South pole bang-time diagnostic on the National Ignition Facility Page: 4 of 8
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Draft
are sensitive to both x rays and neutrons, the signals are separated
by 40 ns due to their relative speeds and can be easily
discriminated.X rays from
implosion
ipso X-ray filters
.
-- HOPG mirror
Hevimet shielding
PCD assembly
E21011J1shots were anticipated to have much higher background x rays,
the SPBT was designed to maximize the x-ray bang signal-to-
background contrast in two ways: the diagnostic view of the
background x-ray source is minimized and crystal
monochromators select the optimum x-ray energy.
The background signal from a NIF hohlraum implosion is
primarily a result of x rays coming from the hohlraum's inside
wall due to thermal radiation and bremsstrahlung radiation
induced by fast electrons from laser-plasma interactions. SPBT
reduces this background by minimizing its view of the
hohlraum's inside wall. The diagnostic head is positioned on the
hohlraum's south polar axis placing the detectors at z = -316 cm
from target chamber center (TCC) with a view of the capsule
implosion through the LEH of the hohlraum, as shown in Fig. 3.(a)
FIG 1. A diagram of the south pole bang-time (SPBT) diagnostic
head shows the layout of the photoconductive detectors (PCD's)
and highly oriented pyrolytic graphite (HOPG) mirrors. The red
arrows illustrate one of the x-ray pathways from an entrance hole
to a PCD.
An individual detector assembly, shown in Fig. 2, is a
modification of an original design of Glebov.10 In our design, 50-
52 impedance is maintained over the transition from the cable
center conductor to the diamond surface by using a coaxial
Macor dielectric disk and a 15-pm-thick electrode on the
diamond lower surface. This eliminates a ringing observed near
the peak of the detector impulse response function.
4tr-
E21171J1
FIG 2. PCD assembly. The Macor disk along with the 15-pm-
thick gold electrode sputter deposited on the diamond bottom
maintain a 50 2 impedance from the diamond to the cable.
A preliminary test of a CVD diamond PCD to record the
x-ray bang showed the necessity of reducing the background.
The x-ray signal from the 840-kJ NIF shot N091030 had the
strongest bang signal-to-background ratio recorded by this
diagnostic but the signal was a factor of -7x smaller than the
background signal. Since subsequent higher laser power NIF(b)
North pole
DIM's PCD view
Hohlraum
Minimum end
thickness 50-pym Au
SPBT viewE19456J1
FIG 3. (a) The SPBT is installed in the NIF with the detector
head on the target chamber's south polar axis viewing upward.
The SPBT detector head is mounted to the target chamber on the
end of a metal arm. The upper part of the arm is removable to
allow one access into the NIF target chamber via a man-lift. (b)
SPBT on-axis view (red) through the lower laser entrance hole
(LEH) ensures that the only part of the hohlraum's inside wall
directly seen by the diagnostic is the opposite LEH rim.
A small portion of the opposite LEH rim is the only inside
portion of the hohlraum directly viewed by the diagnostic [see
Fig. 3(b)]. SPBT is shielded from the rest of the hohlraum's inner
wall by a minimum of 50 pm of gold in the hohlraum's end wall.
Collimating the view further was not considered to keep the
alignment tolerances loose and avoid in-situ adjustments.
SPBT was designed to further improve the signal-to-
background contrast by incorporating monochromators to select x
rays where the capsule x-ray brightness was predicted to be
maximized with respect to the x-ray background. Figure 4 shows
that a peak exists in the simulated x-ray spectrum during the
capsule bang around 10 keV predicted by the radiation-
hydrodynamics code LASNEX.11 The background spectrum for aV
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MacPhee, A.; Edgell, D.; Bradley, D. K.; Bond, E. J.; Burns, S.; Callahan, D. A. et al. South pole bang-time diagnostic on the National Ignition Facility, article, May 1, 2012; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc846110/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.