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Implosion dynamics measurements at the National Ignition Facility
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10.1063/1.4769268
/content/aip/journal/pop/19/12/10.1063/1.4769268
http://aip.metastore.ingenta.com/content/aip/journal/pop/19/12/10.1063/1.4769268

Figures

Image of FIG. 1.
FIG. 1.

(a) Target design showing the position of the zinc backlighter foil and the viewing slots through the hohlraum. Hohlraum dimensions are given for one type of design (see Table III). (b) Schematic of ablator showing dimensions for a graded Si-doped capsule (see Table I).

Image of FIG. 2.
FIG. 2.

Simulated radiograph used to perform the test analysis. Radiograph has 50 ps of temporal blurring and 17 μm of spatial blurring.

Image of FIG. 3.
FIG. 3.

Simulated x-ray intensity profile at various times (black) along with the best fit from the original regularization analysis (dashed red) and the inferred unattenuated intensity (blue). Gaussian density profiles are assumed throughout. The radiograph, with 50 ps of temporal blurring and 17 μm of spatial blurring, has been temporally and spatially deconvolved prior to analysis.

Image of FIG. 4.
FIG. 4.

Comparison of results obtained from analyzing the simulated streak using the previously described forward technique (dashed red) and the new inverse technique (dotted green). Values obtained directly from the simulated density profiles are shown for cases where the ablation front is defined in terms of a radiation temperature cutoff (solid black) and a density cutoff (dotted black). Both forward and inverse radiographic analysis techniques accurately capture the simulated inputs for all parameters.

Image of FIG. 5.
FIG. 5.

Measured radiograph from the Rev5 Ge-doped CH shot, N110625. (a) Raw data on four strips. Each strip, separated by , records 3 images separated by . The center of each image is obscured by the shadow of the tungsten fiducial wire. (b) Stitching the images together sequentially in time produces a “streak,” with time going from left to right. Note the slightly brighter spot in the streak, converging from the bottom left, is likely a thin spot in the ablator.

Image of FIG. 6.
FIG. 6.

Radiographic lineouts from Shot N110625 (black) showing the best forward fit to the data (dashed red) and the inferred unattenuated backlighter intensity (blue).

Image of FIG. 7.
FIG. 7.

Data from 3 ConA shots. (a) Laser power at LEH (solid lines) and backlighter power (dotted-dashed lines) versus time. Data extracted from radiograph: (b) Ablator center-of-mass radius versus time, also showing the laser pulse (dotted) for reference, (c) Ablator center-of-mass velocity versus radius, (d) Ablator versus radius, (e) Ablator mass remaining fraction versus radius, (f) “Rocket curve” of velocity versus mass, and (g) Shell thickness versus radius. Data are given by solid points with error bars. Simulations are shown with solid lines. For these shots, the simulations used laser multipliers to roughly account for shock timing data but use a 100% multiplier on the peak of the pulse. Ignition requirements of 330 μm/ns in shell velocity and 14.5% total remaining mass are marked on the velocity and mass axes with dashed-dotted lines. Note that a timing error resulted in only partial data being acquired on N101220. Color legend is given in (a).

Image of FIG. 8.
FIG. 8.

Data showing the comparison of Si-doped and Ge-doped implosions. Laser pulses for N110625 (Ge) (green) and N110627 (Si) (red, black) were nearly identical and overlap each other. Si-doped capsules have a higher peak velocity and lower remaining mass than Ge-doped capsules indicating a higher ablation pressure is achieved using the lower-Z dopant. Simulations used laser multipliers to fit shock timing data in the foot of the pulse but use a 100% multiplier on the peak of the pulse. The apparent early deceleration of the Si-doped capsules is due to deceleration at the ablation front, rather than the inner edge of the capsule, and is likely due to hydrodynamic growth in these unpolished capsules.

Image of FIG. 9.
FIG. 9.

Data showing the improvement in shell velocity achieved by increasing the peak power duration by 600 ps. The increased acceleration using the extended pulse is apparent in radius and velocity records, along with the reduced ablator mass and thickness. Simulations used laser multipliers to fit shock timing data in the foot of the pulse and an 87% multiplier on the peak of the pulse to attempt to bring the simulations into better agreement with these data.

Image of FIG. 10.
FIG. 10.

Radiographs from 3 different shots ordered sequentially in time. These images span radii from and a time window of .

Image of FIG. 11.
FIG. 11.

A study of implosion dynamics before, during, and after the main laser pulse. Measurements on each shot covered a window set by the backlighter pulse length. To cover the period from the 2nd shock to the end of the main pulse, camera timings were shifted from shot to shot, building a composite history of a characteristic 2 ns rise implosion using shots N111220.2, N111218, and N111219. Note that though N111220.2 used a 1 ns rise pulse and N111218 used a shorter duration pulse, radiographic measurements were performed before these features occurred, and thus were still characteristic of the "standard" 2 ns rise pulse. Shot N111220.1 was the first shot to use a 3 ns rise pulse. Simulations use shock timing multipliers as well as an 85% multiplier in the peak.

Image of FIG. 12.
FIG. 12.

Comparison of measured (red) and simulated (black) density-opacity, , profiles around the time of the 2nd shock (left) and approaching peak velocity (right). At the early time, the inner peak corresponds to the doped region of the shell while the outer peak corresponds to the incoming shock wave in pure CH. Late in time, the observed shell is thicker than simulations.

Image of FIG. 13.
FIG. 13.

A detailed comparison of shell behavior around the time of 4th shock break-out and near peak velocity for the fast (1 ns) and slow (3 ns) rise pulses. Pulses for both 1 ns shots (red and black) and 3 ns shots (blue and green) were nearly identical. Slight differences in simulated results within each pair are mainly due to target variations. Unexpectedly, the fast rise pulse resulted in a velocity that was slower than the slow rise pulse (both rise times have the same peak power and laser energy). This suggests that the slow rise is more efficient at converting laser energy to capsule kinetic energy. Simulations here have an 87% multiplier on the peak.

Image of FIG. 14.
FIG. 14.

Comparison of the performance of an implosion using a longer, lower power () laser pulse with a uranium hohlraum to a previous shot using the shorter, higher power () pulse in a gold hohlraum. Both shots have a similar foot and slope-of-rise. The value of the lower peak power was designed to produce the same peak velocity capsule, taking into account the longer pulse and different hohlraum material. Radius, velocity, and mass appear close to simulations using an 87% multiplier on the peak power. The lengthened pulse reduced shell thickness late in time.

Image of FIG. 15.
FIG. 15.

Streaked radiograph from shot N120408 showing spatial and temporal scales. The central wire provides a background reference. X-ray emission from the stagnating core is visible at . At later times, the radiating, outgoing blast wave is apparent.

Image of FIG. 16.
FIG. 16.

Comparison of shell dynamics to different dopant concentrations and distributions. Data were recorded on a streak camera. By design, the velocity and mass of these shots were similar. The shell thickness of data using the graded dopant (N120408) was slightly smaller than that for the standard graded dopant concentration (N120324), indicating a higher shell density was achieved with more dopant.

Image of FIG. 17.
FIG. 17.

Comparison of the dynamics of a thicker shell implosion (N120418) with the standard-thickness shell (N120408). Both capsules use the 2× graded doped design. The thicker shell is driven with a longer and higher power pulse designed to give a similar peak velocity. The similar fractional mass remaining between the shots means that the thicker shell has a larger absolute final mass (since its initial mass is larger).

Image of FIG. 18.
FIG. 18.

Data for a layered “THDConA” capsule (N120329) in a gold hohlraum as compared to a standard gas-filled ConA capsule (N120306) in a uranium hohlraum. Similar capsule velocities were achieved by design, with the gold hohlraum being driven at higher peak power. Replacing the additional CH payload with the x-ray transparent THD layer results in a lower mass observed in the THD-ConA target as expected.

Image of FIG. 19.
FIG. 19.

Comparison of maximum measured velocity (relative to the ignition requirement) as a function of peak laser power for different shots. Low peak power shots primarily used the more efficient uranium hohlraums with the low-coast pulse, while the higher peak power shots used gold hohlraums, some with a larger LEH diameter. Shots using Ge-doped ablators, the larger LEH, or a 2% uniformly doped ablator resulted in lower velocities for a given peak laser power.

Image of FIG. 20.
FIG. 20.

The path of the rocket curve relating velocity to fractional mass remaining is expected to be weakly dependent on pulse shape and dopant, allowing data from many different experiments to be co-plotted. Here, only data taken at radii larger than 300 μm are shown to focus on the acceleration phase of the implosion. Experimental data generally have lower masses than simulations at a fixed velocity, suggesting a lower-than-expected ablation efficiency throughout the implosion. The simulated curve for Ge-doped capsules, on which the ignition requirements were set, lies at the upper limit of the envelope of curves for Si-doped capsules, indicating that the lower-Z ablator is slightly less ablatively efficient. Note that the dotted-dashed lines represent the Rev5 ignition requirements as established for Ge-doped capsules.5

Tables

Generic image for table
Table I.

Summary of capsule types used in these experiments, showing nominal dimensions, densities, and dopant concentrations at cryogenic shot temperatures (). Individual layer properties for the graded doped designs are listed in order from smallest radius to largest. In the case of the THD target, the mass given is for the ablator only. The actual (measured) outer radius and total thickness on a given shot vary slightly from these nominal values and are listed in Table III.

Generic image for table
Table II.

Radiographic imaging setup for each shot: (a) The primary difference between the two target platforms used was in the length of the hohlraum slots cut around the equator for radiographic viewing. ConA (and THDConA) targets had 1.2 mm long slots, while ConAw targets had 2.4 mm long slots. (b) Experimental data recorded were either gated (GXD) or streaked (DISC); the numbers refer to the specific camera used. (c) Experiments were begun on DIM(90-315) and, to reduce facility transactions, switched to DIM(90-78). (d) Total magnification of the target at the image plane was measured on each shot using parallax of image features. The DISC internal magnification of 1.24 is included. (e) The imaging slit width in the snout was adjusted according to expected resolution requirements. (f) The field of view was 1.2 mm for measurements at late time only, and 2.4 mm for other measurements as fixed by the target platform and the backlighter pointing separation. (g) The distance of the slit to the target determined from the magnification.(h) Backlighter quads were set to be outer cone quads most opposite the DIM being used. (i) The distance of the backlighter foil from capsule center was set by the need for the beams to clear light shields. (j) Separation of the backlighter quad pointing positions was set by the required field of view. (k) Backlighter energies were higher for shots requiring viewing over a longer time window. (l) Backlighter peak powers were generally set near facility limits at the time of the shot.

Generic image for table
Table III.

Essential capsule, hohlraum, and laser parameters for each shot, along with some key neutron and x-ray core emission results. (a) Ablator thickness at shot temperature; (b) Outer radius of ablator at shot temperature; (c) Dopant type; All are graded doped except Si 2%, which is uniform doped. Si2x corresponds to Si concentration in the doped layers; (d) Gas fill in atomic %; all used apart from a single shot and a layered THD shot. (e), (f), (g), (h): Hohlraum material and inner dimensions; Laser wavelength separation between (i) beams and outer beams, and (j) beams and beams; (k), (l): Laser energy and peak power incident at the LEH (thus does not include backlighter beams); (m) Rise time of the main (4th) pulse, as specified for a 420 TW drive (lower peak powers have shorter rise times, such that the slope is preserved); (n), (o) Primary neutron yield and Doppler-inferred ion temperature. For fills, this refers to the D-D yield; for the THD fill this corresponds to the DT yield; (p) X-ray bang time refers to the time of peak x-ray emission as measured by DIM-based framing cameras or by the south-pole bang time diagnostic (or both); (q), (r), (s): Legendre modes of the integrated x-ray core emission (at stagnation), as measured by DIM-based framing cameras.

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/content/aip/journal/pop/19/12/10.1063/1.4769268
2012-12-07
2014-04-19
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Implosion dynamics measurements at the National Ignition Facility
http://aip.metastore.ingenta.com/content/aip/journal/pop/19/12/10.1063/1.4769268
10.1063/1.4769268
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