Inertial Confinement Fusion

The Basics

The NOVA reactor (inertial confinement)
NOVA: ICF reactor

At the Lawrence Livermore labs in Livermore, California. Image courtesy of General Atomics.

Inertial confinement is the other direction or research towards plasma confinement. This technique involves imploding a small fuel pellet (most likely a 50/50 mixture of deuterium and tritium). If it is compressed quickly and hard enough, temperature and density rise, allowing the reaction to reach or exceed the Lawson criterion. It is the inertia of the imploding pellet that keeps it confined momentarily. Because it is confined only by its own inertia, the plasma lasts for about one nanosecond. Therefore, to achieve breakeven point, a very large density is needed, usually around 10²⁴ particles/cm³, which is many times more than lead.

The fuel pellet, or target, is compressed and heated with what are called energy drivers. These high-powered sources of energy are usually either high-powered laser or ion beams, which bombard the target from all sides symmetrically. The outer layer of the pellet vaporizes and moves away from the pellet like a rocket. This projection creates shock waves which go on to compress and heat the core. The compressed fuel then burns, releasing much energy, and expands. This is partially offset by the shock waves, which tend to continue compressing the material. This behavior is known as inertia. The result is an inertal confinement fusion reaction.

Direct and Indirect Drive Fusion Ignition

Image courtesy of the Lawrence Livermore National Laboratory.

There are two types of targets: a direct-drive inertial fusion energy target, and an indirect one. The direct-drive targets are just the spherical pellets containing the fuel (most likely a mix of dueterium and tritium) which will be pounded directly by a laser or ion beam. The indirect-drive targets have the fuel pellet placed inside a hohlraum, which is a small and thin cylindrical container composed of a high atomic number material, like gold or lead. The container will convert the driver beams into x-rays, which subsequently compresses the fuel.

The image to the right is a reperesentation of indirect-drive using a hohlraum. The blue objects in the image are beams entering the hohlraum. The dime is there to show the relative size of the hohlraum used.

Another Promising Advance - The Fast Ignitor

The fast ignitor is a variation on the standard inertial confinement methods. The difference is that an extremely short and intense laser creates a hotspot in the center of the fuel which ignites the core. There are four steps in this process:

The pellet is compressed the standard way as explained above.
An ultra short and intense laser pulse punches a hole through the atmosphere left over from the compression.
An even smaller, intense laser pulse is shot down the newly formed channel, creating a hotspot on the dense fuel for ignition.
The burn spreads throughout the rest of the fuel, releasing large amounts of energy.

The benefits of this scheme is that the size and complexity of the primary compression laser system is reduced, and the amount of energy released to energy absorbed could also increase.

Let's Review:

Briefly describe inertial confinement fusion.

Briefly describe how the fast ignitor works.