The standard approach to ICF with central hot spot ignition puts extreme requirements on the spherical symmetry of the implosion, tolerating deviations of only a few percent. The new concept of fast ignition affords promise for significantly relaxing this requirement, by separating fuel compression from fuel ignition.

The imploded DT fuel with a core density of several 100 g/cc is ignited by a separate petawatt-peak-power laser pulse. At least 10 kJ of ignition energy have to be delivered to the core within 10 picoseconds to initiate fusion burn in a peripheral hot spot from where it then spreads over the whole fuel volume. It is only since recently that laser pulses adequate for this purpose have become available. With envisioned intensities of 10^{20}\:{\rm W}/{\rm cm}^2, the interaction physics involves all facets of relativistic laser plasma dynamics such as hole boring, beam generation and transport, but now in unexplored density regimes far above solid density.

The focus of present fast ignition research is on cone guiding. Here a gold cone is inserted into the capsule to keep a path free of plasma and let the ignitor beam approach the fuel core more closely at the time of maximum compression. The final distance to be bridged by electron transport can be reduced to 100 – 150 μm in this way. Though the cone severely breaks spherical symmetry – just notice the strongly distorted imploded core – , this can be tolerated because ignition will occur by means of the laser/electron beam incident through the cone. This cone-guided fast ignition may be not very practical for future power reactors, but presently it is a way to achieve ignition in single-shot experiments. This promise was impressively demonstrated in first experiments, in which the fusion neutron yield could be increased by more than a factor 100.