Spatially resolved X-ray emission measurements of the residual velocity during the stagnation phase of inertial confinement fusion implosion experiments

Abstract

A technique for measuring residual motion during the stagnation phase of an indirectly driven inertial confinement experiment has been implemented. This method infers a velocity from spatially and temporally resolved images of the X-ray emission from two orthogonal lines of sight. This work investigates the accuracy of recovering spatially resolved velocities from the X-ray emission data. A detailed analytical and numerical modeling of the X-ray emission measurement shows that the accuracy of this method increases as the displacement that results from a residual velocity increase. For the typical experimental configuration, signal-to-noise ratios, and duration of X-ray emission, it is estimated that the fractional error in the inferred velocity rises above 50% as the velocity of emission falls below 24 μm/ns. By inputting measured parameters into this model, error estimates of the residual velocity as inferred from the X-ray emission measurements are now able to be generated for experimental data. Details of this analysis are presented for an implosion experiment conducted with an unintentional radiation flux asymmetry. The analysis shows a bright localized region of emission that moves through the larger emitting volume at a relatively higher velocity towards the location of the imposed flux deficit. This technique allows for the possibility of spatially resolving velocity flows within the so-called central hot spot of an implosion. This information would help to refine our interpretation of the thermal temperature inferred from the neutron time of flight detectors and the effect of localized hydrodynamic instabilities during the stagnation phase. Across several experiments, along a single line of sight, the average difference in magnitude and direction of the measured residual velocity as inferred from the X-ray and neutron time of flight detectors was found to be ∼13 μm/ns and ∼14°, respectively.

Dynamic high energy density plasma environments at the National Ignition Facility for nuclear science research

“…While radiography and imaging of self-emission x-rays are the primary methods for observing the symmetry of the capsule during compression [89][90][91], current radiography methods go blind due to the high capsule density prior to stagnation, and the x-ray images are strongly influenced by the emitting materials and opacity near stagnation. To understand the shape of the burn volume, whether the fuel is burning where it is emitting x-rays, or how the remaining ablator or cold fuel are distributed around the burning core, the 14.1 MeV neutrons that are produced by the burning fuel provide a useful probe.…”

“…Design concept for a gamma imaging system in the NIF neutron imaging line-of-sight. This configuration, or facsimile, will be implemented in the equatorial neutron imaging lines-of-site, located in NIF polar coordinates at (90,315) and (90,213).…”

“…The specific thin disks of activation material, Zr for activation with 14 MeV neutrons from DT in 90 Zr(n, 2n) 89 Zr reactions, are located in insertion devices at specific locations both within the chamber and are also distributed at specific locations on the outside of NIF chamber flanges (FNAD) (figure 6) [56,57]. The distribution of measured neutron yields monitors the absolute fuel ρR and its angular distribution in high yield shots since a larger ρR means less activation in the direction of the corresponding activation target due to greater scattering of the fusion neutrons out of that line-of-sight.…”

“…The design incorporates thick (1-4 cm) tungsten filtering between the PTOF detector and TCC to reduce the x-ray background by a factor of 1000x, and uses a fixed dipole magnet with peak field strength of 1 T to bend protons with energy between 6 and 16 MeV around the shielding and onto the PTOF detector. The MagPTOF has been implemented as a ride-along diagnostic on DIM (90,78), utilizing the same detectors, cables, and recording systems as the PTOF.…”

Subpercent-Scale Control of 3D Low Modes of Targets Imploded in Direct-Drive Configuration on OMEGA

“…Over the last decade, different experiments have shown that targets imploded with the current laser facilities experience significant growth of their low-mode nonuniformities. Mode l = 1 was typically inferred from properties of the final assembly including asymmetry in its areal density [18], variation of its ion temperature along different lines of sight [19], hot-spot motion [20], and asymmetric x ray emission of a Ti layer embedded at the inner surface of the shell [21]. Modes l ≥ 2 were measured from the hot-spot shape [22,23], xray [24] or Compton [25] radiography, x-ray absorption spectroscopy [26], and self-emission shadowgraphy [27].…”