Inertial Confinement Fusion
The most-promising approach to controlled thermonuclear fusion now seems to be via the method called inertial confinement fusion (ICF), or more colloquially “laser fusion.” Edward Teller noted that laser fusion is essentially the internal combustion engine approach to nuclear fusion energy. ICF schemes are based on the following sequence:
- a tiny pellet of deuterium-tritium (DT) isotopes is injected into a blast chamber
- the pellet is compressed to very high densities with an intense laser
- the high density and compression heat induces the ignition of the thermonuclear explosion
- the kinetic energy carried by reaction products–including neutrons, x rays, and charged particles–is deposited as heat in a blanket that acts as a heat source in a steam thermal cycle to produce electricity. An ICF combustion engine would use a series of microexplosions to produce power.
There has been considerable interest in using ICF implosions to simulate thermonuclear weapons. There are two approaches to compress the DT pellet: direct-drive laser fusion and indirect-drive laser fusion. In the direct-drive method, many intense laser beams are focused symmetrically on a hollow plastic pellet filled with a mixture of deuterium and tritium gas. The intense focused energy ablates the shell, and the subsequent rocket action implodes the inner part of the shell, compressing the gas. Substantial nuclear energy gain results if the compressed gas reaches densities of the order of 1000 times that of liquid DT, along with sufficient heating of the gas. In the indirect approach, a pellet is centered within an enclosure (called a “hohlraum”). A few intense laser beams entering the enclosure and striking the walls create x rays. The subsequent “cloud” of x rays then symmetrically ablates the pellet shell and produces the required compression and heating. The world’s leading center for indirect-drive research is the Lawrence Livermore National Laboratory.