General Fusion

A different approach is to fill a small (~1 cm) spherical shell of plastic or glass with a deuterium-tritium gas mixture. Intense lasers are fired all around the sphere, exploding the outside of the shell. The rest of the shell is accelerated inwards by the pressure of this explosion and compresses and heats the gas mixture to fusing conditions. The inertia of the imploding material alone keeps the plasma together for a short time. Because the hot plasma exists for only a very short time, it must be very dense to produce enough fusion reactions to make more energy than the laser energy.

Atmosphere Formation

Laser beam rapidly heats the surface of the fusion target forming a surrounding plasma envelope.

Compression

Fuel is compressed by the rocket-like blowoff of the hot surface material.

Ignition

During the final part of the laser pulse, the fuel core reaches 20 times the density of lead and ignites at 100,000,000 degrees Celcius.

Burn

Thermonuclear burn spreads rapidly through the compressed fuel, yielding many times the input energy.

	Laser energy
	Blowoff
	Inward transported thermal energy

ICF has been experimentally demonstrated to produce more fusion energy than the energy needed to initially implode the shell. Unfortunately, in these experiments, the power to compress the shell came from an atomic explosion, not a very practical system. Nevertheless, it was established that one needs at least ~1 MJ (0.3 kWh, 1000 Btu) of initial energy to compress the shell in order to achieve break-even. This is the energy released in burning 30 g of gasoline, which is a relatively small amount of energy. However, a 1 MJ laser is a very big device. The present experiments use lasers of around 100 kJ, 10 times too small to achieve break-even. However, they are large enough to investigate the physics, compare the experimental results to computer models and drive relatively large numbers of fusion reactions. Scientists working with the Omega laser at the Laboratory for Laser Energy in Rochester NY and the Nova laser at the Lawrence Livermore National Laboratory in California are exploring this alternative.

The US is presently building a $3 billion, 1.8 MJ laser called the National Ignition Facility (NIF) at Livermore (www.llnl.gov/nif). Once completed, it is expected to implode shells and produce more fusion energy than the laser energy. However, the so-called wall-plug efficiency (electricity produced/electricity used) will still be less than one, mainly because of the poor electrical efficiency of the laser. It is built by the department of defense, mainly to test computer simulation of fusionning plasma against experiments. The physics in these implosions is similar to the physics of thermonuclear weapons and the US is forbidden by peace treaties to conduct nuclear explosion tests.