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The effect of laser pulse shape variations on the adiabat of NIF capsule implosions
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10.1063/1.4807331
/content/aip/journal/pop/20/5/10.1063/1.4807331
http://aip.metastore.ingenta.com/content/aip/journal/pop/20/5/10.1063/1.4807331

Figures

Image of FIG. 1.
FIG. 1.

(a) Typical laser pulse shape for NIF layered DT implosions showing the sub-epochs of the pulse (picket, trough, 2nd, 3rd, and 4th pulses). (b) Shock trajectories in mass coordinates through the ablator, DT ice layer, and central DT gas regions. 1st shock (red), 2nd (green), 3rd (blue), and 4th (magenta). The dashed box is the inner 20% of the DT ice layer showing the nominal shock timing sequence. (c) Plot of the fuel adiabat vs. time for the inner 20% of the DT ice layer showing the four distinct pressure jumps from each successive shock.

Image of FIG. 2.
FIG. 2.

(a) Mis-timed shock trajectories for the case of picket and trough laser powers decreased by 2% driving a weaker 1st shock, which merges earlier than nominal within the DT ice layer. Blue dashed curve is the 1st shock trajectory for nominal power. (b) Mis-timed shock trajectories for picket and trough laser powers increased by 2% resulting in increased decompression time at the DT ice-gas interface. (c) Adiabat history averaged over the inner 20% of the DT ice layer for shock trajectories of (a) in red compared to the nominal adiabat history (black). (d) Adiabat history averaged over the inner 20% of the DT ice layer for shock trajectories of (b).

Image of FIG. 3.
FIG. 3.

Plot of adiabat averaged over the full (100%) DT ice layer vs. deviation from nominal power in individual epochs of the laser pulse: picket + trough together (red), picket (green), trough (blue), 2nd (cyan), 3rd (magenta), and 4th (black).

Image of FIG. 4.
FIG. 4.

Plot of neutron Down-Scattered-Ratio (DSR) for layered DT implosions on NIF vs. shot sequence over a two-year period. Red dashed line separates implosions performed without any attempt to tune the shock strengths and timings (shots 1–4) vs. those performed with tuned shocks (shots ≥ 5).

Image of FIG. 5.
FIG. 5.

(a) Laser pulses used for layered DT shots varying the rate-of-rise to peak power: N120126 (3 ns, slow-rise, blue), N120114 (2 ns, nominal rise-rate, green), and N120131 (1 ns, fast-rise, red). (b) Laser pulses varying the peak power level and duration: N120219 (427 TW, 4.57 ns duration, red), N120311 (334 TW, 5.55 ns duration, blue). Unintentional trough power variation is also seen.

Image of FIG. 6.
FIG. 6.

VISAR-measured shock velocity histories in the 3rd and 4th shocks for slow-rise (N120103, blue), nominal rise-rate (N111108, green), and fast-rise (N120106, red) pulses in keyhole tuning shots illustrating the transition from a continuous compression (slow-rise, blue) to a strong shock compression (fast-rise, red).

Image of FIG. 7.
FIG. 7.

Plot of adiabat (a)–(c) for full DT ice layer and corresponding DSR (d)–(f) vs. variation of the laser pulse shape in the 4th pulse: (a) and (d) variation of rise time from 3rd power level to peak power (ns), (b) and (e) variation of 4th power level (±30%), (c) and (f) variation in 4th pulse duration (ns). Inset figures in (a)–(c) show the 4th pulse shape variations.

Image of FIG. 8.
FIG. 8.

(a) Measured DSR history vs. shot sequence colored by 4th pulse rate-of-rise: 3 ns, slow-rise (blue), 2 ns, nominal rise-rate (green), and 1 ns, fast-rise (red). (b) Peak laser power (open square symbols) and 4th pulse duration (closed circle symbols) vs. shot sequence. Vertical dashed line at shot 15 delineates change from relatively higher-power, shorter-duration pulses to lower-power, longer-duration pulses.

Image of FIG. 9.
FIG. 9.

Deviation of delivered laser energy from requested value in (a) picket, (b) trough, (c) 2nd pulse, (d) 3rd pulse, and (e) 4th pulse. Dashed line is the absolute energy goal and shaded region is the r.m.s. “stretch” requirement from Ref. . Symbol color legend in (e) identifies target type: Keyholes (blue), Symcaps (yellow), Convergent Ablator (ConA, green), layered DT capsules (red), and other laser-only shots (black). (f) Correlation between picket and trough deviations from request.

Image of FIG. 10.
FIG. 10.

Plot of measured neutron Down-Scattered-Ratio (DSR) vs. average deviation of foot (picket + trough) laser power from request. 3ns, slow-rise (blue), 2 ns, nominal rise-rate (green), and 1 ns, fast-rise (red). Dashed line is from 1-D simulation accounting for the average measured difference in picket and trough deviations between keyhole tuning shots and layered DT shots as shown in Figure 11 .

Image of FIG. 11.
FIG. 11.

Correlation between picket and trough deviations from request for all keyhole tuning shots (blue) vs. all layered DT shots (red) illustrating a statistical difference between the two. The average deviation from request for keyhole shots in (picket, trough) is (−0.8, −1.0%). The average deviation from request for layered DT shots in (picket, trough) is (+1.4, +1.7%). Thus, DT shots are statistically over-driven by 2.2% in the picket and 2.7% in the trough. Inset shows difference in 4th pulse between nominally identical keyhole and DT shots.

Image of FIG. 12.
FIG. 12.

DSR history normalized for foot laser power deviation vs. shot sequence. Symbols are colored by 4th pulse rate-of-rise: 3 ns, slow-rise (blue), 2 ns, nominal rise-rate (green), and 1 ns, fast-rise (red). Open symbols highlight the lower-power, longer-duration pulses.

Tables

Generic image for table
Table I.

Allowable laser energy deviation for Δα = +0.05.

Generic image for table
Table II.

Summary of DSR vs. 4th pulse slope, power, and duration.

Generic image for table
Table III.

Summary of r.m.s. shot-to-shot laser deviations.

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/content/aip/journal/pop/20/5/10.1063/1.4807331
2013-05-31
2014-04-21
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The effect of laser pulse shape variations on the adiabat of NIF capsule implosions
http://aip.metastore.ingenta.com/content/aip/journal/pop/20/5/10.1063/1.4807331
10.1063/1.4807331
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