Inertial Confinement Fusion
In a fusion reaction, deuterium and tritium combine to produce helium, a neutron, and excess energy
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.
Laboratory for Laser Energetics
University of Rochester, Rochester, NY
The world's leading experimental facility for the direct-drive approach is here at the Laboratory for Laser Energetics (LLE) of the University of Rochester. Our OMEGA laser system creates 60 laser beams, yielding a total energy of over 30,000 J focused on a pellet for an interval of about 10–9 seconds. Pellets have already been compressed to 100 times liquid density and to temperatures in excess of 4 keV (4 x 107 K). Substantial nuclear yields have been recorded. The OMEGA laser system is also used for studies of indirect-drive and high-energy-density physics.
The OMEGA laser system was built at a cost in excess of $50 million, and its operation and initial cost are all supported by the US Department of Energy. The OMEGA laser is the most powerful UV laser system in the world and will remain so until the completion of an even more powerful laser (the $3.5 billion National Ignition Facility or NIF) at LLNL in the year 2009. Even then, vigorous research will continue, using OMEGA, on ICF, high-energy-density physics, laboratory astrophysics, and related technologies.
National Ignition Facility
Lawrence Livermore National Laboratory
For more information please contact Prof. Riccardo Betti.