Page 436 - High Power Laser Handbook

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

for four quads. This was sufficient to cause the 48-quad rms power
variation to fall outside the specification during the early part of the
drive and for a small portion of the peak. If those quads are excluded
from the analysis, the specification is met. NIC experiments will pro-
vide the opportunity to repeat these tests, gather more statistics on NIF
performance, and continue to improve our pulse-shaping techniques.
Figure 14.41 displays the results of a 96-beam pointing measure-
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ment made on January 14, 2009. 48 beams from each hemisphere of
NIF were aimed at an 8 × 6 rectangular array of target locations on a
flat metal target, and the resulting x-ray emission pattern was
observed with two static x-ray imagers. An additional six beams—
four from below and two from above—illuminated small holes in the
target plate, enabling precise collocation of the upper and lower
images and providing a global measurement of the beam-to-target
pointing accuracy. The red (blue) squares in each image surround the
holes illuminated by beams hitting the holes from the far (near) side
of the target. The x-ray emission centroids differed from their intended
locations on the target by an rms error of 64 ± 4 μm, compared with
the current Rev. 5 point design specification of less than or equal to
80 μm. The worst beam missed its mark by ~120 μm, twice as good as
the Rev. 5 requirement of less than or equal to 250 μm.
NIF’s ability to achieve target-chamber-center (TCC) pulse syn-
chronization is illustrated in Fig. 14.42. Short (88-ps) impulses are
fired at a flat target positioned at TCC, and the x-ray emission is mea-
sured with one of two streaked x-ray detectors (SXDs). The target can
be oriented either normal to the SXD field of view, thus maximizing
the number of beams that can be viewed simultaneously, or at an
angle, allowing cross timing between upper and lower hemispheres.
Spatially separating the beam target points and temporally stagger-
ing the impulse times in the MOR allowed as many as 96 beams to be
observed on the two SXDs on a single shot. Using this method, the
192 NIF beams were timed to within 64 ps rms, which is worse than
the 30-ps specification but sufficient for the 2009 target shots.
In early 2010, a set of four fiber optic cables was installed on a
diagnostic manipulator that could be placed at the center of the NIF
target chamber. Low-energy laser pulses (from the NIF regenerative
amplifier) can be directed onto these fibers, allowing the relative
timing of each beamline on NIF to be measured, four beamlines at a
time, at a high repetition rate. Using this new capability has allowed
us to adjust the timing of NIF to less than 30 ps rms, which meets both
the NIF Functional Requirements and Primary Criteria and the NIC
ignition requirements.
NIF’s pulse-shaping capability is exquisite. A combination of
model-based shot setup, an advanced arbitrary wave form generator
pulse-forming network, and careful attention to stabilized front-end
operation have given the laser the ability to generate pulses varying
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from 88 ps quasi-gaussians to 20 -ns shaped pulses. The current target