Impact of imposed mode 2 laser drive asymmetry on inertial confinement fusion implosions

Abstract

Low-mode asymmetries have emerged as one of the primary challenges to achieving high-performing inertial confinement fusion implosions. These asymmetries seed flows in the implosions, which will manifest as modifications to the measured ion temperature (T ion ) as inferred from the broadening of primary neutron spectra. The effects are important to understand (i) to learn to control and mitigate low-mode asymmetries and (ii) to experimentally more closely capture thermal T ion used as input in implosion performance metric calculations. In this paper, results from and simulations of a set of experiments with a seeded mode 2 in the laser drive are described. The goal of this intentionally asymmetrically driven experiment was to test our capability to predict and measure the signatures of flows seeded by the low-mode asymmetry. The results from these experiments [first discussed in M. Gatu Johnson et al., Phys. Rev. E 98, 051201(R) (2018)] demonstrate the importance of interplay of flows seeded by various asymmetry seeds. In particular, measured T ion and self-emission x-ray asymmetries are expected to be well captured by interplay between flows seeded by the imposed mode 2 and the capsule stalk mount. Measurements of areal density asymmetry also indicate the importance of the stalk mount as an asymmetry seed in these implosions. The simulations brought to bear on the problem (1D LILAC, 2D xRAGE, 3D ASTER, and 3D Chimera) show how thermal T ion is expected to be significantly lower than T ion as inferred from the broadening of measured neutron spectra. They also show that the electron temperature is not expected to be the same as T ion for these implosions.Published under license by AIP Publishing. https://doi.

Impact of stalk on directly driven inertial confinement fusion implosions

“…This is currently believed to be due to fill fraction uncertainties; nominally identical implosions have previously been demonstrated to give DT and D 3 He yields in excellent agreement with expectation. 20 The stalk effect implications of the results will be discussed in Sec. IV.…”

“…Shell qR is inferred from the KO-p yield, 58 and total qR from the D 3 He-p energy downshift. 20,59 The proton spectrum measurements also give D 3 He-p yield (Table III). The qR measurements are discussed in Appendix A.…”

“…3 Low-mode asymmetries have emerged as a primary challenge to achieving highgain ICF implosions [4][5][6] and have been the subject of significant recent work (e.g., Refs. [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.…”

“…22 In this case, the stalk was found to significantly impact the mixing of the shell material into the central cavity for 15-lm thick plastic targets shot with a 1-ns square laser pulse, and good agreement between simulated and measured yields was obtained with the inclusion of the stalk in the 3D simulations. Recent work 15,20 suggests the significant impact of the stalk on implosion dynamics and the flow field in the burning fuel. This raises the question of exactly how the stalk perturbs the implosion and what the stalk impact is on signatures of implosion asymmetry.…”

Direct-drive target designs as energetic particle sources for the Laser MégaJoule facility

“…2015; Gatu Johnson et al. 2019). D 3 He gas produces a large variety of energetic particles such as protons at a birth energy of 3.03 MeV and 14.68 MeV, Tritons at 1.01 MeV, 4 He at 3.67 MeV, 3 He ions at 0.82 MeV, and neutrons at 2.45 MeV.…”