Description
With the achievement of ignition at the National Ignition Facility (NIF), the focus of inertial confinement fusion research is turning to schemes relevant to inertial fusion energy. In particular, the move from indirect drive currently used at NIF to direct drive in which lasers illuminate the fusion fuel directly. This is expected to increase the coupling efficiency and reduce driver energy requirements. Novel implosion designs are being sought that can robustly achieve high fusion energy gain.
In the shock-augmented ignition (SAI) approach, the implosion is driven at reduced velocities that provides hydrodynamic stability and is more energetically efficient. With this reduced velocity though, ignition requires the supplemental energy of a late-timed shock wave that is launched by a sudden increase in laser intensity at the end of the pulse. However, high laser intensities stimulate laser-plasma instabilities that direct energy away from driving the shock. The novel feature of SAI is that the igniting shock wave can be launched with substantially reduced laser intensities by introducing a drop in laser power before rapidly increasing again.
The effectiveness of the scheme was experimentally demonstrated on the OMEGA laser system by the performance increase achieved by optimally timing the shock launch. Key performance metrics, such as fusion yield and hot-spot pressure, saw substantial increases when the shock was timed to arrive at the implosion centre at the point of peak convergence. The optimal timing range and trend in performance were closely matched by simulations.