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FAST IGNITION

The research on thermonuclear fusion, mainly inertial confinement fusion (ICF), is an important mission within the pulsed-power center of Sandia National Laboratories. The parameters achieved with the Z-Accelerator get close to this scenario, and the undergoing ZR-upgrade will get us even closer. The conventional ICF scenario achieves ignition by the heat and density resulting of pure compression of a hydrogen (more precisely: a mixture of deuterium and tritium) capsule. However, symmetric compression of the hydrogen is difficult. There is an alternative concept known as Fast Ignition: In this concept an ultra-intense laser (fast ignitor) shoots into an already compressed target and increases the temperature locally to surpass the needed energy density for ignition. People like to compare the two ignition scenarios with a diesel engine (pure compression) and a gasoline engine (where the fast ignitor is the equivalent to the spark plug). The fast fast ignitor can relax the extremely tight compression conditions needed for ICF; refs.: Tabak 1994, Slutz 2005.


Simple description of ablative compression until ignition.


Simple description of fast ignition.

REPLAY

While the Z-Accelerator can provide substantial compression on a fusion pellet, local capsule heating with the upcoming Z-Petawatt laser can get us closer to ignition and increase the neutron yield and the maximum energy density. Fast ignition efforts at Sandia will include studies of ways to inject the petawatt pulse (including the study of imploding asymmetric capsules which allow easy access of the laser to the compressed fuel, laser heating of materials and eventually integrated fast ignition experiments.
Common fast ignition concepts usually rely on laser generated electrons, but scenarios based on laser generated proton beams are in discussion as well:


Cartoon of a proton based fast ignition scenario; ref.: Geissel 2005

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