fastignition

Fast ignition is a concept in inertial confinement fusion that proposes separating the steps of compressing the fusion fuel from igniting it. In a fast-ignition scheme, the fuel pellet is first compressed to high density by a conventional, longer-duration energy drive, and then a secondary, ultra-short, high-intensity pulse delivers energy to a small region of the compressed core to ignite fusion reactions. The idea is to create a localized hot spot that initiates burning in the surrounding fuel, potentially increasing the overall energy gain or reducing the total energy required from the driver.

Mechanism and variants: The ignition drive is typically a very short, high-intensity laser pulse or a particle beam that generates a jet of energetic electrons or ions. These fast particles travel into the dense core and deposit energy, raising the central temperature to ignition conditions. Variants include electron-driven fast ignition and ion-driven fast ignition, each with different transport and coupling challenges. A central technical issue is how efficiently the energy from the heating beam can be transported through the outer layers and deposited into the central region without preheating or dispersing the fuel.

Rationale and challenges: Fast ignition aims to reduce the energy required to achieve ignition and to ease some mechanical and symmetry constraints of compression. It faces significant challenges in energy coupling efficiency, transport through dense plasma, and precise timing and location of energy deposition. Preheating, laser-plasma interactions, and hydrodynamic instabilities complicate the path to reliable ignition.

History and status: The concept was proposed in the 1990s as a two-step approach to inertial confinement fusion. Since then, experimental programs have explored energy transport and coupling, but a definitive demonstration of ignition with net energy gain via fast ignition has not yet been achieved.