Page 437 - High Power Laser Handbook

404 So l i d - S t at e La s e r s The National Ignition Facility Laser 405

modeling indicates that ignition will require pulses with 3ω contrast
(maximum:minimum power) slightly in excess of 200:1. The first
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attempt at generating such a pulse with the full NIF facility, fired on
August 21, 2010, actually delivered 1.04 MJ at a contrast of 187 and
matched the requested peak power to within less than 5 percent.
Because only NIF target experiments will be able to determine
the precise pulse shape needed for low-adiabat, high-convergence
capsule implosion at the scale of interest, design requirements include
the ability to make subtle pulse-shape adjustments. Figure 14.43
shows PDS measurements from 2007 that illustrate this flexibility.
Two pulse-shape requests are shown by the dotted lines—a baseline,
or “unshifted,” pulse in red and a “shifted” pulse generated by
increasing the power at 8 ns by 10 percent and delaying the point at
10 ns by 100 ps. In both cases, these were full 1.3 MJ, 385 TW FNE
(7 kJ, 2 TW per beamline) pulse shapes; the scale has been expanded
to emphasize the low-power portion, because that is where the
changes were made. The two solid lines are averages of multiple single-
beam measurements. The laser was first adjusted to give a good fit to
the unshifted shape; then 16 shots were taken without changing the
front-end setup. The result is the blue solid curve. The front-end setup
was then changed, based solely on numerical prediction, and 12 shots
with this modified setup were taken, producing the solid red curve.
NIF’s success in accurately achieving the requested pulse-shape
modification is apparent.

Conclusion
The NIF laser is by far the largest and most complex optical system
that has ever been built. It has about 3 times as many beams and
60 times the energy or power of its nearest competitor. It can do point-
ing, timing, pulse shaping, and power balancing with unprecedented
precision. While work continues to improve the laser performance,
important plasma physics experiments are progressing, and early
results compare well with expectations. Backscatter has been measured
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at less than 10 percent. Full-ignition-scale (1 cm length × 5.4 mm
diameter) hohlraums have been heated to 285 eV with megajoule
drive energies, in agreement with radiation-hydrodynamics simula-
tion. High-convergence implosion of ignition-scale capsules has been
achieved, and the ability to tune the symmetry has been demon-
strated. As of the date of this writing (July, 2010) we are on track to
begin the first-ever inertial fusion ignition campaign in late 2010. The
first cryogenic layered tritium, hydrogen, deuterium shot was fired
in October 2010, and the shock-timing campaign began in November
2010.
NIF has also begun to fulfill its other two missions: ensuring the
safety and reliability of the U.S. nuclear deterrent and serving as an