NIF-Scale Hohlraum Asymmetry Studies Using Point-Projection Radiograph of Thin Shells Page: 4 of 18
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specifications on flux asymmetry as a2/a0 < 1.0%, a4/a0 < 0.5%, a6/a0 < 0.3
%, and a8/a0 < 0.25%. [3]
Asymmetry Diagnostics
A variety of different techniques measure the symmetry in hohlraums
using surrogate capsules. The reemission ball [4] is a solid Bi ball with the
same radius as an ICF capsule. Thermal radiation on the ball heats it up, and
it reemits the radiation at its characteristic temperature, which will vary from
point to point on the ball if the incoming flux is asymmetric. When viewed
in 2 KeV x rays, the emission from the ball is highly sensitive to the
temperature, and thus is sensitive to the incoming flux. Thus small
variations in temperature are magnified, and can be measured as a function
of time.
- Another asymmetry diagnostic is the foam ball [5]. This is a solid sphere
of low-density SiO2 or CH. A converging shock produces a visible limb,
and the radius of the limb as a function of angle gives information about the
incoming flux asymmetry as a function of time. The speed of the shock
varies as the square root of the incident flux.
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symmetry capsule. For example, spherical hohlraums with tetrahedral
illumination (four laser entrance holes) have yielded triangular implosion
images [6], showing that the part of the capsule that lies under a laser
entrance hole feels a reduced flux.
In this paper, we will concentrate on the thin shell diagnostic. Table 1
below compares the demonstrated accuracy of the various techniques for P2,
P4, P6 and P8.
Table 1. Experimental accuracy demonstrated at Nova/Omega, scaled to
NIF, for the four asymmetry diagnostics discussed in this paper. The first
column is the accuracy to a P2 perturbation that lasts for 2 ns. The
remaining columns show both the accuracy to a perturbation on the foot
only, and the response to the perturbation being constant for all time.
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Pollaine, S.; Bradley, D.; Landen, O.; Wallace, R. & Jones, O. NIF-Scale Hohlraum Asymmetry Studies Using Point-Projection Radiograph of Thin Shells, article, October 6, 2000; California. (https://digital.library.unt.edu/ark:/67531/metadc718422/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.