Fast Ignition

"Fast ignition" is the name given to the process in which a sharp energy spike is used to ignite a small region of fuel that has been pre-compressed. This two-step process is much more efficient. Although compression still requires substantial energy, the energy requirement is reduced by relieving the compression process of the responsibility to also achieve ignition temperature.  Fast ignition by lasers is held to be "futuristic" by the leadership of laser fusion, in consequence of the exquisitely complex plasma physics. In contrast, fast ignition with HIF will rely on the "classical" physics by which energetic ion beams stop in matter, first described by Neils Bohr in 1926.

LLNL graphic shows benefit - if National Ignition Facility were able to do fast ignition

The more comprehensive plots in the figure below demonstrate the scaling of the gain curves more fully toward higher input beam energy. The essential fact is the reliability of ignition once the fuel is compressed. The overwhelming advantage of FPC's approach to ignition is our use of values for fuel compression over normal solid density that are practical as shown by the fact that these values have already been demonstrated. Using only realistic values of compression follows the conventional logic of scaling devices downward in manageable steps from the parameter regimes where their operation is well known and documented. In the case of ICF, the scaling concerns the known physics and technology of fusion bombs. The plain challenge has been to replace the fission ignition source with some kind of clean energy source. The great importance of the discovery of heavy ion fusion in the mid-1970s was that heavy ion beams generated with mainstream RF accelerator technology would be capable of delivering ignition pulses with parameters that would achieve ignition with realistic fuel compression.
Fast ignition first began to be explored in the 1990s based on creative ideas arising from the complex plasma physics arising from the interaction of intense laser light with matter. The above figure is from an early engineering assessment of the potential advantage for fusion power systems: "The Case for IFE Fast Ignition as an IFE Concept Exploration Program, R.B. Stephens et al., UCRL-JC-135800, presented at the 1999 Fusion Summer Study Conference in Snowmass, Colorado, July 11-23, 1999. Although the complex plasma physics of the laser-driven fast ignition processes is highly uncertain, the straightforward processes whereby ion beams interact with matter involve no such uncertainty.