OPTIMIZATION OF EXPERIMENTAL DESIGNS BY INCORPORATING NIF FACILITY IMPACTS Page: 4 of 7
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in modeling debris and shrapnel is presented in
a companion paper. [2]
In section II, we discuss the use of relatively
inexpensive borosilicate glass as disposable
debris shields (DDS's) to protect the more
expensive fused silica main debris shields
(MDS's). We show results on the first use of
these DDS's on a series of NIF shots using four
beams. In section III, we discuss how damage
to a NIF detector was mitigated by making a
change in a diagnostic component, tilt of an x-
ray pinhole, to direct shrapnel away from the
detector. In section IV, we discuss the potential
problem of narrow sprays of molten shrapnel on
DDS's. We conclude in section V.
II. DISPOSABLE DEBRIS SHIELDS
We shield the most expensive optical
components in the final optics assembly, such as
the wedge focus lens, from the hostile target
chamber environment, using a combination of
DDS's and MDS's. This is in contrast to the
majority of other laser facilities that use a single
set of debris shields. The large-scale production
of borosilicate glass for flat panel monitors,
allows for their use as relatively inexpensive
DDS's. However, this glass has sufficient
nonlinear absorption of 3w light to limit the
thickness to 1-3 mm. This thickness is not
sufficient to stop the largest and fastest
fragments so an additional 1-cm thick fused
silica MDS is placed behind the DDS. In
addition, to providing additional protection from
penetrations, the use of two debris shields helps
with the problem of damage growth. By
limiting the number of damage sites on the
MDS's, they can remain in the chamber for
many more shots than an unprotected MDS. The
shot limit for an unprotected MDS is primarily
set by the total scattering area of a very large
number of relatively small damage sites. These
damage sites are caused by laser damage, which
is enhanced by debris on the surface, and
shrapnel impact. In contrast, the shot limit for a
protected MDS is primarily limited by growthof a relatively few damage sites making sure
their size does not exceed the thickness of the
optic. The shot limit for DDS's is determined
by shrapnel penetration and to a less extent by
debris induced transmission loss.
The use of DDS's in a target chamber was
tested for the first time during a series of NIF
shots in September and October of 2004. We
placed DDS's on three of the four beam lines.
On one of these beam lines, we changed the
DDS a number of times during the series of 21
shots. In Fig. 1, we show the MDS that was
unprotected by DDS's. The beam footprint is
clearly seen, where debris is not removed by
beam cleaning around the edge of the optic. A
very large number of damage sites are seen in
the optic associated with a combination of
debris and modulations in the beam associated
with upstream optics. Modeling predicted that
any shrapnel generated during these shots would
be directed away from the optics[2] and no
shrapnel induced damage sites were observed on
the unprotected MDS or any of the DDS's. One
of the protected MDS's is shown in Fig.2.
Essentially no debris reached this MDS so the
beam footprint is visible and the number of laser
induced damage sites is substantially reduced.
The transmission for all DDS's placed in the
chamber was measured at the completion of the
experiments and compared to DDS that were
not used. The loss of transmission for the two
DDS's that were in for all 21 shots was
measured to be -2% less than the DDS not
placed in the chamber. The loss in transmission
for the DDS's that were in for fewer shots was
measured to scale roughly with the number of
shots they were in the chamber.
III. DIRECTING SHRAPNEL
One of the early hydrodynamic experiments on
NIF showed the need to direct shrapnel away
from sensitive components. During early
operation the use of Be x-ray shields was
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Eder, D. C.; Whitman, P. K.; Koniges, A. E.; Anderson, R. W.; Wang, P.; Gunney, B. T. et al. OPTIMIZATION OF EXPERIMENTAL DESIGNS BY INCORPORATING NIF FACILITY IMPACTS, article, August 31, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc873803/m1/4/: accessed April 20, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.