FY2005 Progress Summary and FY2006 Program Plan Statement of Work and Deliverables for Development of High Average Power Diode-Pumped Solid State Lasers, and Complementary Technologies, for Applications in Energy and Defense Page: 56 of 67

                                
with thicker ablators that have ~30-40% left at ignition; however, the latter will be burdened
with a higher energy content debris flux.
The laser drive pulse shape is shown in Fig 5.1.2 and is distinguished by a high intensity
"picket" prepulse to set the ablator on a high adiabat, thereby enhancing its Rayleigh-Taylor
stability through increased ablation velocity. The pulse shape is modeled as a prepulse plus four-
shock system, i.e., picket, foot, 2nd shock, 3rd shock, and main shock. Pedestals were used for
ease of locating shock convergence with rise/fall times of 100 ps (the assumed risetime
capability of DPPSL and KrF). The rise/fall times can be increased to -200-300 ps providing
timing centroids and integrated energies are maintained; a slight timing retune would be
required. Shocks 1-3 are timed to coalesce at inside edge of fuel at time of breakout of picket
shock. The 4h (main) shock is timed to reach fuel/ablator interface at time when rarefaction
shock from 1-4 coalescence reaches there.
Power                             zoom-2
zoom-t 1
1.0       0.25Pmax                             .425TW
0'1       prepulse                                -350TW
0.01       4
57:1 foot
Time              22.4ns
Fig. 5.1.2. Laser pulse shape for the baseline target.
Two zooms are used where spot size is reduced to critical radius at that time. The zoomed laser
absorption fraction is -98%, whereas the unzoomed absorption fraction would be ~90%
The new baseline target performance is shown in Table 5.1.3 under both KrF (0.248 pm)
and DPSSL (0.349 pm at 3o) illumination, and obtained from LASNEX, our standard radiation-
hydrodynamic-burn code. Note from the pulse shape above that the peak power for KrF was
reduced from a maximum of 425 TW at the second zoom point down to 350 TW in order to
maintain the peak laser intensity at the critical surface consistent with a nominal LPI limit of
I?2~l.Ox10" Wcmhpm2 at k=0.248 pm. The second and third columns show the DPSSL
performance at the same peak power and same LPI limit, respectively.
Gains of -150 at drive energies of -2.5 MJ are obtainable with this target design.
Without zooming, the gains would decrease to -121 at a drive energy of -3 MJ. Note that at the
same peak power, the DPSSL-driven target is very similar to that of the KrF target. However, if
we operate it at the same LPI Ik2 limit, the gain decreases from -150 to -130.

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