Gigabar Spherical Shock Generation on the OMEGA Laser

This Letter presents the first experimental demonstration of the capability to launch shocks of several-hundred Mbar in spherical targets—a milestone for shock ignition [R. Betti et al., Phys. Rev. Lett. 98, 155001 (2007)]. Using the temporal delay between the launching of the strong shock at the outer surface of the spherical target and the time when the shock converges at the center, the shock-launching pressure can be inferred using radiation-hydrodynamic simulations. Peak ablation pressures exceeding 300 Mbar are inferred at absorbed laser intensities of $\sim 3\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. The shock strength is shown to be significantly enhanced by the coupling of suprathermal electrons with a total converted energy of up to 8% of the incident laser energy. At the end of the laser pulse, the shock pressure is estimated to exceed $\sim 1\mathrm{Gbar}$ because of convergence effects.

Figure 1

Experimental setup used to infer the shock and laser ablation pressure at SI-relevant intensities.

Figure 2

An x-ray framing camera captured a short x-ray flash at the time when the shock converged in the center. The timing in each frame gives the peak time of the electrical gating pulse relative to the start of the laser pulse. The color bar indicates the measured emission intensity in arbitrary units.

Figure 3

Total energy converted into suprathermal electrons versus laser energy. Blue squares indicate that SSD was on, whereas red circles indicate that SSD was off. Up to $\sim 8\%$ of the total laser energy was converted into suprathermal electrons at moderate temperatures (50–100 keV). The error bars represent half the difference between HERIE and BMXS.

Figure 4

Integrated x-ray (2–6 keV) intensity lineouts from the center of the target for two shots with 27 kJ of laser energy and 2.2 kJ of suprathermal electron energy (upper curves) and 24 kJ of laser energy and 0.8 kJ of suprathermal electron energy(lower curves).

Figure 5

Comparison of experimental (solid lines) and simulated (dashed lines) curves for shot 71589 with incident laser power (gray line), laser absorption (blue lines), and hard x-ray emission (red lines) resulting from suprathermal electrons (in arbitrary units). The experimental hard x-ray emission was averaged over the three highest HXRD channels and has a $\pm 5\%$ uncertainty due to noise and deconvolution error. The absorption data are averaged over two scattered light diagnostics each with an uncertainty of $\pm 5\%$.

Figure 6

Simulated ablation pressure (blue lines) and shock pressure (red lines) as a function of time for shot 72679. The solid lines indicate a simulation that matches all experimentally observed quantities using a flux limiter of 5%. The dotted lines are the simulation results in the absence of suprathermal electrons (flux limiter of 5%). The dashed lines indicate a simulation that also matches the x-ray flash time but in the absence of suprathermal electrons (the flux limiter was increased to 8%). For reference, the solid gray line indicates the laser pulse.

Figure 7

Scaling of the inferred maximum ablation pressure with suprathermal electrons (solid red circles and solid line) and effective maximum ablation pressure without suprathermal electrons (open blue circles and dashed line) versus the maximum laser intensity that is absorbed at the critical surface for simulations matching all of the experimental observables. The black dash-dotted curve indicates the ablation pressure scaling for nonconstrained simulations without suprathermal electrons and a constant flux limiter value of 6.5%.