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The velocity campaign for ignition on NIFa)
a)Paper BI3 2, Bull. Am. Phys. Soc. 56, 25 (2011).
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Image of FIG. 1.
FIG. 1.

Delivered laser pulses for shots to explore impact of increased laser energy on velocity at constant power using convergent ablator targets. Laser energy was increased from 1.25 MJ (shot N111011) to 1.47 MJ (shot N111009). Pulse is compared to a DT shot (N110914) and shows the reduction in peak power because 8 beams are used for backlighter beams in the convergent ablator shots.

Image of FIG. 2.
FIG. 2.

With convergent ablator target, 12 backlit snapshots are taken of the shell as it implodes. These can be put together to get a radius versus time for the Imploding capsule.

Image of FIG. 3.
FIG. 3.

The velocity of the center of mass of the shell and the mass remaining is extracted from the convergent ablator data.

Image of FIG. 4.
FIG. 4.

Hydra calculations show that the fuel is moving faster than the center of mass of the capsule.

Image of FIG. 5.
FIG. 5.

Calculations are used to relate the center of mass velocity and mass remaining to the fuel velocity.

Image of FIG. 6.
FIG. 6.

Hydra calculations show that the 11 keV x-ray yield is a sensitive function of velocity. Data from the convergent ablator experiments agree with this trend.

Image of FIG. 7.
FIG. 7.

South pole bangtime diagnostic shows 160 ps earlier bangtime with and 1.67 x higher x-ray yield with depleted uranium (DU) hohlraum compared to gold (Au) hohlraum.

Image of FIG. 8.
FIG. 8.

Gated x-ray detector images ofthe hotspot for symcap experiments using a uranium hohlraum (P2/P0 =35.5% ± 3%) and a gold hohlraum (P2/P0 = 31.2% ± 6.9%) show no significant change in symmetry with uranium hohlraum. Neutron yield was 1.5 x higher for uranium than gold hohlraum.

Image of FIG. 9.
FIG. 9.

Hohlraum aspect ratio was re-optimized after the first experimental campaign on NIF. New hohlraum (“575 hohlraum”) is shorter but with a larger radius, which was intended to put more laser energy near the waist of the capsule.

Image of FIG. 10.
FIG. 10.

All shock-timed implosions with 544 hohlraums had negative P2 (oblate or “pancaked” implosions). 575 hohlraum allows P2 to be zero.

Image of FIG. 11.
FIG. 11.

(a) Orientation of the m = 4 polar asymmetry relative to the location of the inner cone beams. 3-d Hydra calculations show a reversal—the image should be large where the beams are brighter. This image suggests the 30 degree beams are brighter than the 23.5 degree beams. (b) The m = 4 asymmetry can be corrected by changing the wavelength separation between the 23.5 and 30 degree beams.

Image of FIG. 12.
FIG. 12.

Hydra calculations show that the P2 symmetry can swing as a function of time over the emission profile if the 2nd and 3rd cone fractions are not properly tuned.

Image of FIG. 13.
FIG. 13.

Visar data using the mirrored keyhole target show that tuning the cone fraction in the 2nd and 3rd pulses improved the pole to equator symmetry of the shocks.

Image of FIG. 14.
FIG. 14.

P2/P0 symmetry of the imploded capsule has a significantly smaller swing after tuning the cone fraction in the 2nd and 3rd pulses than before tuning.

Image of FIG. 15.
FIG. 15.

After tuning the cone fraction in the 2nd and 3rd pulses, the dP2/dt is close to zero when P2 is zero.


Generic image for table
Table I.

Comparison of capsule performance metrics show improved performance with depleted uranium hohlraum compared to gold hohlraum.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The velocity campaign for ignition on NIFa)