Mini 'H-bomb' blasts: a key to the feasibility of fusion power use?

While the leading concept for a hydrogen fusion power reactor features some form of magnetic container, a radically different approach is not far behind. "It's been a banner year for inertial fusion," says, Gregory Canavan, director of the Office of Inertial Fusion at the US Department of Energy (DOE). This is a form of fusion in which the hydrogen fuel is contained in tiny pellets , smaller than a pinhead. Compressed by intense beams of laser light or of particles, they explode like miniature hydrogen bombs.

Although it currently has further to go before demonstrating its worth as a power source, inertial fusion research has progressed enough in the past year for experts such as Dr. Canavan to feel confidence that they will have a significant fusion reaction running in laboratory experiments by mid-decade. What may be even more important from the researchers' point of view: They expect by then to be in a position to set up the experiments that would establish the ultimate feasibility of their process.

Thus, as the United States and other nations -- Japan, Western Europe, and the Soviet Union -- gear up to push fusion power development more swiftly, physicists have more than one major strategy they can follow. "this is what technologists like," says Richard L. Schriever, Canavan's deputy, "two drastically different approaches with two different sets of problems. If we can't solve it one way, maybe we can solve it another."

Inertial fusion is different from magnetic confinement schemes in more than its technical aspects. It arose, in the US, out of nuclear weapons research. One of the main objectives of the program remains the development of a system by which the mini-explosions can simulate the effects of full-scale hydrogen bombs. Thus, the program retains a substantial military aspect.

Inertial fusion takes its name from the nature of the confinement used to hold the fuel together long enough to produce significant amounts of energy. The fuel must be compressed to something like 1,000 times the density of liquid hydrogen and heated to around 100 million degrees.

Laser beams heat the outside of the pellet causing it to vaporize and blow off at high speed. This creates a rocket-like reaction that almost instantaneously compresses the fuel, heating it in the process. If the fuel can be compressed to a high enough density and temperature, fusion will ignite and "burn" through the fuel before the compressed mass flies apart. It takes a small, but crucial, amount of time for the fuel particles to accelerate because of their inertia. This is an inertial effect, hence the term inertial fusion.

Experiments in the US have not yet achieved what researchers call a "significant thermonuclear burn," meaning a mini-explosion producing as much energy as it takes to compress the fuel pellet. Canavan says this should occur with a new device using laser "drivers," called NOVA, now being built at the Lawrence Livermore National Laboratory (LLNL) in California. That calls for a compression to 1,000 times liquid density. Compressions to 100 times liquid density already have been achieved with LLNL's Shiva Laser installation.

As the NOVA system is brought up to full power, it should go beyond the "significant burn" stage to achieve yields of about 10 percent more energy than it takes to ignite the fusion. For a magnetic fusion device, that would be encouraging. But for inertial fusion, Canavan says, it is not good enough.

He explains that the compression process has inherent inefficiencies so that much higher yields are needed for inertial fusion to be commercially attractive. This, he says, means that inertial fusion has to go through an extra step of demonstrating high efficiency after it demonstrates fusion ignition -- a step that magnetic fusion does not face. That will take a more advanced facility than NOVA.

This is the main reason inertial fusion is considered to be less advanced than magnetic fusion, even though both aproaches are expected to demonstrate significant fusion processes within the next four or five years.

Nevertheless, Canavan believes that igniting fusion with NOVA will be a "pivotal" demonstration because it will establish the feasibility of burning small spark plugs of fuel. These will be needed to start an effective burn in the larger fuel pellets that will be used for high energy yields.

It is important to demonstrate conclusively that high energy gains can be produced from small masses of fuel, Dr. Schriever notes. The most important issue in doing this, he explains, is to find the best driver -- whether it be a laser or a particle beam accelerator. Much of the research in the next few years will be aimed at this. There are complex technical questions of energy efficiency, of how beams (light or particle) interact with fuel pellets, of how to focus particle beams, and so forth.

Part of the reason for optimism, Schriever says, is that experiments over the past year give encouraging indications that some of the critical technical difficulties can be overcome.

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