Symmetry control of an indirectly driven high-density-carbon implosion at high convergence and high velocity

Abstract

We report on the most recent and successful effort at controlling the trajectory and symmetry of a high density carbon implosion at the National Ignition Facility. We use a low gasfill (0.3 mg/cc He) bare depleted uranium hohlraum with around 1 MJ of laser energy to drive a 3-shock-ignition relevant implosion. We assess drive performance and we demonstrate symmetry control at convergence 1, 3–5, 12, and 27 to better than ±5 μm using a succession of experimental platforms. The symmetry control was maintained at a peak fuel velocity of 380 km/s. Overall, implosion symmetry measurements are consistent with the pole-equator symmetry of the X-ray drive on the capsule being better than 5% in the foot of the drive (when shocks are launched) and better than 1% during peak drive (main acceleration phase). This level of residual asymmetry should have little impact on implosion performance.

Thermal Temperature Measurements of Inertial Fusion Implosions

“…Tens of thousands of 2D HYDRA capsule simulations were run covering a 9dimensional parameter space including drive amplitude variations of ±25%, drive asymmetry variations including Legendre modes 1, 2, and 4 with respective variations of ±2%, ±10%, and ±5%. The simulations were based on a high density carbon ablator design [18][19][20][21]. In particular, these 2D simulations spanned hotspot peak temperatures from ∼ 3 − 9 keV, fuel areal densities from ∼ 0.01 − 4 g/cm 2 , hotspot masses from 1 − 50 µg, and confinement times from ∼ 70 − 130 ps.…”

Impact of stalk on directly driven inertial confinement fusion implosions

“…[7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Low-mode asymmetries have been found to be seeded by factors external to the imploding target, including the engineering features used to support the target, [21][22][23][24][25] target offsets, 26 and laser drive asymmetry, 13,27,28 and are also known to seed directional, non-thermal flows in the assembled fuel, which can be diagnosed through their impact on measured neutron energy spectra (see, e.g., Ref. 8).…”

Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment

“…High-foot designs used the same hohlraum as the low foot, albeit with an even higher helium gas fill of 1.6 mg=cm 3 and suffered from similar hohlraum problems. A parallel effort exploring exploding pushers in near vacuum hohlraums, for neutron diagnostic flat fielding, mitigated losses due to LPI leading to a transition away from high gas-fill hohlraums [58,59]. Without a high gas fill, the hohlraum needed to be larger diameter to delay the ingress of hohlraum plasma into the path of the laser beams.…”