Burning DT Plasmas with Ultrafast Soft X-Ray Pulses

Fast ignition with narrowband, coherent ultrafast soft x-ray pulses\footnote{S. X. Hu, V. N. Goncharov, and S. Skupsky, ``Burning Plasmas with Ultrashort Soft-X-Ray Flashing,'' to be published in Physics of Plasmas.} has been investigated for cryogenic deuterium--tritium (DT) plasma conditions achieved on the OMEGA Laser System. In contrast to using hard x-rays (\textit{h$\nu $} = 3 to 6 keV) proposed in the original x-ray fast-ignition proposal, we find that soft x-ray sources with \textit{h$\nu $} $\approx $ 500-eV photons can be more suitable for igniting the dense DT plasmas. Two-dimensional radiation--hydrodynamics simulations have identified the breakeven conditions for realizing such a ``hybrid'' ignition scheme (direct-drive compression with soft x-ray heating) with 50-\textit{$\mu $}m-offset targets: an $\sim $10-ps soft x-ray pulse (\textit{h$\nu $} $\approx $ 500 eV) with a total energy of 500 to 1000 J to be focused into a 10-\textit{$\mu $}m spot size. A variety of x-ray pulse parameters have also been investigated for optimization. It is noted that an order of magnitude increase in neutron yield has been predicted even with x-ray energy as low as $\sim $50 J. Scaling this idea to a 1-MJ large-scale NIF target, a gain above $\sim $30 can be reached with the same soft x-ray pulse at 1.65-kJ energy. Even though such energetic x-ray sources do not currently exist, we hope that the proposed ignition scheme may stimulate efforts on generating powerful soft x-ray sources in future. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.