Sunday, October 19, 2014

Progress in Fusion Energy

Lockheed engineers recently claimed to have a breakthrough idea for a fusion reactor. Cheap, plentiful fusion power is only 20 years away!

Cynical observers familiar with the long history of fusion power research contained their enthusiasm. 50 years or so ago, when peaceful fusion power had already frustrated the expectations of its devotees for a while, a prescient wit observed that fusion power was fifty years away - and always would be. If that's been updated to twenty years away - with the same qualification - that might be considered progress of a sort.

The possibility of extracting energy from nuclear fusion was first understood in the 1920, and the great astrophysicist Eddington quickly appreciated that such fusion must power the stars. The Wikipedia article cited above notes that the first fusion reactor patent dates to 1948.

Getting hydrogen atoms to fuse to form helium is a multi-step process, but the basic idea is simple: get two protons close together and they can fuse to form deuterium (plus a positron, a neutrino, and a photon). A couple of more steps gets one to helium. The hard part is getting those protons really close together - about 10^-13 cm apart. It's hard because protons carry electrical charge and consequently repel each other.

It's a pretty easy lab experiment though - just accelerate protons to a few tens of thousands of electron volts (or so) and slam them into a proton rich target, and some tiny fraction will fuse in one of the stages of helium formation. Stars manage the feat by getting their internal temperatures up to about 15 million K. At that temperature, an incredibly tiny fraction of their protons will react. Because stars are really big - about 24,000 Earth masses, minimum - it takes a while for the energy released in fusion to leak out, and enough remains behind long enough to keep the temperature up and the reaction going.

Aside from stars, we really only know one way to keep the fusion energy from leaking out too fast to sustain thermonuclear fusion - take a big mass of hydrogen isotopes and drastically compress it to super high temperature with the blast of X-rays from a nuclear bomb. At the super high temperatures and densities achieved, a whole lot of fusion takes place before it has time to blow itself to smithereens.

Neither the gravitational confinement of the star nor the inertial confinement of the thermonuclear bombs can be domesticated on human scale, so other means of confinement must be sought. Such efforts have been failing now for about 2/3 of a century.