Polar Direct Drive implosions on the Laser MegaJoule

We present the first two direct-drive implosion experiments with symmetric laser beam illumination on the LMJ laser facility, performed in the last two years. Two different implosion regimes have been investigated, both using 10 laser bundles, i.e. 20 quads and 80 beams. In both cases, we used square laser pulse shapes and the laser pointing has been optimized with the Polar Direct Drive (PDD) technique as well as power balance between irradiation cones.

The 2025 experiment investigated the exploding-pusher regime using D2-filled 1-mm diameter few-micrometer thick glass capsules and PDD settings on the quad level. We obtained a well-characterized and robust platform that can be employed for particle generation, though with a small convergence ratio ≈ 3 and a limited yield of up to 2x1011 neutrons. While PDD optimization improves implosion symmetry, the implosion performances appear to be robust against low-mode asymmetries.

The 2026 experiment was aimed at improving the implosion performances by exploring a more compressive (or ablative) implosion regime. For this purpose, larger targets were used, namely D2-filled 2-mm diameter 20-µm thick plastic capsules. To deal with the capsule size, we tested a more advanced PDD scheme, with moderate quad defocusing as well as quad splitting in addition of PDD on the quad level. In this configuration, convergence ratios were between 5 and 10, and neutron yields > 5x1011 were reached. Important in-flight as well as hot-spot deformations have been measured. Nevertheless, the neutron yield remained relatively robust, indicating that the implosion regime was not yet fully compressive.

These first experiments pave the way for direct-drive experiments upcoming in the next years, either through academic access or in the frame of the French TARANIS project for Inertial Fusion Energy.