Measurements of the Conduction-Zone Length and Mass Ablation Rate in Cryogenic Direct-Drive Implosions on OMEGA

Measurements of the conduction-zone length ($110\pm 20\mu \mathrm{m}$ at $t=2.8\mathrm{ns}$), the averaged mass ablation rate of the deuterated plastic ($7.95\pm 0.3\mu \mathrm{g}/\mathrm{ns}$), shell trajectory, and laser absorption are made in direct-drive cryogenic implosions and are used to quantify the electron thermal transport through the conduction zone. Hydrodynamic simulations that use nonlocal thermal transport and cross-beam energy transfer models reproduce these experimental observables. Hydrodynamic simulations that use a time-dependent flux-limited model reproduce the measured shell trajectory and the laser absorption but underestimate the mass ablation rate by $\sim 10\%$ and the length of the conduction zone by nearly a factor of 2.

Figure 1

(a) The laser pulse shape is shown (black curve) along with a comparison of the time-resolved scatter light power measured (green curve), calculated with hydrodynamic simulation using the nonlocal thermal transport and cross-beam energy transfer models (red curve) and using the time-dependent flux-limiter model (blue curve). (b) Self-emission x-ray image calculated after the laser has burned through the outer CD layer. The image contains two rings. The inner ring corresponds to the emission at the ablation surface (dashed curves), and the outer ring corresponds to the emission at the CD-DT interface (dotted-dashed curve). (c) Comparison of the density profile (the blue curve corresponds to DT and the orange curve corresponds to CD), normalized temperature profile (green curve), and normalized self-emission lineout (black curve) calculated 460 ps after the laser has burned through the outer CD layer. In (b),(c), the x-ray self-emission was calculated by postprocessing the hydrodynamic simulation with SPECT3D [27].

Figure 2

Self-emission images (a),(c),(e) and profiles azimuthally averaged over 360° [black line (b),(d),(f)] measured at $t=2.2\mathrm{ns}$ (a),(b), $t=2.5\mathrm{ns}$ (c),(d), $t=2.6\mathrm{ns}$ (e),(f). The position of the radial shifts added to the peak intensity determined in the 360° averaged profile are plotted [black line in (a),(c),(e)]. The self-emission profiles (dashed red lines), the position of the ablation front (small dashed blue line), and the position of the CD-DT interface (dotted-dashed green line) calculated with the hydrodynamic simulations using the nonlocal thermal transport and cross-beam energy transfer models (b),(d),(f). The calculated profiles were convolved using the point spread function of the diagnostic [28, 31].

Figure 3

The CD is ablated at 2.34 ns when the CD-DT interface separates from the ablation surface (dotted-dashed line). The conduction-zone length is determined from the distance between the measured ablation surface (squares) and CD-DT interface (diamonds) at the time when the interface crosses the critical density surface (2.8 ns). The rapid increase in wavelength shift at 2.8 ns is a result of the CD-DT interface crossing the critical density surface. The 5% intensity contour is used to determine the maximum wavelength shift (dashed curve). Third-order polynomials fit to the ablation front (solid curve) and to the CD-DT interface (dotted-dashed curve).

Figure 4

(a) Comparison of the measured ablation front (squares) and CD-DT interface (circles) trajectories with ablation front (solid red curve) and the CD-DT interface (upper dashed red curve) calculated using a simulation that includes nonlocal thermal transport and cross-beam energy transfer models ($\mathrm{NL}+\mathrm{CBET}$) and the CD-DT interface trajectory calculated using a simulation with a time-dependent flux-limited (FL) thermal transport model (lower dashed red curve). The flux limiter was adapted to have the ablation front radius match the measured ablation front at each time. The laser pulse (black curve) corresponds to the right axis. (b) Comparison of the averaged mass ablation rate of the CD (solid squares) and the size of the conduction zone (open circles) measured at $t=2.8\mathrm{ns}$ with simulations that use nonlocal thermal transport and cross-beam energy transfer models and time-dependent flux-limiter model.