensitive to the flux limiter as the time of neutron production. Improved measurement of the absorption fraction is being developed. TC5651

5 Shots during the same week give reproducible NTD results 900 µm 950 µm 1000 µm Rates (1/s) 1100 µm 18-µm CH, 20 atm D 2 1-ns square pulse, varying diameter TC5652

6 The neutron burn history shows details of the shock arrival and the stagnation phase of the implosion Radius (µm) Compression yield Neutron rate (s 1 ) Experimental conditions Pulse 1-ns square DD fill (atm) Shell thickness (µm) Shock yield 400-ps square TC5659

7 Two different zoning schemes are possible when using the hydrocode LILAC Equal mass zoning Equal radius zoning E10632

8 The size of the central zone affects mainly the shock yield for the 400-ps square pulse CH/fuel interface trajectory 400-ps square pulse 1-ns square pulse Distance (µm) Equal mass smaller central zones Equal mass Equal R Neutron rate (1/s) Equal R = 4.6 µm Experiment Equal mass R = 98 µm Equal mass R = 58 µm Equal mass Equal R Experiment TC5660

9 For a 1-ns square pulse and a 20-µm shell, a flux limiter of 0.07 (sharp cutoff) gives the best agreement with experiment for timing of NTD Neutron rate (1/s) Shot 22862: 15-atm D 2 Shot 22864: 3-atm D 2 f = 0.070, 46 µm f = 0.070, 46 µm f = f = Exp. f = Exp. f = TC5653

10 For the 400-ps-pulse case, a flux limiter of is not large enough to reproduce the experimental measurement Vary the flux limiter f = Vary the inner-zone size Flux limiter = Neutron rate (1/s) Exp. f = f = µm 98 µm 58 µm Exp TC5662

11 A flux limiter of may be too large for 28-µm-thick shells Neutron rate (1/s) Neutron rate (1/s) Diameter = 900 µm Diameter = 1000 µm Diameter = 950 µm Diameter = 1100 µm Exp. f = f = f = TC5654

12 For a 20-µm shell and 1-ns square pulse, increasing the flux limiter brings the absorption fraction into the experimental error range Absorption fraction Experimental range Yield over clean Flux limiter Flux limiter TC5663

13 Summary/Conclusion Simulations of experiments on the OMEGA laser suggest slightly larger values than currently used for the flux limiter The increased precision of the neutron time-of-flight (NTD) diagnostic and the reproducibility of laser shots permit a more precise determination of the flux limiter and central cell zoning. The timing and the level of both the shock yield and the onset of the compression yield are sensitive to the flux limiter. A flux limiter of (sharp cutoff) gives general agreement with experiment for 1-ns square pulses and 20-µm CH shells. Differences for the 400-ps square pulse and the 28-µm CH shell imply that other factors affect target dynamics. Diagnostics are being developed to measure the flux-limiter in crogenic targets. TC5661