Role of Hot-Electron Preheating in the Compression of Direct-Drive Imploding Targets with Cryogenic D2 Ablators

The compression of direct-drive, spherical implosions is studied using cryogenic D₂ targets on the 60-beam, 351-nm OMEGA laser with intensities ranging from ~3×10¹⁴ to ~1×10¹⁵ W/cm². The hard-x-ray signal from hot electrons generated by laser-plasma instabilities increases with laser intensity, while the areal density decreases. Mitigating hot-electron production, by reducing the laser intensity to ~3×10¹⁴ W/cm², results in areal density of the order of ~140 mg/cm², in good agreement with 1D simulations. These results will be considered in future direct-drive-ignition designs.