Plasma Physics: Revolution in Power

The possibility of generating useful amounts of electricity by blowing hot gas between the poles of a magnet has long intrigued adventurous physicists. Many have tried to turn the trick, but until Dr. Arthur Kantrowitz, chief of Avco Corp.’s Everett, Mass.. laboratory, reported impressive progress last week, large-scale magnetohydrodynamic power seemed as improbable as its nonstop name.

Nothing Simpler. Avco’s Mark II generator has no moving parts; yet it generates 1,350 kilowatts of electric power. Dr. Kantrowitz likes to say that his creation is just like an ordinary rotary generator, but simpler. In one sense he is correct: a conventional generator has electrical conductors (copper wires) that are spun in a magnetic field by a steam turbine. Their motion causes a current to flow through them. In an MHD generator the conductor is a hot gas (plasma) that has been ionized by having electrons knocked off some of its atoms. When the plasma squirts rapidly between the poles of a powerful magnet, a current flows across its stream and is picked up by electrodes.

But though MHD generators may be simple in principle, they are devilish devices to design, mostly because of the extremely high temperature that must be reached to ionize their gas and make it conductive. Avco does the job by preheating the incoming air that eventually becomes plasma, adding extra oxygen to it, and burning oil with it in a chamber that looks like a rocket engine. This produces a flame with a temperature of 5,300° F. Spiked with a little powdered potassium carbonate to increase the ionization, the flaming stream of gas shoots between the poles of a magnet at 3,000 to 4,000 ft. per second, about three times the speed of sound at ordinary temperatures. The plasma cools down considerably as it flows, and a good part of its residual heat is used to raise the temperature of the incoming air.

Minor Expense. Racing gas at 5,300° quickly destroys almost anything it comes in contact with, but Avco claims to have licked this problem by building the wall of its MHD generator out of water-cooled metal pegs separated by heat-resistant ceramic. This material has already lasted through six days of steady use. Dr. Kantrowitz feels sure that in a full-scale plant it will last long enough to make replacement a minor expense. Nor is he worried about the current that is consumed by resistance in the coils of the generator’s magnet—a problem that has plagued experimenters in the past. Avco scientists are working on coils made of newly developed superconducting material that loses all electrical resistance when cooled to the temperature of liquid helium. A magnet with such coils can maintain a powerful field while consuming no current at all.

MHD research is expensive and may become more so, but Avco is backed by a consortium of eleven powerful electric companies that sense a power revolution not many years away. If a full-scale generator works out as well as Dr. Kantrowitz expects, it will turn the chemical energy of coal or oil into electrical energy with 56% efficiency. The most efficient modern steam-driven generators do not get better than 40%.